CA1304169C

CA1304169C - Dry film photoresist for forming a conformable mask and method of application to a printed circuit board or the like

Info

Publication number: CA1304169C
Application number: CA000591968A
Authority: CA
Inventors: Frederick John Axon; James John Briguglio; Amedeo Candore; Lawson Lightfoot; Clancy Patrick Crooks; Leo Roos
Original assignee: Morton International LLC
Current assignee: Eternal Technology Corp
Priority date: 1988-02-26
Filing date: 1989-02-24
Publication date: 1992-06-23
Anticipated expiration: 2009-06-23
Also published as: EP0330339A2; EP0330339A3; DE68928654T2; ES2116967T3; DK89089D0; AU2996189A; KR920008720B1; ATE165674T1; DK89089A; EP0330339B1; JPH026960A; DE68928654D1; SG52247A1; US4992354A; AU625637B2; JPH0666031B2

Abstract

DRY FILM PHOTORESIST FOR FORMING A CONFORMABLE
MASK AND METHOD OF APPLICATION TO
A PRINTED CIRCUIT BOARD OR THE LIKE

ABSTRACT OF THE DISCLOSURE

A dry film for forming a solder mask includes a cover sheet, a photoimageable composition layer which is curable to form a solder mask and a top coat interposed between the cover sheet and the photoimageable composition layer which is selectively adherent to the photoimageable composition layer.
The dry film is applied to a surface of a printed circuit board with a minor portion of the area of the photoimageable composition layer tacked to the printed circuit board. The cover sheet is peeled away. With heat and vacuum, the photoimageable composition layer is laminated to the irregular surface of the printed circuit board, conforming the photoimageable composition layer to the contours thereof and leaving the top coat as a protective covering over the photoimageable composition layer. The photoimageable composition layer is exposed to patterned actinic radiation, developed and cured to form a hard, permanent solder mask.

Description

PATENT
~ 416~
DRY FILM PHOTORESIST FOR FO~MIN~ A CON~ORMABLE
MASK ~ND METHOD OF APPLICATION TO
A PRINTED CIR~UIT BOARD OR THE LIKE

The pre3ent invention is directed to dry ~ilms which orm mask~ that conform to irregular surfaces and a method of forming such conformable ma~ks. One aspect of the present in~ention is directed to a dry film which includes a photoimageable layer which i~ developable and curable to form a permanent solder mask over a printed circuit board or the like and to a method of for~ing a solder ma~k on a printed circuit board or the like using the dry film.

BACKGROUN~ 0~ THE INVENTION
A solder ma~k is a hard~ permanent layer o~
non-conductive material which co~er~ the surface of a printed circuit board or the likeg ~ncapsulatin~ the traces of the printed circuit it~elf. By 301der ms~k i~ meant herein a hard, permanent layer which meets ~he abrasion resistance te ts as defined in IPC-SM-840A, Table 12, Summary of Criteria for Qualification/Con~ormance (Institute for Int~rconnecting and Packagin~ Electronic Circuit9). The solder mask i~
patterned to ~ully cover the circuitry, e~cept for those portions intended to be exposed, e.g., for soldering to another component. Sold~r masks are typically fonmQd ~rom a layer o~ photoimageable composition ~hich i9 applied to a ~ur~ace o the printed circui~ board. The photoimageabl~
layer is expo~ed to actinic radiatio~ whioh is patterned by mean~ of a template or art~ork. Subsequent to exposure, the photoimageable layer is developed'in an organic solvent or an aq~eous ~olution which wa~hes away either exposed or unexpo~ed portions o~ the layer (depending upon whether the photoimageable material i~ positive acting or negative acting). The portion of ~he layer which remain~ on the surface i8 then cured7 e.g. with hea~ and/or W light to form 1763 ~ 35 - Zl~
13~ ! PATENT

a hard, permanent solder mask intended to prote:ct the printed circuitry for the life of th~ board.
One prior art method of applylng che layer of photoimaE~eable composition to the circuit board ~urfac~ is to apply the material in liquid form and then either allow it to dry or partially cure the material to form a semi-~ta~le layer.
There are a number of ad~an~ages to applying a photoimageable layer to a circuit board a~ a dry film rather than as a liquid. In particular, dry films are free of organic ~olvent and therefore eliminate this hazard from the workplaee and eliminate the need for apparatu~ to protect the immediate work environment and the more general environment from organic ~olvent emi~sions. Typically, a dry fil~
compri3e~ a coYer ~heet of support material which is somewhat flexible but which has sufficient rigidity to provide structure to a layer of photoimageable compo~ition which overlies one surface of the cover sheet. Typically, the cover ~heet is formed of polye~ter matcrial, such as polyethylene terephthalate tPET), ~uch as ~hat sold as M~LINE ~.
To protect the photoimageable layer and to enable the dry film to be ro:Lled, it is conventional that the expo~ed surface of the photoimageable layer be covered with a removable protective shQet e.a., a 8heet of polyethylene~ An example of such a dry film is sold aF9 LAMINAR D ~ by the Dynachem Division of Morton Thiokol, Inc.
The method of u~e o~ such a prior ar~ dry ~ilm is ~enerally a~ :Eollows. The protective sheet i~ removed from the photoimageable composition layer immediately prior to application of the dry film to ~he surface of ~he printe~
circuit board. Thi~ ma~ be accomplishet, iEor e~ample9 u~ing automated apparatu~ which peels away and rolls up the protective sheet as the dry ~ilm i~ unrolled from a reel. The try film is applied to the surface of the circuit board with the photoimageable layer in direct contact with the board surface. Using heat, ~acuum and mechanical pressure 5 the pho~oimageable layer i8 immetiately laminated to the surface , "

17~3-35-24 ~3~41 69 PATENT

of the board. The cover sheet remains overlying the photoimageable 1 yer, protecting the photoima~e~ble layer from exposure to oxygen ant from handling damage. The cover sheet also permits an artwork (or template) to b~ laid directly on top of the dry film for contact printing, if contact printing i9 to be used (as i3 usual~y preferred from the standpoint of obtaining optimal image re~olution). The dry film is exposed to patterned actinic radiation throug~ the PET cover aheet.
At thi~ time, the PET support cheet is removed, permitting acce~s to the exposed photoimageable layer by developer.
Depending upon the compo~ition of the photoimageable layer, the photoimageable layer is developed with organic 301vent, aqueous developerg or ~emi-aqueous developer. By semi-aqueou~
developer is meant herein a developer which is about 90% or more by volume aqueous 801ution, balance an organic solvent such a~ 2-buto~y ethanol and other glycol ether8. The photoimageable layer may either be po~itive acting, in which case the e~po~ed portions are removed by developer, or negative acting, in which case ~he unexposed portion~ are removed by developer. Most photoimageable layer~ for preparing solder masks are negative actin~. Most photoimageable composition layers require aome cure subsequ~nt to development to render the layer hard and permanen~ so as to sarve as a solder mask. Depending upon the composition of the photoimageable layer, curine may be effected with heat and/or UV light.
Althou~h a nu~ber of advantages accrue through the use o~
dry films as opposed to coating the c~rcu~l~ board with a liquid compo~ition, there are some problems with ~older ma~k-forming dry film3 which the present inventio~ addre~ses.
Because the cover sheet of a conventional dry film is relatively rigid, the photoima~eable layer cannot Pully conform to the irregular contour~ of the printed circuit board, from which circuitry traoeq rise from the flat aurface of the board itsel~. Becau~e of this, the thickne3s of the photoimageable layer must be ~lightly greater than the elevation of the traces above the surface o the board. For .

:~, . " , _ , ... .... .... .. .....

1763-35~2~
~30416~ PAT~NT

example, i~ the traces rise 75~ above the surface of the board, the photoimageable layer will typically be about 100 thick; when laminated to the board, approximately 25~ of photoimageable composition overlies the upper sur~aces of the trace~.
One unfortunate result of the solder magk being thicker than the elevation of the traces is that elevated regions of solder mask e~ist between those portions of the circuitry from which photoimageable composition is removed during the development of the solder mack~ e.g. those portions of the traces to which a component is to be soldered. Elevated regions are particularly problematic with respect to components which are surface-mounted to the printed circuit board. Components typically contain a plurality of pins which are each to be; soldered to an exposed portion of a trace of th~ circuit board. If the component does not fit closely to the board, it may rest on the surface of photoimageable composition without adequate bonding being established between the pins and the traces. It would be desirable if low regions 3 or "valleys", rather than elevated regions existed between exposed trace portions.
Some common traces are narrower at the foot than at the top, in which;case small o~erhangs extend outward along ~ach side of each trace ~an eRfect known as "mushrooming"). With both liquid compo9itions and conventional dry film9, there inharently remains air-filled void~ below ~uch overhangs. The air trapped in such voids may produce some oxidation o the traces and may lead to~eventual defects in the solder mask.
The present invention is di.rected to an improved dry film ~or forming a;solder magk whieh eliminates some of ~he problems of both~liquid application and conventional dry films. The present invention is also directed to a method of using a dry film to apply a photoimageable layer, which may be one which forms~a solder mask or may be a primary imaging photoresist, to a surface, particularly an uneven surface.
Dry film carrying a primary imaging resist may be applied by ~3~16~ 1763-35-2~
PATENT

the method of the present invention, providing excellent con~ormation for tab plating operations.

SUMMARY OF THE INVENTION

In accorclance with one aspect of the invention, there i8 provided a dry film for formin~ a solder ma3k on a printed circuit board or the like. The dry film includes a cov~r sheet formed of flexible, but generally non-conforming material, giving the dry film shape while allowing it to be rolled into a reel. On one surface of the co~er sheet is a thin layer of material, referred to herein a~ a top coat, which is preferably soluble in the developer for the photoimageable composition. The photoimageable composition form~ a layer over the other ~urface of the top coat. A
removable sheet (which is optional, but highly preferred) protects the photoimageable composi~ion layer. The material which is u~ed to form the top coat is selectively adherent to the photoimageable composition layer relative to its adherence to the cover sheet, whereby the cover sheet may be removed from the top coat and thereby leave the top coat as a protective coYering for the layer of photoimageable compo~ition.
To form a solder mask in accordance with one aspect of the present invention, the protective, removable sheet of the dry film is peeled away, and the exposed sur~ace o~ the photoimageable composition layer i9 tacked to a minor portion of the surface of the printed circuit board. ~he cover sheet i~ remo~ed, lea~ing the top coat as a prot~ctive covering on the layer of photoimageable compoqition. At this ~ime9 the layer of photoimageable compo ition is firmly laminated to the surface of the board with the use of heat, vacuum, and mechanical pressure, conforming the photoimageable composition layer to the contours of the board surface and substantially encap~ulating the traces. The photoimageable composition layer is then exposed to patterned actinic radiation through ~3~ 7~3_ 35 ~6, PATENT

the top coa~c. A developer is used to remove either expo~ed or non-e~posed portions of the photoimageable composition layer, leaving the remaining portion of the layer laminated to the circuit board. 5ubsequently, the portion~ of the photoimageable composition layer remaining on the cirruit board are cured, e.g., with heat and/or UV light. A similar proces~ i~ useful for applying and deYeioping a conformable primary imaging photoresist layer, although heat and UV curing are not used.
In accordance with a modified aspect of the present in~ention, after the photoimageable composition lay~r i~
tac~ed to a minor portion of the surface of the boar~, e.g., along the leading and trailing edges, the dry film is laminated with heat, vacuum and mechanical pressure to the board. The cover sheet i3 then removed. At thi~ time, the remaining layer~ of the dry film are again vacuum laminated with heat, vacuum and mechanical pres~ure to conform the remaining layers of the dry ~ilm to the contours of the board sur~ace.
U ually, the s~cond lamination is carried out for a ~horter period o time.
In accordance with another variation of the present invention, ~fter the photoimageabl~ composition layer i~
tacked to a minor portion of the 3urface o~ the board, e.g., along the leading and trailing edges, the dry ~ilm is laminated with heat, vacuu~ and mechanical pre~ure to the board. Th~
cover ~heet i~ then removed. At th~ time, the remaining layer~ o~ the dry ~ilm are conPormed to the contours of the board by mechanical pr~ure supplied by heated roller~ whi.ch pass acro~s the sur~ace of the bo~rd.
The present invention, there~ore/ in one aspect, resides in a method o~ forming a solder mask on the surface of a printed circuit board or the like, the method comprising (A~ providin~ a dry iilm compri~in (1) a cover ~heet, (2~ a top coat on said rover ~hee~, and ~ 6 -., ",', ~o~

~3) a layer of photoimageable composition on said top coat~ which photoimag~able compo~ition i9 curabla after e~pDsure a~d development to provide a hard, permanent solt~r mask overlying the printed circuit board; said top coat being selectively adherent to said photoimageable compo~ition relative to it~ adherence to ~aid cover ~heet;
(B) tacking ~aid photoimageable compo~ition layer to the surface oP said printed circui~ board;
(C) removi~g ~aid sover sheet from ~aid top coa~, leaving ~aid pho~oimageable composition layer tack2d to the surface of ~aid circuit board with said top coat co~erln~ and protectin~ the same;
(D) laminating said photoimageable composition layer to the ~urface of said printed circuit board, u~ing heat and vacuum to conform said phntoimageable compo~ition layer and ~aid top coat to the ~urface ther~of;
(E) e~posing said photoimageable compo~ltion layer to patterned actinic radiation;
(F) developing said pho~oimageable compcsition to remove ~i~her expo8~d or unexp~ed por~ions from said print~t circui~ boart, l~a~lng either unexpos~d or exposed portion~ remaining on ~aid pri~ed oircuit board; and ~G) curing ~aid remaining portions o~ ~aid photoimageable compo~ltion layer to form a hard, permanent solder mask ~rotecting ~aid printed ~ircuit board.
The pre6ent invention, in an~ther aspect, re~ides in a printed circuit board comprising a flat, - 6a -~ ~30~

n~n-conducting or semi-conducting board, conducting traces elevated above the ~urface of said board, ~aid traces having flat upper surface~ and lateral knee~, and a solder ma~k covering a ma~or portion o~ ~aid circuit board, including ~uch traces, said solder mask having an average thickne~s of about equal to or less than the elevàtion o~
said traces, the thickness of ~aid solder mask in the region of said lateral knee~ being at.lea~t about 70% of the thicknes~ of said ~older ma~k ov2r the top surfaces of said trace~ over about ~0% of the linear e~tent of ~aid trace~.
The present invention further provides a method of forming a photoimaged layer on a substrate surface, the method comprising (A) providlng a dry film comprising (1) a cover sheet, (2) a top coat on said cover sheet, snd (3) a layer of photoimageable compo~ition on said top coat; said top coat bein~
selectively adherent to said photoimageable compo~ition relative tv it~ adherence to sald cover sheet;
(B) tacking 8aid photoimagesble co~po~ition lager to said 8ubstrate ~urPace;
(C) remo~ing sait cover ~heet from said top coat, leaving 3aid photoimageable composition layer tacked to said sub~trate sur~ac~ with said top coat covering and protec~ing said photoimageable compos:ition layer;
(D) laminating ~aid photoimageable compo9ition layer to said:substrate ~urface, using heat ~nd vacuum to conform said p~otoimageable compo~ition layer and said top coat to 8aid substrate surface;
~) e~posing ~aid photolmageable co~po~i~ion l~yer to patterned actinic radiation; and - 6h -f~

~L3~

( F ) developlnE~ ~a~d photoimage~ble compo~ition to remove either exposed or unegposed portions from ~aid printed circuit board, leaving e~ther unexpo3ed or expo~et portions remaining on ~ald sub~trate surface.
The present invention still further provides a meth~d of forming a solder m~sk on the surface of a print~d circ~it board or the like, the method comprising:
(A) providing a dry film comprising (1) a cover sheet, (2) a~top coat on ~aid sheet9 and (3) a lsyer of photoimageable co~position on ~aid top coat, which photoi~ageable compo~ition i~ curable after e~posure and tevelopment to provide a hard, permanent ~older ma~k overl~ing the printed circuit board; said ~op coat bein8 ~electively adh~rent to ~aid photoimage~ble compo~ition relative to it~ adherence to ~aid cover sheet;
(B) tacking said photoi.ma~eable composltion layer to the.surface of said printed circuit board;
(G) laminatlng said photoimag~able laye~ to the ~ur~ace of ~aid printed circui~ board u~in~
heat, vacuum and meohanical pressure;
(D) re~oVing said coVer ~hee~ from ~aid top coat, leaving said photoimage~ble compo~ition layer tacked to the 8urface o~ ~aid circui~ board with said top coat coverin~ and protec~ing the same;
(E) laminatin~ said photoimsgeable compo~ition layer to the surface of said printed circuit board, u~ing heat, ~acutim and ~echanic:al pr~ure to -- 6c --. ~ ~

~3~4~69 conform ~ait photoimageable compo~ition layer and 3aid top coat to the surface thereo~;
(F) exposing ~aid photoimageable composition layer to patterned actinic radiation;
(G) developing said photoim~geable composition to semo~e either e~posed or unexpo~ed portions from ~aid printed circuit board, leaving either unexposed or exposed portions r~maining o~ ~aid printed circuit board; and (H) curing ~aid remaini~g portion~ of ~aid photoimageable composition layer to form a hard, parmanent ~older mask protecting ~aid printed circui~ board.
According to the pre~ent inv~ntio~ there i6 also provided a method o~ forming a photoimaged layer on a substrate surface, the method comprising:
(A~ pro~iding a dry fil~ compri~ing (1) a cover ~heet, (2) a top coat on ~aid cover sheet, and (3) a la~er of photoimageable composition on 3aid ~op coat; ~aid top coat being selectiv~ly adherent to ~aid photolmageabla compo~ition lay~r relative to it~ adherence to ~aid cover ~heet;
(B) tacking said photoimageable compo~ition layer to ~aid ~ub~trate surface;
(C) laminating ~ait photoimageable compo~ition layer to ~ait ~ub~trate surface u~ing heat, vacuum and mechanical pre~ure;' (D) removing ~aid cover ~heet from sa~d top coat~
lea~ing ~aid photoimageable compo~ition layer tacked to ~aid ~ub~trate ~ur~ac~e with ~aid top - 6d -\\ :

. , `~- 1.3~4~

coat c:overing and protecting ~aid photoima~eable compo~.ition layer;
(E) laminating said photoimageable composition l~yer to said subst2ate ~ur~ace, using heat ~ vacuum and mechanical pre~sure to coTl~orm ~ald photoi~nageable compo3i~ion layer and said top coat to ~aid sub~trate ~urface;
(F) exposing ssid photolmageable compo~itlon layer to patterned actinic radia~iorl; and (G) d~veloping ~aid photoimageable composition layer to remove either expo~ed or unexpos2d por'cions from ~aid ~ubstrate surface, leaving either une~cposed or expo~ed portions remaining on said sub~trate surface.
The present invention additionally provides a composite comprising:
a non-conducting substrate haYing a flat surface, circuitry traces extending upward of said ~lat surface of said non-conducting substrate, a photoimageable composition layer having an average thickness equal to or less than the height of said circuitry traces above said non-conducting substrate, said photoimageable composition layer overlying said non~
conducting substrate and being conform~d s~ a~ to ~ully encapsulate said circuitry traces to a thickness of at least about 18~, said photoimageable composition layer being soluble in a developer and capable of being rendered insoluble to the developer when exposed to actinic radiation, said photoimageable layer being curable after se~uential steps of exposure to patterned actinic radiation and development in the developer to provide a hard, permanent solder mask overlying said non-conducting substrate and encapsulating said circuitry traces, and - 6~ --- ~ r~
' '. '" ~, ' ~ ) 13~ g a continuous oxygen-barrier layer overlying said photoimageahle composition, said oxygen-barrier layer having a thickness of between about 1 and about 5~, said oxygen-impermeable layer being soluble in a developer for said photoimageable composition layer.

BRIEF DESCRIPTION OF THE DRAWINGS
_ _ FIGURE 1 is a cros~-~ectional view of a con~entional dry film used for forming a solder mask, shown being applied to a printed circuit board.

.

-6f-~ 3~34 ~ ~ 9 P~T~MT' FIGURE 2A is a cross-sectional view of a dry film in accordance with the invention used or formin~ a solder mask.
FIGURE 2B ~hows removal of the cover sheet from a dry film tacked to a printed circuit board.
FIGURE 3A i~ a cross-~ectional view of a trace on a prin~ed circui~ board covered by a solder mask formed ~rom a prior art~ liquid photoimageable composition.
FIGURE 3B is a cross-sectional view of an alternative trace on a printed circuit board covered by a solder mask formed from a prior art, liquid photoimageable composition.
FIGURE 4A is a cross-sectional view of a trace on a printed circuit board covered by a solder mask ~ormed from a prior art, conventional dry film.
FIGURE 4B is a cross-sectional view of an alternative trace on a printed circuit board over which a solder mask formed from a prior art, conventional dry film is applied.
FIGURE 5A is a cross-qectional view of a trace on a printed circuit board encapsulated by a solder mask formed from a dry film in accortance with the present invention.
FIGURE 5B is a cros~-sectional view of an alternative trace on a printed circuit board encapsulated by a solder mask formed from a dry film in accordance with the pre~ent invention.
FIGURES 6-9 are each photographs o~ a perpendicular slice of a printed circuit board having a solder mask pr~pared using a dry film in accordance wi~h the present inve~tion, the photographs being taken at an angle of 60 relative to the plane o~ the board 50 as to ~how both the cross-sectioll of the board and the outer sur~ace of the photoma~k~ Figure 6 is a 300x magnification of a 50~ trace covered wlth a 50~ solder ma~k. Figure 7 i3 a 750x magnification of a 75~ trace covered with a 50~ solder mask. Figure 8 is a 300x magnifica~ion of a 50~ trace having la~eral overhang~ and being covered with a 50~ ~older mask. Figure 9 is a lOOx magnification of 50 traces covered with a 50~ solder mask.

1763-~5..zl~
~3~4~9 PAT~NT

FIG~RE 10 is a 300x magniication of a primary photoresist applied by the method of the pre~ent invention over a circuit board havin~ traces.
FIGURE l:L i~ a diagrammatic illustration of the use of roller apparatus to tack down a dry film to a board.
FIGVRE 12 is a diagrammatic illustration of th~ use of a roller to conform layers of dry film, from which the cover sheet has been removed, to the surface of a board.
FIGURE 13A, 13B and 13C illustrate diagramatically a vacuum applicator and a platen operation sequence thereof.

DET.AILED DESCRIPTION OF CERTAIN PRE~ERRED EtSBODIMENTS

Illustrated in FIGUR~ 1 is an illustration of a circuit board 10 being covered with a prior art dry film 12. The circuit board compri~es a board 14 of non-conductive material, e.g., formed of epoxy, and the conductive tracas 16 which comprise the printed circuit. The dry film comprise~ a cover sheet 18 of flexible, but generally non-conforming material, typically a polyester, such as polyethylene terephthalate, a layer 20 of photoimageable material which eventually i~
processed to become the solder ma~k and a protective sheet 22 ~ormed, for example, of polyethylene.
Typically the dry film is supplied from a reel 2 (Figure ll) to the circuit board 10. Just prior to application to the surface of the circuit board, the protective sheet 2X i9 peeled away, a~ shown, e.g., by rolling onto a take-up reel 4 (Figure 11). As the photoimageable layer is applied to the ~urface of the circuit board 10, it is laminated thereto with heat, mechanical pressure and vacuum. Although the cover sheet 18 is flexible, it i~ non-conforming and ~herefore retains its flat configuration when the dry film 12 is applied to the circui~ board 10. The photoimageable composition layer 20 during lamination confonms at its contact sur~ace to the circuit board from which the trace~ 16 protrude; however, the ~3~4~69 1 763-35-24 surface of the photoimageable composition layer which is in contact with the cover sheet 18 remains flat. In order that the photoimag~able composi~ion layer 20 may fu1y enGapsulate the traces 16, it must be thicker than the elevation of the traces above the boa:rd 14 of non-conductive material.
Standard requirements are that the thickness of the solder mask over all portions of the traces be at lea~t 18~. Because the great majority of trace~ are elevated 50~ or more above the surface of the board 14, the photoimageable composition layer must usually be 68~ or thicker. Conventional dry films 12 for forming solder masks typically have photoimageable layers 100~ thick.
Illustrated in FIGURE 2A is a dry film 21 in accordance with the present invention. This dry film 21 also includes a cover sheet 18, a layer 20 of photoimageable composition which is processable to form a hard, per~anent solder mask and an optional, but preferred, protective sheet 22. In addition, the dry film 21 includes a top coat 24 intermediate the cover sheet 18 and the photoimageable composition layer 20. The material for the top coat 24 i9 selected to have selective adherence to the photoimageable composition layer 20 relati~e to its adherence to the cover sheet 18, whereby the cover sheet may be peeled away from the remaining layers 20, 24 of the circuit board-adhered dry ilm, l~aving the top coat 24 protecting the photoimageable composition layer 20. The top coat 24 is formed o material which al~o is selected for oxygen-impermeability, whereby it protects the underlying layer of photoimageable composition 20 from oxygen inhibition during free radical photopoly1nerization in the exposure step.
Pre~erably, the top coat is also ~elected to be sQluble in the developer for the photoimageable composition, whereby the top coat is remo~ed during the development process. It is possible 3 however, that the top coat not be soluble in the developer, in which case the top coat must be removed in a predevelopment step. The top coat material is further selected for transparency 3 strength and flexibility.

3 ~ 9 ~ 17~3-35-24 PATENT

~ ry film for applying photoresist layers having a ~imilar configuration are described in the U.S. Patent No. 4,530,896 issued July 23, 1985 to Christensen et al. U.S.
Patent N~. 4,530,896 i~ limited to application of very thin resist coating~. Primary photoresists are typically applied to a metal-covered substrate in order to deiine the pattern o circuitry traces on a printed circuit board. In U.S. Patent No.

4,530,896, a dry film i~ described having, in order; a cover ~heet, an intermediate layer which acts a~ a top ~oat after rcmoval o~ the cover sheet, a photoresi~t composi~ion layer and an (optional) protective sheet.
Although the orter of layers are similar to those of the dry film of the present i~vention, there are ~eve~al reasons why it would not be obvious to one with ordinary skill in the art that there would be advantages to a solder mask-forming dry film o ~imilar laminate arrangement.
It i3 a major advantage in a dry film ~or applying a primary imaging photoresi~t layer to a ~ubstrate that the relatively thick cover sheet be removed prior to imaging, thereby ~ubstantially i~proving the resolution of tha ima~e which may be achieved. Although, ~imilar advantages o resolution are obtained, in fact~ with a ~older mask-forming dry film, reso:Lution o~ image i~ oX ~ubstantially le~s concern with ~older mtsk~ than with primary photoresists.
The ~ain advantage~ of a solder mask dry ~ilm having ~his laminate arrangement are achieved through the application of the ~older ma~k by the novel proces~ of the pre~ent invention.
Th~ novel proce~8 of the present invention clo~ely con~orms the photoimageable composition layer 20, which eventually ~orms the solder mask, to the contour~ of a circuit board su~face which are irregular due to the elevated trace In contrast, a primary imaging photoresist l~yer is usually applied to a ~ubstantially flat surface. By con~orming the photoimageable composition layer 20 closely to the circuit board surface, excellent éncapsulation of the trac~s 16 is ! l763-35 z4 ~ 3 04 1 6 9 PATENT

achieved. Als~, a much thinner photoimageable composition layer 20 may be used than has been used in prior art, soldér mask-forming dry films.
A~ in the case with the conventional dry film 12, the protective ~heet 22 i~ i~itially peelet from the photoimageable composition layer 20, and the exposed surace of the photoimageable compo~ition layer i8 applied-over a 3urface of the printed circuit board as ~een in Figure 2A.
Unlik~ the case with the conventional dry film 12, the dry film 21 in accordance with one a~pect of the pre~ent invention may no~ be fully lamina~ed to tha board at the time of application. Rather, slight pre~sure is used to tack the dry ~ilm 21 to the board. Either cold-tacking or hot-tacking may be u~ed. By cold-tacking is generally meant at room temperature, e.g. 9 20-25C. Hot-tacking may be at temperatures up to about 70C. In accordance with one aspect of the invention, the dry film 21 i~ not tacked to the entire surface, but only to a portion ~uffi~ient to provide adequate adherence to the board so as to allow removal of the cover sheet 18 without removal of the remaining layers 20, 24 of the dry film 21 from the circuit board 10. By tacking only a small portion, typically 5% or less of the entire ~urface area of the dry iilm which lies over the board, passageways are left batween the dry film 21 and the board lO ~or subsequen~
removal of air by application of vacuum. If the dry film is applied by machine, edges, e.g., the leadin~ and trailing edge, may be tacked. I~ manual~y applied, tacking at the corners i~ sufficient. Mechanical pressure~ such as that which may easily be provided by a human ~inger, is ~uf~icient ~or tacking.
As an e~ample of how tacking ~a~ be perfor~d mechanically, reference i~ again made to Figure 11. A circNit board 10 i carried from left to right by drive roller~ 23 and the dry film~:21 are eaused to travel in synchronous motion along the ~ur~ace thereof. Non-driven rollers 25 are reciprocable upward and downward in the direction of double arrow~ ~ between vertical po~ition~ in contact with the dry . ~ ,j . PATENT

film and ~ertical pogition~ remote from the sur~ace. When ~he leading edg~ of the board is align~d with the rollers 25~ the rollers 25 are brought into contact with the dry film, pre~ing the dry film onto and tacking the dry film to the leading edge of the board. The roller~ 25 are immediately reciprocated away from the board to allo~r the middle regions of the boart to pass thereby untouched by the rollers. The rollers 25 are again reciprocat~d into contact with the board to tack the dry films 21 to the trailing ed8e of the board 10.
At this time, the cover sheet~ 18 are removed. Because only a minor portion of each dry film 21 area is tacked to the circuit board 10, Figure 2~ show~ a preferred mode of removal in which ~he cover sheet 18 is peeled back in a plane parallel to the plane of the circuit board lO (as opposed to lifting in a generally perpendicular direction which would tend to remove the photoimageable layer 20 from the circui~ board 10).
With the cover sheet 18 removed, the photoimageable layer 20 with the ~op coat 24 the~ecn i~ now vacuum-laminated at elevated temperature to the board. A ~acuum of at l~a3t about 0.3 millibar i9 u~ed and preferably a vacuum of at lea~t about 1 millibar. Laminating temperaturc~ range from about 40C to about lO0C, a range of about 55C to about 80C being pre~erred. Becau8e the cover ~heet 18 is removed, the tWD
remaining layers (photoimageable layer 20 and top coat 24) con~orm ea~ily to the contours of the irregular sur~ace of the circuit board lO.
In con~ntional vacuum laminating~ in addition to heat and vacuum, mechanical pressure is brought to b~ar against the dry film. In ~rhat i3 known a~ a "slap-down" (Figure 13) proce~s, a blanket and/or platen i9 used to press the dry ilm against the board. Commercial examples of laminators u~ing mechanical pre~sure in addition to heat and vacuum are models 724 snd 730 ~old by Dynachem Division of Morton Thio~ol~ Inc.
Such vacuum lamina~ing apparatus lO0 i9 briefly described with re~pect to Figure~ I3A, 13B and 13C; The apparatus includes an upper ~tationary platen 102 and a lower movable platen 104. A~ociated with the upper platen 102 i~ an upper 1763-35-2~
i30~6~ PATENT

resilient blanket 106; associated with the lower platen 104 is a lower resilient blanket 108. Th~ lower platen 104 ha~ a well 109 for carrying a try film-covered circuit board 10.
5e~1ing means in the form of an 0-ring 112 are provided on the lower platen 104 for hennetically sealing the well 109 when the lower platen is brought into contact with the upper platen 102 and its associated ~lanket 106. Also ~hown in Figure~
13A-13C~is a ~him 108. The shim i9 al90 ~ormed of resilient material and i9 used to accommodate board~ of different thickne~s. Several shims may be used, including ~hims interpo~ed between the upper platen 102 and upper blanket 106.
Typically the lower platen 104 is reciprocable both horizontally and vertically. With the lower platen 104 out of horizontal alignment with the upper platen 102, the dry ilm-covered circuit board 10 is placed in the well 109 on top o~ the lower blanket 108. Then the low~r platen 104 is mo~ed vertically into alignmen~ with the upper platen 102, the condition shown in Figure 13A. Next 3 the lower platen is mo~ed upward until contact is effected between the 0-ring 112 and th upper platen 102 and its associated blanket 106 (Figure 13B~. Not shown are means to apply a vacuum in the region of the well 109 which contains the circuit board 10. Also, not ~hown are means to provite a vacuum between the upper platen lOZ and the upper blanket 106. A~ter the well region 109 has been ~ealed, vacuum i~ applied both to the well reg~on 109 and between the upper platen 102:and upper blanket 106. During thi~ time9 the circuit board 10 merely rests on the lower blanket 108. The vacuum in the well region lG9 ~erves to remove air from - between the dry film layers 20, 24 ~nd ~he surfaoes of the circuit board and to draw the layer~ of the dry ~ilm to the surface~ of the circuit board. For a shor~ periot at the end of $he cycle, the vacuum i9 relea~ed from between the upper platen 102 and the upper blanke~ 106. The vacuum in the well region 109 causes the upper resilient blanket 106 to "slap down" against the board 10, pressing the board and dry film layers between the upper and lower blankets 106, 108 as seen in Figu~e 13C. Finally, the vacuum is released from the well ,. .~ ,`, 3 ~ 1763-35-24 ~ ` ~ PATENT

region 109 and the lower platen 104 i~ removed vertically and then horizontally from the upper platen 102 ~o allow removal of the laminated board.
During slap-down, mechanical pres~ure is conveyed to the dry film by the blanket8 formed of resilient material. The re~ilient material of ~he blanket~ i~ a type o~ ru~ber, e.g.,.
silicone rubber, or other type of resilient polymeric materlal.
The ~urface of each blanket which contacts the dry film is conYentionally texturized to prevent a vacuum bond from forming between the blanket and the cover ~heet. In prior art proce~ses, ~uch a ~ond would cause the cover ~heet and possibly underlyinK layer3 to t~ar away ~rom the board when the platen and blanket are withdra~m. Te:c'curizatioll of the blankets may be in ~he form of surface~roughness. of the polymeric materlai itself or it may be pro~ided by a rough, rubberized fabric at the dry fil~ contsct surface. In the process of the pre~ent invention wherein soft remaining lay~rs 20, 24 sf the dry fil~ 21 are laminated to the boart after removal of the cover sheet, conventional blanket~ with rough contact surfaces have been found ~o be ~isadvantageous in that they leave a negative image imprinted on the remaining layer~
of dry film. To avoid this, new ~emi-matte blankets ha~e been de~i~ed which have a very fine texture. Agai~, the texture prevent~ a vacuum bond ~rom being ~ormed between the blanket Yurface and the remaining lay2rs o~ the dry ~ilm; however, ~he fine surface texture creates no ~ignificant imprint on the remaining, ~oft laye~s 20, 24 o~ thc dry ilm. Pre~erred blanket material i9 silicone rubber prefera~ly r~inforced with fabric, such a~ ~iberRlass.
l)uring vacuum lamination, the remaining layers 20, 24 conform very clo~ely to the circu~t board and provide ~everal advantage~ which can be~t be appreciated with reYpect to FIGURES 3A, 3B~ 4A, 48, 5A, and 5B and the pho~ograph~ which are FIGURES 6-9. A~ it turn~ out, the thin layer which compri~es the top coat 24 remains fully intact during the conforming process, a result which would not nece~sarily have been predictable in sd~ance.

. - 14 -S `
r~ .

~L3~ 6~ 1763-35-2~
PATE~T

FIGURE 3A illustrates a solder ma~k which was initially applied as a liquid photoimageable compo~ition. Bein~ a liquid, the layer ~enerally con~orms to the uneven contours of the circuit bo~rd. On th~ top of each trace 16, the layer i~
of generally uniform thickneqs; however, in thç r~gion~ 26 of the knees of the traces, i.e., the lateral edge~, the layer greatly thin~ out, typically being only about 50% a~ thick as the portion of the lay~r which cover~ the flat upper ~urface of the trace. F~equeR~ly, the l~ye~ 2U in the region~ 26 of the knees i~ even thinner relative to th~ p~rti~ns o the layer on the flat upper surface of the trace.
Illustrated in FIGURE 3B i~ a ~older ma~k 20 covering a "mushroomed" trace 16' having overhangs 28 on each side. Not only is ~he ~older mask thin in the region~ 26 of the knees, but a ~oid, air-illed ~pace 30 is left below each o~erhang 28.
FIGURE 4A illu~trates a solder ma~k formed from a photoimageable compo~ition layer 20 of a conventional dry film 12. The outer ~urface of the solder mask ha~ the flat configuration of the cover sheet 18 which was removed subsequent to lamination, whil~ the lamin~ed sur~ace oonform~
generally to the irregular contours of the oircuit board 10.
The solder ma3k which is ~hown in ~IGURE 4A i~ sati~factory from the standpoint of adequately encapsulating the trace~ 16..
It ls to be noted, however, that the thickne9s o the photoimageable compo~ition layer 20 and solder ma~k formed therefrom ~, o~ neces9ity, greater than the elevation of the trace, requiring that a rela~iv~ly large amount of photoimageable compo~ition be u~ed to form th~ layer. This thickne~s represent3 3 l:Lmitation to the resolution o~ photoimaging which can be achieved.
FIBURE 4B illustrates a ~older ma~k ~esulting from a photoimageable layer 20 of a conventional dry film and covering "mushroomed" ~race3 16' ha~ing lateral overhangs 28.
Although thickne~s of the solder ma~k above the trace 16' is adequate, air-Eilled void~ 30 exi~t below ~he overhangs 28.

,' ~i 1763~3~-2~
~3~4~ PATRNT

FIGURES 5A and 5B illustrate solder ~a9k~ formed from photoimageable composition layers 20 of dry film3 21 produced in accordance with the present invention. During the lamination, which ollows cover sheet removal, the remaining layers 20, 24 of the dry ~ilm 21 very ~ubstantially conform to the irreg~lar surface contours of the circuit board In each case ~5A and 5B), the thickness of the phctoimageable layer 20 which was used to form the photomask was eq~al to or less than the elevation of the trace 16 or 16'; howe~er, the surface of the circuit board is ~ully covered by the solder ~a~k with the traces 16 or 16' fully encapsulated and encapsulated to an adequate thickness. It is to be noted that the photomask portions in t~e regions 26 of the knees of the traces are substantially as thick as photomask portions over the upper flat surfaces nf the traces When using a dry film in accordance with the present inven~ion, the resulting solder mask has thicknes~ in the region~ 26 of the knees of the traces which is at least about 70% of the thickness of the solder mask over the upper flat surfacç of the traces (over more than about 90% of the linear length of the traces) It is also to be noted in FIGURE 5B where a trace 16' has overhangs 28, coniormance oi the layer 20 during vacuum/heat lamination is suficient to substantially remove air-filled voids from below the overhangs 28. Even though substantial deformation o the photoimageable composition layer 20 is required to achieve this confo~mance, the top coat 24 maintains its integrity during lamination.
Subseyuent to lamination, the solder mask i5 formed by processin~ the lay~r of photoimageable composition in a conventional manner. Artwork is laid over the remaining layers 20, 24 of the dry film, the top coat 24 prevents the artwork from ~ticking.
The photoimageable composition layer 20 is expo~ed to patterned actinic radiation through the protective top coat 24 Although resolution i~ genera~ly not as critical when developing solder ma~k~ as it is in developing photoresist layers, the thinness of the top coat 24, typically in the 1304~9 1763~35-2~

ran8e ~ abo~t 2-3~ (as opposed to a cover sheet, typically in the range of about Z5Y1 through which photoimageable compo~itio~ layer~ 20 of conve~tional sold~r mask-f~ming dry film3 12 must be exposed) contributes to improved re~olution.
Further contributing to enhanced resolution is the relative thinness of the photoimageable compo~ition layer 2V itself.
As noted above, with a conventional solder mask-forming dry film 12, which i intended to be exposed through the cover sheet 18, the photoimageable composition layer typically must cover traces 16 with elevation of 50~ or more, and the photoimageable composition layers are typically about lO0~
thick. Becau~e the photoimageable composition layer 20 of the dry film of the present invention i~ conformable to the surface contours of the circuit board 10, a thickne~s of 50~
or less is sufficient to coYer traces elevated 50~ and even up to about 75~ above the flat surface of the board 14. Of course, a further advantage is substantially reduced levels of e~pensive photoimageable composition used to form the layer 20.
An additional advantage of a thin9 conforming photoima~eable layer i9 that the solder mask which ~orms in the regions between the traces 16 is generally at or below the level of the traces. With conventional solder masks, slight elevated re~ions exist between the traces. Such elevated region~ may result i~ the component, which is being ~oldered, sitting high on the ~older mask and preventing pinq o~ the component from being adequately ~oldered to expos~d trace portions. Alternatively, the elevated regions may, during reflow, result in some o~ the pin~ o~ a ~oldered component breaking contact, causing the component to stand up at one end, an ef~ect known as "tombstoning". Inadequate soldered contact a~d "tomb~toning" are avoided when "valley~" instead of elevated re~ion~ exist between exposed portions of the traces .
After e~posure to actinic radiatio~l the layers 20 and 24 are developed, the developer removing the top coat 24 and appropriate portions of the photoimaged layer ZO.

1763-3~-24 . PATENT
~ 6 9 In addition, the dry ~ilm ~older mask of this lnvention ha~ the ~ame ability to "tant" via hole~ a~ conventional primary i~aging or ~older masks fiims. Thi~ i9 a Yery important part of the proce~s and cannot be duplicated with ~imple liquid processe~.
Generally, an additional cure follows development in order to harden the solder mask. Typically, heat and/or W
lieht are used to effect this cure.
The present inv~n~ion is not primarily directed to materials from which the various sheets and layer~ are formed;
and referenoe is again made to U.S. Patent No. 4,530,896 of Christensen et al~ The c~er ~heet 18 is generally formed of a polye~ter, such as PET and is generally at least a~out 25~, or thicker, and up to about 75 thick.
PET, if used for the support sheet, is preferably surface-treated to provide adequate hydrophilicity or wetting angle so that it does not adhere strongly to the top coat.
Preferably, the wetting or contact ang~e of the cover sheet material is at least about 60~ and i~ preferably 75 or less.
One suitable PET cover sheet is sold under the trade designation Melinex-393 by ICI. This i8 a PET that is surface-treated on one side to enhance its hydrophilicity, i.e., inorease its ~etting angle.
Other suitable cover 3heet materials include, but are not limited to polyamides, polyoleins, ~inyl polymers, and cellulo~e est~rs.
Any photoimageable compo~itio~ suitable or forming solder masks is suitable for practice of the invention.
The protecti~e sheet ~2 is selected to be remoYable from the photoimag~able composition layer 20, and i8 typically a poly~thylene ~heet, between about 15~ and about 35~ thic~.
The top coa~ 24 is form~d of a ~a~rial which provide~ the requisite selective adherence. The top coat is typically from about 1~ to about 12~ thick, typically between about 2~ and about 5~. Wh~n using a PET cover sheet and a photoimageable material which is developable in aqueous or al~aline aqueou *Trad~rk . - ~8 -f~ ~
,~

176~-35-2 ~ 3 0 ~ 1 6 9 PATENT

deYeloper, a ~ultable top coat con~i~ts essentially of (~3 between about 10 weight percent and 100 weight percent of either polyvinyl alcohol hydrolyzed to at least about 75% or a copolymer formed by the copolymerization of betw~en about 95 and about ~9 molar perc~nt of polyvinyl acetate and betw~e~
abou~ 1 and about 5 molar percent of an alXyl ester of acrylic acid and being hydrolyzed to at least about 75 pcrcent 9 (b) up to about 90 weight percent of hydroxyethyl cellulose; and (c3 up to about 10 wei~ht percent o~ plasticizer. Preferably, hydroxyethyl cellulose i9 u~et at a l~vel of at least 25 wt.
percent and more preferably a~c a level of at least about 50 wt. percent.
The above described copolymers are ~old under the trade *
designation VINOL-118 M by Air Products, and 4 weight percent solution~ of these copolymers and have visco~ities ranging from about 5 to about 65 centipoi~e at 2ûC. PVC homopolymers of ~imilar vi~cosity are useful as well. Con~nercially suitable PVA's are al~o ~old by Air Products.
A plasticizer, i used, tends to lowar the tack temperature of the top coat compo~ition, and tac~ temperatures below about 100C have been achieved ~ing ~uch plasticizers.
A top coat composition having a lower tack temperature is easier to adhere tc the pho~oimageable layer. Pref2rred plasticizers are low molecular weight compounds, e.g., compounds having a molecular weight below about 200, having ~wo ox more hydroxyl groups, such as glycerine, ethylene glycol. and propylene glycol. While plasticizers desirable lower the tack temperature, they also increas~ oxygen permeability o~ the top coat;
accordingly amounts o~ plasticizers in excess of about 10% by weight are desirably avoided.
Other suitable ~aterial~ ~or top coat u~e include, but are not limited to, the group con~isting of polyvinyl ether-msleic anhydride copolymers, water-soluble cellulose ethers, water-soluble salts of carbo~yal~ylcellulose, water-soluble salt3 of carboxyalkyl ~tarch~ polyvinyl alcohol, polyvinyl pyrrolidone, variol~s polyacrylamides, v~rioug water-soluble polyamides9 wat2r-soluble ~lts of polyacrylic *Trad~E~k ~.",~

PATF,NT
13~4~

acid, gelati.n, ethylene o~ide polymers, various ~tarche~ nd the like.
The method of forming the dry film is according to known procedures. The top coat and photoimageable composition layers are succes~ively layered on the cover sheet by method~, such as roller coating or by spraying so~utions or disper~ion~
of the materials.
In a variation of the ~ethod of practice of the invention described above, a dry film m~y be laminated both before and after removal o the cover sheet. In the case where the photoimageable layer is about equal in thic~nes~ or of lesser thickness than the traces (or other unevennes~ of the board), one migh~ expect the traces (or other unevenness of the board) to penetrate the photoimageable layer. However, it is found in such cases that a conventional lamination of ~uch a dry film using heat, vacuum and ~echanical slap-down in a conventional manner re~ult~ in tacking of the photoimageable layer to the upper surfaces of the trace~ (or other clevations) without penetration of the trace~ (or elevations) through the photoimageable layer. The tacking of the dry film oYcr a greater area of the board facilitates its adhesion to the board during cover ~heet removal. A second vacuum lamination with heat, vacuum, and mechanical slap-down after cover sheet removal acts to conform the photoimag2able compo~ition layer (and top coat) to the contour~ of the uneven b~ard ~urace. In a typical procedure the lamination subsequent to cover sheet removal is for about hal~ the time as the lamination prior to cover sheet removal~ e~g., 30 seconds for the ~econt lamination compared to 60 seconds for the ~irst laminations. The additional tackin~ a~forded by the initial lamination may prove to be particularly ad~antageous when using automated equipment or cover sh~et re~oval.
As a further variation to the method of the invention~
instead of u~ing a heat/vacuum lamination process, subsequent to removal of the protective sheet 22 and the cover sheet 18, to conform the photoimageabl~ layer 20 (and top coat 24~ to the con~ours of the board surface, a roller applying '. ~ 3 . ~ ;''~ , .

~ 169 ~ 1763-35 24 mechanical pre3~ure may be used to accomplish the same purpo~e. Illustrated diagrammatically in Figure 12 is the use of the roller uset to conforn a photoimagcable layer 20 and top coat 24 to the contours of the board. In thi~ diagram, the board 10 and layers 20, 24 of a dry film are moved ~rom left to right below a non-driven roller 45 with the roller in pressure bearing contact with the top coat 24. A~ in the processes described above, the dry film i~ first tacked down.
Preferablyl the dry film is then vacuum laminated to the board 10. In the case where the dry film ha~ a thin photoimageable layer 20 relative to khe height of the traces, the layer~ 20, 24 of the dry film for the most part ~it on the top surfaces of the trace~ 16 as seen at the left-hand ~ide of ~igure 12.
After pa3~ing below the roller 45, the photoimageable composition 20 and top coat 24 conform to the contours of the board 10 with tr~ces 16 a~ seen on the right-hand side of Figure 12. The layers 20, 24 are pres~ed down acro~s the full surface of the board 10 as the board pa~ses below the roller.
Alternatively, a roller could be drawn across the surface of a stationary board.
The roller 45 used for this purpose should apply between about 1 bar and about 6 bar pre~sure~. The roller 45 i~
heated depending on th~ particular application to between 40 and 110C, generally in the range of 60 to 80C. The surface of the con~orming roller 45 ha~ ~ome resiliency, preferably being a polymeric material having a Shore A hardne3s o b~tween about 60 and about 80. The surface of the roller is ~mooth. The relati~e ~peed o~ the roller 4~ and board 10 will vary according to the application, generally between absut 1 and about 10 meter~ per secont, 3 meters per second being typical.
The invention will now be described in greater tetail by way of specific e~ample~.

. . , ~304~69 i ~ 63-35-24 A dry film i9 prepared in acco~danc~ with the invention having a cover sheet which is a 50~ thick sheet of PET, a 2.5 top coat which i~ 10 weight percent of the hydrolyzed polyvinyl acetate/acrylic acid copolymer desoribed above, and 90 wei~ht percent hytroxyethyl cellulose, a photoimageable compo~ition forming a layer 50~ thick and a polyethylene protective 3heet Z~ thlck. The photoimageable composition i~
ormulated a~ follow9:
Ingredients:
Chemical Na~e Weight % or Trate Name " .. *
36.5 Scripset 540 (styrene/Maleic Anhydride Co-Polymer 25.8 MEK
1.2 Pigment CI 7426 0.0~ St~rically hindered phenol anti-oxidant 7.54 Tripropylene glycol diacrylate 3.76 Tripropylene propane triacrylate 0.33 Adhe~ion Promoter of the Thiazole type 2.81 N-Methylolacrylamide 2.26 Dimethoxyphenyl Acetophenone 0.56 Coating Aid 1.18 Methyl Ethyl Ke~one 18.01 Fillers 100%

A dry film i8 prepared in accordance with Example 1, using LAMIN~R ~ DM to form the photoimageable compo~ition layer.

*Trad~k . - 22 -, ~ .
`, ' 176.3-35-2~
~ 1304~6~ I PATENT

E ~ PLE 3 Panels having copper traces and plates having tin/lead traces were each cleaned in a con~entional mannex. The dry film of E~ample 2 was applied to the boards both manually and with an automated 8y8tem. Manually, the polyethylene protective ~heet wa~ removed a~d $he four corners of the film wera tacked down with only finger pr~e~r~. U~ing ~ape to o~ain a grip, the cover sheet was remo~ed b~ drawing off the PET c~v~r sheet in a plane parallel to the board ~urface. In the aut~ate~ ~ar~ation, a ~yn~chem DFSM Model 360 laminator was used. The Dynachem appa~atu~ u3e~ an "off contact'7 method which automatically remove~ the polyethylene pro~ective ~heet, tack~ down the leading-edge o the dry film and then tack~
down the traili~g ed~e o~ the dryr film. Application wa~ to both ~ides of the boards. Then the cover ~heet was removPd.
Lamination wa5 effected at a platen temperature of 70C
with a cycle time of 60 seconds and a ~acuum of 2 millibars.
Mechanical slapdown was for 6 ~econds at a ~acuum o~ 0.3 millibar. The boards wer~ held 20-30 ~inute~.
Artwork wa~ laid over the bo~rds. U~in3 Colight DMVL-824, exposure was for 25 seconds at 100 mJ/cm2 ~easured by Dynachemts integrating radiometer model 5009 achieving a Stouffer step of 8 to 9. The boards were held for an additional 20-30 minutes~ The boards were developed in 1.0 weight percent Na2C03'~20 which g~ve a 2~ break point. The board~ were dried u~ing a Chemcut 547 circuit board tryer.
Curing was done u~ing a bynachem W E~ curing unit, an XL-390 unit and a h~gh temperature oven. Curing ~teps are a~
~ollow~:
r U.Y. CURI~G
1. W E~-3 lamp (~00 watt/inch).
2. 3.5 Joules/cm (IL-390) ~ide A.

* Trad~Erk (Each mstance) ~ 23 -.

~, .. , , . , . ~, ...

1304~69 PATENT

3. Cool to room ~emperature.
4. 3.5 Joule~/cm (IL-390) side B.

5. Cool to room temperature.
TH~RMAL CUR
1. Oven b~ke at 150C ~or 60 minute~.

EgAMPLE 4 FIGURE 6 show~ a solder mask formed from a 50~ layer encapsulating a 50~ trace.
FIGURE 7 ~hows a solder ~ask formed from 50~ layer encapsulating a 75~ high trace. Excellent coverage at the region~ of the knees is seen.
FIGURE 8 ~hows 2 solder ma3k fonmed ~rom a 50~ layer encapsulating a 50~ hi~h trace having overhangs. Solder mask material filling the regions below the overhangs is seen.
FIGURE 9 show~ a solder ~na~k formed from a 50~ layer encapsulating 50~ high trace. Conformance of the solder mask to the contour~ of the circuit board i~ seen.

Although the invention is most pertinent to and has been described primarily in respect to a dry film carrying a photoimageable composition layer which is curable to form a solder mask, the invention is generall~ appllcable to a dry film ha~in~ any primary imaging product, including primary photoresist~, to be applied to an irregular sur~ace, particularly a sur~ace ~rom which traces rise ~rom an otherwi~e ~lat sur~ace. It may alsa be de~irahle to use the method of the present invention to apply a layer of primary photoresi~t to a copper ~urface with unevenne~ caused by "fatglass". There are instance~, ~or e~ample, where it is desirable to lay a photoresis~ layer over ~ pa~terned circuit which is already formed and then either add or subtract exi~ting copper patterns or tab plate gold fingers.

) 1763 3S-2 P~TENT
~L3~4~1~9 A dry film i~ prepared in accordanc~ with Example l;
., ~, 1 using Lamina~ A~ to form a photoimageable composit~on layer.
The resist was laminated over rais~d connector tab~, a~ per E~ample 3, and exposed to an image to a 301id copper 7-9 on a Stouffer ~ 21 Step Sen~itivity guide. A covered trace i~ ~een in Figure 10. The expo3ed panel wa~ then developed in a "Chemcut 547'spray developer containing 91 liters water, 3% by weight pota~3i~m carbonate and 1 to 5% Butyl Cello~olY ~ which bared about 3~4 o~ ea~h c~nnector pad. De~eloper temperature wa held between 27-33C. A~ter thorough rin~ing, the board was given a 0.30 mil nickel ~trike followed by a 3V min. gold plating cycle in a Technic RC 80 bath. The resist wa~ then 3tripped in a 3-5% NaOH at 54-60C. The gold plated tab~ were then inspectet for wicking in the areas around which the resist had conformed. No traces of Au or Ni were found anywhere above the gold plated areas. A control, not vacuum laminated in this ~ashion, showed wicking on about 25% of the tab~ .
The excellent cono~mability achi~ved by the laminated dry film in aecordance with the pre~ent invention mi8ht also be achieved with a dry film compri~ing a layer of photoimageable mat~rial backed ~y a cover sheet o~ de~ormable m~terial, e.g., heat-con~orming vacuum .~ilm. Such a dry ~ilm ~ould be applied ln the manner o~ conventional il~, heat/~acuum laminating being effected ei~her during or ~ubsequent to appli~ation to ~he circuit board. The cover ~heet, in thi~ case, would provide the desired con~ormanee;
handlin~ proble~ of the more flexible ~heet might be encounter¢d.
In a Yariation of ~he method of the pre ent i~vention~
after the cover ~heet i~ removed and prior to vacuum/heat lamination, a fle~ible sheet, e.g., Tedlar or Teflon5 is laid * Txad~rk (Each ~ tance) . - 25 -1 ,", 1763-35-2~t ~ ~304~69 ~ PATRNT

over the top coat. During vacuum lamination, thc flexible ~heet conforms to the top coat, but can be and is removed prior to or after ~xposure to allow development. A reason for applying ~uch a flexible sheet is to lengthen the permis~ible period of time between lamina~ion and exposure of the photoimageable layer. Fle~ible she~t material may be a polymeric material or a thin sheet of paper.
While the method of the invention ha~ been described primarily in terms o a d~y film compri~ing a cover sheet, a top coat on the cover ~heet, and a lsyer o~ photoimageable composition on the top coat, the m~thod of the inv2ntion may b~ practiced u~ing a conventional try film compri$ing a cover sheet and a photoi~ageable composition layer, e.g., either a primary imaging photore$ist or a solder ma~k-fvrming photoimageable composition which i3 in direct contact with the cover ~heet. ~uch a conventional dry film may optionally contain a protective 3heet covering the photoimageable comparison layer. The method may be practiced with conventional dry ~ilm, provided that the layer o photoimageable compo~ition tac~s to a ~urface with sufficient adhe~ion relat:iYe to the adhesion between the photoimageable composition and the cover sheet that the cover sheet may be removed from the photoimageable composition layer, leaving the photoimageable composition layer tacked to the 3urface. A~ter the co~er ~heet i~ removed, the photoimageable composition layer may be covered by a ~heet of ~lexible material, ~uch as "Tedla~'or"Te~lon (polytetra~luoroethylene). ~uch a flexible material iacilltates contact printing and prevents the photolmageable composition layer ~rom ~ticking to parts of the vac~um laminator. ~lternatively, blanket~ in the vacuum laminator may con~ist of fle~ible material~. The phctoimageable compo~ition lay~r is ~aminated to the surace with heat and vacuum, conforming the photoimageable composition 12yer to the aurface. The photoimageable *fflad~K

, 1763-35-~4 ~3i~4~6~ P~ rENT

compo3ition layer is expo~ed to patterned actinic radiations e.g., by of~-contact printing, such as by means of a Tamarack E~posure source. If a fle~ible sheet has been applied, it is removed, either be~ore or ater exposure. The photoimageable composition layer is then developed. If the photoimageable layer is a snlder ma3k-forming composition, it is cured.

The following i~ a specification of a finely te~tured resilient blanket in accordance with the present invention.
Product Construction: Silicone rubber (methylvinyl) balanc~d coated onto fiberglass fabric, matte finish one side.

Product Color: Red:

Product Feature~: a. Fine matte finish on one side, smooth the other.
b. Dimensional stability.
c. High break strength.
d. High temperature resistance and release propertie~ of silicone.

1763~35-2 PAT~NT
~3~411Ei~

Thickness, in. (~ .005") 1~16 Fib~rgl~s~ Style 1564 Fi~rglass Thickn~ss, in. .015 Rubber Durom~ter, Shore DA" 60 Br~ak Strensth, ppi SWarp Direction) 450 Hydrostati~ Burst Strength, p5i 750 Waight L OS5 ~ X
CHe2t Ased 4 hours ~ 400F)1.5 While the invention has been described in terms of certain preferred embodiments, modification~ which ~ould be obvious ~o one with ordinary ~kill in the art may be made without departing from the scope of the invention.
Various features o~ the present invention are set forth in the following claims.

/~

Claims

1. A method of forming a solder mask on the surface of a printed circuit board having a non-conductive surface and circuitry traces elevated therefrom, the method comprising (A) providing a dry film comprising (1) a cover sheet, (2) a top coat on said cover sheet, and (3) a layer of photoimageable composition on said top coat, which photoimageable composition is curable after exposure and development to provide a hard, permanent solder mask overlying the printed circuit board; said top coat being selectively adherent to said photoimageable composition relative to its adherence to said cover sheet;
(B) tacking said photoimageable composition layer to the surface of said printed circuit board;
(C) removing said cover sheet from said top coat, leaving said photoimageable composition layer tacked to the surface of said circuit board with said top coat covering and protecting the same;
(D) laminating said photoimageable composition layer to the surface of said printed circuit board, using heat and vacuum to conform said photoimageable composition layer and said top coat to the surface thereof;
(E) exposing photoimageable composition layer to patterned actinic radiation;
(F) developing said photoimageable composition to remove either exposed or unexposed portions from said printed circuit board, leaving either PATENT

unexposed or exposed portions remaining on said printed circuit board; and (G) curing said remaining portions of said photoimageable composition layer to form a hard, permanent solder mask protecting said printed circuit board.

2. A printed circuit board comprising a flat, non-conducting or semi-conducting board, conducting traces elevated above the surface of said board, said traces having flat upper surfaces and lateral knees, and a solder mask covering a major portion of said circuit board, including such traces, said solder mask having an average thickness of about equal to or less than the elevation of said traces, the thickness of said solder mask in the region of said lateral knees being at least about 70% of the thickness of said solder mask over the top surfaces of said traces over about 90% of the linear extent of said traces.

3. A method of forming a photoimaged layer on a substrate surface, the method comprising (A) providing a dry film comprising (1) a cover sheet, (2) a top coat on said cover sheet, and (3) a layer of photoimageable composition on said top coat; said top coat being selectively adherent to said photoimageable composition relative to its adherence to said cover sheet;
(B) tacking said photoimageable composition layer to said substrate surface;

PATENT

(C) removing said cover sheet from said top coat, leaving said photoimageable composition layer tacked to said substrate surface with said top coat covering and protecting said photoimageable composition layer;
(D) laminating said photoimageable composition layer to said substrate surface, using heat and vacuum to conform said photoimageable composition layer and said top coat to said substrate surface;
(E) exposing said photoimageable composition layer to patterned actinic radiation; and (F) developing said photoimageable composition to remove either exposed or unexposed portions from said printed circuit board, leaving either unexposed or exposed portions remaining on said substrate surface.

4. A method of forming a solder mask on the surface of a printed circuit board or the like, the method comprising (A) providing a dry film comprising a method sheet and a layer of photoimageable composition on said cover sheet, which photoimageable composition is curable after exposure and development to provide a hard, permanent solder mask overlying the printed circuit board;
(B) tacking said photoimageable composition layer to the surface of said printed circuit board;
(C) removing said cover sheet from said photoimageable composition layer, leaving said photoimageable composition layer tacked to the surface of said circuit board;
(D) laminating said photoimageable composition layer to the surface of said printed circuit board, PATENT

using heat and vacuum to conform said photoimageable composition layer to the surface thereof;
(E) exposing said photoimageable composition layer to patterned actinic radiation;
(F) developing said photoimageable composition to remove either exposed or unexposed portion. from said printed circuit board, leaving either unexposed or exposed portions remaining on said printed circuit board; and (G) curing said remaining portions of said photoimageable composition layer to form a hard, permanent solder mask protecting said printed circuit board.

5. A method of forming a photoimaged layer on a substrate surface, the method comprising (A) providing a dry film comprising a cover sheet and a layer of photoimageable composition on said cover sheet;
(B) tacking said photoimageable composition layer to said substrate surface;
(C) removing said cover sheet from said photoimageable composition layer, leaving said photoimageable composition layer tacked to said substrate surface;
(D) laminating said photoimageable composition layer to said substrate surface, using heat and vacuum to conform said photoimageable composition layer to said substrate surface;
(E) exposing said photoimageable composition layer to patterned actinic radiation; and PATENT

(F) developing said photoimageable composition to remove either exposed or unexposed portions from said printed circuit board, leaving either unexposed or exposed portions remaining on said substrate surface.

6. A method of forming a solder mask on the surface of a printed circuit board or the like, the method comprising:
(A) providing a dry film comprising (1) a cover sheet, (2) a top coat on said sheet, and (3) a layer of photoimageable composition on said top coat, which photoimageable composition is curable after exposure and development to provide a hard, permanent solder mask overlying the printed circuit board; said top coat being selectively adherent to said photoimageable composition relative to its adherence to said cover sheet;
(B) tacking said photoimageable composition layer to the surface of said printed circuit board;
(C) laminating said photoimageable layer to the surface of said printed circuit board using heat, vacuum and mechanical pressure;
(D) removing said cover sheet from said top coat, leaving said photoimageable composition layer tacked to the surface of said circuit board with said top coat covering and protecting the same;
(E) laminating said photoimageable composition layer to the surface of said printed circuit board, using heat, vacuum and mechanical pressure to PATENT

conform said photoimageable composition layer and said top coat to the surface thereof;
(F) exposing said photoimageable composition layer to patterned actinic radiation;
(G) developing said photoimageable composition to remove either exposed or unexposed portions from said printed circuit board, leaving either unexposed or exposed portions remaining on said printed circuit board; and (H) curing said remaining portions of siad photoimageable composition layer to form a hard, permanent solder mask protecting said printed circuit board.

7. A method of forming a photoimaged layer on a substrate surface, the method comprising:
(A) providing a dry film comprising (1) a cover sheet, (2) a top coat on said cover sheet, and (3) a layer of photoimageable composition on said top coat; said top coat being selectively adherent to said photoimageable composition layer relative to its adherence to said cover sheet;
(B) tacking said photoimageable composition layer to said substrate surface;
(C) laminating said photoimageable composition layer to said substrate surface using heat, vacuum and mechanical pressure;
(D) removing said cover sheet from said top coat, leaving said photoimageable composition layer tacked to said substrate surface with said top PATENT

coat covering and protecting said photoimageable composition layer;
(E) laminating said photoimageable composition layer to said substrate surface, using heat, vacuum and mechanical pressure to conform said photoimageable composition layer and said top coat to said substrate surface;
(F) exposing said photoimageable composition layer to patterned actinic radiation; and (G) developing said photoimageable composition layer to remove either exposed or unexposed portions from said substrate surface, leaving either unexposed or exposed portions remaining on said substrate surface.

8. A method of forming a photoimaged layer on a substrate surface, the method comprising:
(A) providing a dry film comprising a cover sheet and a layer of photoimageable composition on said cover sheet;
(B) tacking said photoimageable composition layer to said substrate surface;
(C) removing said cover sheet from said photoimageable composition layer, leaving said photoimageable composition layer tacked to said substrate surface;
(D) causing a heated roller to pass across said photoimageable composition layer applying pressure thereto, thereby causing said photoimageable layer to conform to said substrate surface;

PATENT

(E) exposing said photoimageable composition layer to patterned actinic radiation; and (F) developing said photoimageable composition to remove either exposed or unexposed portions from said substrate surface; leaving either unexposed or exposed portions remaining on said substrate surface.

9. A method of forming a solder mask on the surface of a printed circuit board or the like, the method comprising:
(A) providing a dry film comprising (1) a cover sheet, and (2) a layer of photoimageable composition on said cover sheet, which photoimageable composition is curable after exposure and development to provide a hard, permanent solder mask overlying the printed circuit board;
(B) tacking said photoimageable composition layer to the surface of said printed circuit board;
(C) removing said cover sheet from said photoimageable composition layer, leaving said photoimageable composition layer tacked to the surface of said circuit board;
(D) causing a heated roller to pass across said photoimageable composition layer applying pressure thereto, thereby causing said photoimageable composition layer to conform to the surface of said printed circuit board;
(E) exposing said photoimageable composition layer to patterned actinic radiation;
(F) developing said photoimageable composition to remove either exposed or unexposed portions from PATENT

said printed circuit board, leaving either unexposed or exposed portions remaining on said printed circuit board; and (G) curing said remaining portions of said photoimageable composition layer to form a hard, permanent solder mask protecting said printed circuit board.

10. A method of forming a photoimaged layer on a substrate surface, the method comprising:
(A) providing a dry film comprising (1) a cover sheet, (2) a top coat on said cover sheet, and (3) a layer of photoimageable composition on said top coat; said top coat being selectively adherent to said photoimageable composition relative to its adherence to said cover sheet;
(B) tacking said photoimageable composition layer to said substrate surface;
(C) laminating said photoimageable composition layer to said substrate surface using heat, vacuum and mechanical pressure;
(D) removing said cover sheet from said top coat, leaving said photoimageable composition layer tacked to said substrate surface with said top coat covering and protecting said photoimageable composition layer;
(E) causing a heated roller to pass across said photoimageable composition layer applying pressure thereto and causing said photoimageable composition layer to conform to said substrate surface;

PATENT

(F) exposing said photoimageable composition layer to patterned actinic radiation; and (G) developing said photoimageable composition layer to remove either exposed or unexposed portions from said substrate surface, leaving either unexposed or exposed portions remaining on said substrate surface.

11. A method of forming a solder mask on the surface of a printed circuit board or the like, the method comprising:
(A) providing a dry film comprising (1) a cover sheet, (2) a top coat on said cover sheet, and (3) a layer of photoimageable composition on said top coat, which photoimageable composition is curable after exposure and development to provide a hard, permanent solder mask overlying the printed circuit board; said top coat being selectively adherent to said photoimageable composition relative to its adherence to said cover sheet;
(B) tacking said photoimageable composition layer to the surface of said printed circuit board;
(C) laminating said photoimageable layer to the surface of said printed circuit board using heat, vacuum and mechanical pressure;
(D) removing said cover sheet from said top coat, leaving said photoimageable composition layer tacked to the surface of said circuit board with said top coat covering and protecting the same;
(E) causing a heated roller to pass across said photoimageable composition layer applying PATENT

pressure thereto and causing said photoimageable composition layer to conform to the surface of said printed circuit board;
(F) exposing said photoimageable composition layer to patterned actinic radiation;
(G) developing said photoimageable composition to remove either exposed or unexposed portions from said printed circuit board, leaving either unexposed or exposed portions remaining on said printed circuit board; and (H) curing said remaining portions of said photoimageable composition layer to form a hard, permanent solder mask protecting said printed circuit board.

12. A composite comprising:
a non-conducting substrate having a flat surface, circuitry traces extending upward of said flat surface of said non-conducting substrate, a photoimageable composition layer having an average thickness equal to or less than the height of said circuitry traces above said non-conducting substrate, said photoimageable composition layer overlying said non-conducting substrate and being conformed so as to fully encapsulate said circuitry traces to a thickness of at least about 18µ, said photoimageable composition layer being soluble in a developer and capable of being rendered insoluble to the developer when exposed to actinic radiation, said photoimageable layer being curable after sequential steps of exposure to patterned actinic radiation and development in the developer to provide a hard, permanent solder mask overlying said non-conducting substrate and encapsulating said circuitry traces, and a continuous oxygen-barrier layer overlying said photoimageable composition, said oxygen-barrier layer having a thickness of between about 1 and about 5µ, said oxygen-impermeable layer being soluble in a developer for said photoimageable composition layer.

13. A composite according to Claim 12 wherein said photoimageable layer is developable in alkaline aqueous solution and said oxygen-barrier layer is soluble in alkaline aqueous solution.

14. A composite according to claim 12 wherein said circuitry traces have flat upper surfaces and lateral knees, the thickness of said photoimageable composition layer in the region of said lateral knees being at least about 70% of the thickness of said photoimageable composition layer over the top surfaces of said circuitry traces over about 90% of the linear extent of said circuitry traces.

15. A composite in accordance with claim 12 wherein said photoimageable composition layer and said continuous oxygen-barrier layer are vacuum-conformed to said nonconducting substrate and said upwardly extending circuitry traces.

16. A method of forming a solder mask on the surface of a printed circuit board, the method comprising (A) providing a dry film comprising a cover sheet and a layer of photoimageable composition on said cover sheet, which photoimageable composition is curable after exposure and development to provide a hard, permanent solder mask overlying the printed circuit board;
(B) tacking said photoimageable composition layer to the surface of said printed circuit board;
(C) removing said cover sheet from said photoimageable composition layer, leaving said photoimageable composition layer tacked to the surface of said circuit board;
(D) laminating said photoimageable composition layer to the surface of said printed circuit board, using heat and vacuum to conform said photoimageable composition layer to the surface thereof;
(E) exposing said photoimageable composition layer to patterned actinic radiation;
(F) developing said photoimageable composition to remove either exposed or unexposed portions from said printed circuit board, leaving either unexposed or exposed portions remaining on said printed circuit board;
and (G) curing said remaining portions of said photoimageable composition layer to form a hard, permanent solder mask protecting said printed circuit board, wherein the improvement comprises between steps (C) and (D), a flexible sheet is laid over said photoimageable composition layer, said flexible sheet removably laminating to said photoimageable composition layer during step (D), said flexible sheet being removed from said photoimageable composition layer either between steps (D) and (E) or between steps (E) and (F)

17. A method of forming a photoimaged layer on an uneven substrate surface, the method comprising:
(A) providing a dry film comprising a cover sheet and a layer of photoimageable composition on said cover sheet;
(B) tacking said photoimageable composition layer to said substrate surface;
(C) removing said cover sheet from said photoimageable composition layer, leaving said photoimageable composition layer tacked to said substrate surface;
(D) causing a heated roller to pass across said photoimageable composition layer applying pressure thereto, thereby causing said photoimageable layer to comform to said substrate surface;
(E) exposing said photoimageable composition layer to patterned actinic radiation; and (F) developing said photoimageable composition to remove either exposed or unexposed portions from said substrate surface, leaving either unexposed or exposed portions remaining on said substrate surface.

18. A method according to claim 17 wherein said heated roller applies a pressure of between about 1 and about 6 bars.

19. A method according to claim 17 wherein said roller is heated to a temperature of between about 40°
and about 100°C.

20. A method according to claim 17 wherein the surface of said roller is resilient material of between about 60 and about 80 Shore A hardness.

21. A method of forming a photoimaged layer on an uneven substrate surface, the method comprising:
(A) providing a dry film comprising (1) a cover sheet, (2) a top coat on said cover sheet, and (3) a layer of photoimageable composition on said top coat; said top coat being selectively adherent to said photoimageable composition layer relative to its adherence to said cover sheet;
(B) tacking said photoimageable composition layer to said substrate surface;
(C) laminating said photoimageable composition layer to said substrate surface using heat, vacuum and mechanical pressure;
(D) removing said cover sheet from said top coat, leaving said photoimageable composition layer tacked to said substrate surface with said top coat covering and protecting said photoimageable composition layer;
(E) causing a heated roller to pass across said photoimageable composition layer applying pressure thereto and causing said photoimageable composition layer to conform to said substrate surface;
(F) exposing said photoimageable composition layer to patterned actinic radiation; and (G) developing said photoimageable composition layer to remove either exposed or unexposed portions from said substrate surface, leaving either unexposed or exposed portions remaining on said substrate surface.

22. A method of forming a photoimaged layer on an uneven substrate surface, the method comprising:
(A) providing a dry film comprising (1) a cover sheet;
(2) a top coat on said cover sheet, and (3) a layer of photoimageable composition on said top coat; said top coat being selectively adherent to said photoimageable composition layer relative to its adherence to said cover sheet;
(B) tacking said photoimageable composition layer to said substrate surface;
(C) laminating said photoimageable composition layer to said substrate surface using heat, vacuum and mechanical pressure;
(D) removing said cover sheet from said top coat, leaving said photoimageable composition layer tacked to said substrate surface with said top coat covering and protecting said photoimageable composition layer;
(E) laminating said photoimageable composition layer to said substrate surface, using heat, vacuum and mechanical pressure to conform said photoimageable composition layer and said to coat to said substrate surface;
(F) exposing said photoimageable composition layer to patterned actinic radiation; and (G) developing said photoimageable composition layer to remove either exposed or unexposed portions from said substrate surface, leaving either unexposed or exposed portions remaining on said substrate surface.

23. A method according to claim 6 wherein said conforming laminating step E is carried out for about half the time as said laminating step (C).

24. A method according to claim 6 wherein in step B
said photoimageable composition layer is tacked to the leading and trailing edges of the printed circuit board.

25. A method according to claim 6 wherein said lamination steps C and E are each carried out at a temperature of between about 40°C. and about 100°C. and a vacuum of at least about 1 millibar.

26. A method according to claim 6 wherein the thickness of said photoimageable layer is about equal to or less than the elevation of traces on said printed circuit board.

27. A method according to claim 1 wherein less than 5% of the area of said photoimageable composition layer is tacked to the surface of the printed circuit board.

28. A method according to claim 1 wherein said lamination is carried out at a temperature of between about 50°C and about 90°C. and a vacuum of at least about 1 millibar.

29. A method according to claim 1 wherein said photoimageable composition layer is cured with heat and/or UV.

30. A method according to claim 1 wherein the thickness of said photoimageable layer is about equal to or less than the elevation of traces on said printed circuit board.

31. A method according to claim 1 wherein said circuit board has thru-holes, and in step (D), the vacuum is adjusted according to the strength of said photoimageable composition layer such that portions of said photoimageable composition layer tenting such thru-holes prior to vacuum exposure remain tenting said thru-holes.

32. A method of forming a solder mask on the surface of a printed circuit board having a non-conductive surface and circuitry traces extending upward therefrom, the method comprising (A) providing a dry film comprising (1) a cover sheet, (2) a top coat on said cover sheet, and (3) a layer of photoimageable composition on said top coat, which photoimageable composition is curable after exposure and development to provide a hard, permanent solder mask overlying the printed circuit board; said top coat being electively adherent to said photoimageable composition relative to its adherence to said cover sheet;
(B) tacking said photoimageable composition layer to the surface of said printed circuit board;
(C) removing said cover sheet from said top coat, leaving said photoimageable composition layer tacked to the surface of said circuit board with said top coat covering and protecting the same;
(D) laminating said photoimageable composition layer to the surface of said printed circuit board, using heat and vacuum to conform said photoimageable composition layer and said top coat to the surface thereof;
(E) laying artwork directly on said top coat and exposing said photoimageable composition layer to patterned actinic radiation through said top coat;
(F) developing said photoimageable composition to remove either exposed or unexposed portions from said printed circuit board, leaving either unexposed or exposed portions remaining on said printed circuit board;
and (G) curing said remaining portions of said photoimageable composition layer to form a hard, permanent solder mask protecting said printed circuit board.

33. A method according to claim 32 wherein said circuitry traces extend at least about 50µ above said nonconductive surface, said photoimageable composition has a thickness about equal to the height of said circuitry traces above said non-conductive surface or less, and said top coat has a thickness of between about 1µ and about 5µ.

34. A method according to claim 32 wherein said circuit board has thru-holes, and in step (D), the vacuum is adjusted according to the strength of said photo-imageable composition layer such that portions of said photoimageable composition layer tenting such thru-holes prior to vacuum exposure remain tenting said thru-holes.

35. A method of forming a solder mask on the surface of a printed circuit board having a non-conductive surface and circuitry traces extending upward therefrom, the method comprising (A) providing a dry film comprising (1) a cover sheet, (2) a top coat on said cover sheet, and (3) a layer of photoimageable composition on said top coat, which photoimageable composition is curable after exposure and development to provide a hard, permanent solder mask overlying the printed circuit board; said top coat being selectively adherent to said photoimageable composition relative to its adherence to said cover sheet;
(B) tacking said photoimageable composition layer to the surface of said printed circuit board;
(C) removing said cover sheet from said top coat, leaving said photoimageable composition layer tacked to the surface of said circuit board with said top coat covering and protecting the same;
(D) laminating said photoimageable composition layer to the surface of said printed circuit board, using heat, vacuum and mechanical slap down applied directly to said top coat to conform said photoimageable composition layer and said top coat to the surface thereof;
(E) exposing said photoimageable composition layer to patterned actinic radiation;
(F) developing said photoimageable composition to remove either exposed or unexposed portions from said printed circuit board, leaving either unexposed or exposed portions remaining on said printed circuit board;
and (G) curing said remaining portions of said photoimageable composition layer to form a hard, permanent solder mask protecting said printed circuit board.

36. A method according to claim 35 wherein siad circuitry traces extend at least about 50µ above said non-conductive surface, said photoimageable composition has a thickness about equal to the height of said circuitry traces above said non-conductive surface or less, and said top coat has a thickness of between about 1µ and about 5µ.

37. A method according to claim 33 wherein in Step (E), artwork is laid directly on contact with said top coat and said photoimageable composition layer is exposed to patterned actinic radiation through said artwork.

38. A method according to claim 33 wherein said circuit board has thru-holes, and in step (D), the vacuum is adjusted according to the strength of said layer of photoimageable composition such that portions of said layer of photoimageable composition tenting such thru-holes prior to vacuum exposure remain tenting said thru-holes.