CA1258013A

CA1258013A - Method and apparatus for dispensing viscous materials

Info

Publication number: CA1258013A
Application number: CA000529902A
Authority: CA
Inventors: David Schoenthaler; Thadeus Wojcik
Original assignee: American Telephone and Telegraph Co Inc
Current assignee: AT&T Corp
Priority date: 1986-02-18
Filing date: 1987-02-17
Publication date: 1989-08-01
Also published as: US4622239A; JPH0376191B2; JPS62201672A

Abstract

Abstract An apparatus for dispensing solder paste into openings in a stencil comprises a housing having a piston slidably mounted therein. A manifold, having a successively increasing cross-sectional area, is attached to a side wall of the housing for admitting a viscous material, such as solder paste therein through successively larger sized openings so the paste is distributed evenly beneath the piston. A pair of elastomeric blades are secured to, and depend from the housing on opposite sides of a slot to contact the stencil. Upon the application of a fluid pressure against the piston, paste will be expelled from the slot within a working region between the blades.
The blades force paste into the openings in the stencil when the housing is moved thereacross.

Description

~5~ 3 METHOD AND APPARAT~S FOR DISPBNSING
VISCOUS MATERIALS

Technical Field This invention relates generally to a method and apparatus for dispensing a viscous material/ such a~
solder paste, through openings in a substrate~
Background of the Invention ~ . .
Recent advances in the field of se~iconductor technology have led to the development of chip carrier packagesO Chip carrier packages are generally comprised of a planar housing, havinq a semiconductor chip therein. Conductive members are provided on the chip carrier for bonding to metallized areas on the surface of a printed circuit board. In addition to chip carrier packages, other electronic components, such as resistors and capacitors, have been developed for mounting directly on the s~lrface of a printed circuit board. I'he term surface mounted component is commonly used to generically described any type of electronic component which is adapted for mounting on the surface of a printed circuit board.
The technique now in common use for mounting such components on the surface of a printed circuit board comprises the steps of: ~a) applying a controlled amount of a bonding material, typically solder paste, on the metallized areas on the circuit board; (b) placing the component on the surface of the circuit board such that the conductive members thereof are in contact with the solder paste-coated metallized areas; and (c) heating the solder paste to bond the conductive members of the component to the metallized areas.

~a~$

5~ 3 There are various methods by which solder paste may be deposited on the metallized pads on a printed circuit board. One approach is to use the dispensing tool described in V~ S. Patent: 4,515,297.
The dispensing tool described in the aforesaid patent comprises a chamber into which solder paste is drawn via a vacuum~ Once the chamber is filled~ a piston is placed within the chamber on top of the solder paste. A
pressure i5 applied against the piston to force the solder paste out of the chamber through a nozæle having a plurality of apertures each in registration with a separate one of the metallized areas on the printed cir~uit board. In order to dispense a different pattern of solder paste, the nozzle must be replaced with one having the proper arrangement of apertures, which is a time consuming process.
Solder paste can also be deposited (printed) onto metallized areas on the printed circuit board by forcing the paste through openings in a substrate (e.g., a stencil) placed in intimate contact with the circuit board. The stencil has openings therein arranged in the same pattern as metallized areas on the circuit board, so that only the metallized areas are exposed through the stencil openings. Solder paste is applied to the stencil and a squeegee blade is moved across the surface thereof to force the paste into the stencil openings and onto the metallized areas on the printed circuit board.
All of the metallized areas exposed through the stencil openings can be advantageously coated with solder paste during one or two passes of the squeegee blade across the stencil.
High quality printing of solder paste onto the printed circuit board requires a consistent volume of paste to be uniformly distributed across the leading edge of the squeegee blade and a minimal volume to be distributed across the trailing edge of the blade. In the past, prior art dispensing apparatus have incorporated only one squeegee blade. After a single pass of the blade across the s~encil, a quantity of paste will ~ypically remain proximate the leading edge of the blade. Before making the next pass in the opposite direction, the blade is made to hop or jump over this quantity of paste so the paste is now distributed along the leading edge of the blade.
Unfortunately, solder paste has a high viscosity and tends to stick to the blade. thus, paste wi-ll often cling to what now becomes the trailing edge of the blade once the blade is displaced in the opposite direction during a second pass across the stencil. The result is streaking of solder paste on the stencil and an uneven depc,~it of solder paste on the circuit board.
lS To a lesser degree, the disadvantages attendant in printing of solder paste onto a substrate using a single sque0gee blade are also present in printing other less viscous materials. In an effort to improve the printing of less viscous conductive inks and the like on a printed circuit boaed, U.S. Patent 3,464,351 discloses a apparatus embodying a pair of squeegee blades which serve to contain the material to be printed therebetween. In practice, the material is supplied into the region between the blades from a

2~ remote source under pressure. While the DeHart et al.
apparatus may be suited for materials having a moderate viscosity, substantial difficulties are likely to arise in attempting to use the apparatus to print materials having a high viscosity, such as solder paste. In ord&r to force solder paste from a remote source into the region between the blades of VeHart et al., a very high pressure is required which can cause the solder paste to separate. Further, when the pressure is removed, there will be a long lag time before the paste forced from the region between the blades stops flowing Accordingly, there is a need for a technigue for dispensing a highly viscous material, such as solder paste, onto a substrate.
Summary of the Invention In accordance with an aspect of -the invention there is provided a method for dispensing viscous material through apertures in a substrate comprising the steps of:
directing a viscous ma~erial into a housing; simultaneously compensating for fluctuations in the pressure of the material during the flow thereof into the housing; applying pressure against the vlscous material to force it from the housing through an opening therein; confining the viscous material forced from the housing hetween a pair of flexible members extending from the housing on opposite sides of the opening so as to be in contact with the substrate; and imparting a relative motion between the substrate and the housing so that the members urge the viscous material forced from the housing through the substrate apertures.
In accordance with another aspect of the invention there is provided an apparatus for dispensing viscous material into apertures in a substrate comprising: a housing having a first opening therein and a plurality of spaced second openings of successively increasing size; a piston slidably mounted within said housing for movement to and from said first opening; conduit means, having a successively increasing cross-sectional area in communi-cation with said second openings in said housing for carrying viscous material into said housing while simultaneously compensating for fluctuations in the pressure of the viscous material during the flow thereof;
means for applying pressure against said piston to cause said piston to force the viscous material ou-t from said first opening in said housing; and a pair of flexible members depending from the housing on opposite sides of the first opening for confining the viscous material forced from the first opening therebetween.
~ n~

- 4a -Brief Descriptic~n of the Drawing FIG, 1 is a perspective view of a prior art circuit board having chip carriers mounted therein;
FIG. 2 is a perspective view of an apparatus, according to the present invention for applying .solder paste onto the circuit board of FIG. l;
FIG. 3 is a cross-sectional end view of the apparatus of FIG. 2;
FIG. 4 iS a partial view of the apparatus of FIG. 2 in cross-section;
FIG. 5 iS a front view of the apparatus of FIG. 2 in cross sec~ion; and FIG. 6 is a side view of the apparatus of FIG. 2 showing a pivot mechanism therefor.

5~ 3 Detailed Description . ~ . .. .. .
FIG~ 1 is a partial perspective view of a circuit package 10, according to the prior art, which is comprised of a printed circuit board 12 having a plurality of components such as chip carriers 14-14 mounted on a major surface 16 thereof. In an exe~plary embodiment, each chip carrier 14 has electrically conductive pads 18-18 adapted for solder bonding to corresponding metallized areas 20-20 on the surface 16 of the circuit board 12. The metallized areas 20-20 are arranged on the surface 16 in the same pattern as the conductive pads 18-18 on the chip carriers 14-14.
Metallized foils 22-22 are provided on the surface 16 to selectively interconnect the metallized areas 20~20.
To facilitate solder bonding of the pads 18 18 on the chip carriers 14-14 to the metallized areas 20-20 on the surface 16, the metallized areas are typically each coated with a layer of solder paste (not shown).
When heated, the solder paste, together with whatever solder is present on the pads 18 18, melts to provide a solid electrical and mechanical bond between the pads and the metallized areas 20-20.
One way in which the metallized areas 20-20 on the surface 16 can be coated with solder paste is to print the paste thereon. Referring to FIG. 2, a substrate 24, typically taking the form of a thin metal stencil having a plurality of openings 26-26 therein, is placed in inti~ate contact with the surface 16 of the circuit board 12. The openings 26-26 are sized on the order of, and are arranged in the same pattern as the metallized areas 20-20 (see FIG. 1) on the surface 16 so that each metallized area i5 exposed through a separate - one of the openings in the stencil 24. Overlying the stencil 24 is a solder paste dispensing head 28, constructed in accordance with the teachings of the present invention, for forcing solder paste through the openings 26-26 and onto the metallized areas 20-20 (see _ 5 -FIG. 1) on the surface 16 of the circuit board 12.
The solder paste dispensing head 28 comprises a prismatic (rectangular) housing 30 attached to a pair of arms 32-32 which are connected to a mechanism (not shown) which serves to reciprocate the housing across the stencil 26 ai!ong a single a~is, represented by the double-ended arrow 34. The housing 30 has an opening 36 in the top thereof which is sealed by a cover 38 secured to the top of the housing by fasteners 40-40.
Referring to FIGS. 2 and 3 the housing 30 is comprised of a pair of parallel, spaced side walls 42-; 42, each joined at their ends to a separate one of a pair of end walls 43-43 (FIG. 2). Each of the end walls 43-43 is pivotally attached to one of the arms 32-32 as seen in FIG. 2 thereto. Extending from the lower edge of each of the side walls 42-42, respectively, is one of a pair of inwardly and downwardly directed bottom walls ~-44, hest illustrated in FIG. 3, which provide the housing 30 with a "V"-shaped bottom. The bottom walls 44-44 do not intersect because a gap or slot 46 is provided therebetween which runs the length of the side walls 42-42~ Typically, the slo~ 46 is on the order of .08-.10 inch wide. An end seal 47, best illustrated in FIGS. 2 and 4r is provided in each end wall 43 to seal the opening into each end of the slot 46.
Referring to FIu~ 3, each of the bottom walls 44-44 has a pocket 48 in the lower (exposed) surface thereof in communication with the slot A6. Æach of the pockets 48-48 runs horizontally the length of the side walls 42-42 and is sized to receive a separate one of a pair of bar-shaped~ elastomeric squeegee blades 50~50 therein so that an edge Sl on each blade projects vertically downwardly from the pocketO The blades 50-50 are of a width slightly larger than each of the pockets 48-A8 so that the spacing between the blades is less than the width of the slot 46. Although not shown, the ends of the blades 50-50 abut the end seal 47 in each of - 7 - ~25~3 the end walls 43-43 of FIGS. 2 and 4O
The blades 50-50 are each held within a separate one of the pockets 48-48 by a separate one of a pair o clamping a~:ms 52-52 which run along the length of the bottom walls ~2-42. Each of the clamping arms 52-52 is shaped complimentary to the lower surface of a separate one of the bottom w311s 44-44. The clampin~
arms are secured to the bottom walls 44-44 by a set of bolts 54-54 so a portion of each clamping arm overlies each pocket 48. A boss 56 runs along on each clamping arm 52 for seating in a complimentary notch 58 running in each blade 50 to assure that each blade is securely held in its corresponding pocket 48 by the clamping arm.
Referring to FIGS. 2 and 3, slidahly mounted within the housing 30 is a piston 60. The piston 60 is generally prismatic in shape and has a "V"-shaped bottom 62. A pair of spaced, parallel, continuous lips 64-64, each extend hori~ontally outwardly from the sides and ends of the piston 60 proximate the top and bottom thereof a distance of approximately .06 inch thereof.
The lips 64-64 on the piston 60 serve to reduce the area on the piston 60 which may contact the interior surface of the side and end walls 42-42 and 43-43, respectively, minimizing the friction therebetween~ In practice, a .0025 inch clearance is provided all the way around between the lips 64-64 and the inside surface of the side and end walls 42-42 and 43-43, respectively.
Between the lips 64-64, there exists a U-shaped channel 66. After the piston 60 has been inserted in the housing 30, the channel 66 is filled with a fluid (not shown)t typically a commercially available wave soldering oil or grease to create a liquid/fluid seal between the housing and the piston.
The sealing liquid/fluid prevents air Erom leaking in between the piston 60 and the side and end walls 42-42 and 43-43, respectively. To facilitate the admission of the fluid into the channel 66, small openings, in the - 8 _ ~2S~3 form of notches or the like (not shown~, are provided in the upper one of the lips 64-6~.
A viscous joining material (e.g., solder paste)~ which may have a viscosity, typically as 700,000 centipoisel is admitted into the housing 30 below the piston 60 via hollow ~anifold 70 attached to one of side walls 42-42 of the housing. Referring to FIGS. 4 and 5, the manifold 70 has a cavity 72 therein which progressively tapers both in height and breadth from the ends thereof towards the center, making the cavity "bow tie" in shape. Stated another way, the cavity 72 has a successively increasing cross-sectional area from the center towards the ends thereof. The cavity 72 is sealed by way of plate 73.
At the center of the manifold 70 is an inlet 74 (see FIG. 5) in communication with the cavity 72 to allow the solder paste to be admitted into the cavity at relatively high pressure (greater than 50 psi).
plurality of spaced apertures 76-76 are provided in the plate 73 so as to extend through the one side wall 42 of the housing 30 to allow the solder paste admitted into the cavity 72 to enter the housing. The apertures 76-76 are of successively increasing size from the center of the manifold 70 towards the ends thereof. The advantage of directing the viscous solder paste through the cavity 72 and into the spaced, progressively larger sized apertures 76-76 is that the pressure drop which is experienced by the solder pas~e during its flow is compensated for by the increasing cross-sectional area of the cavity and the progressively increasing dia~eter of the apertures. In this way, the solder paste evenly fills the void in the housing 30 beneath the piston 60 without voids or bubbles. To insure that the paste is admitted into the housing 30 below the piston 60, stops (not shown) may be provided on either the side or end walls 42-42 and 43-43 to prevent the piston from descending below the apertures 76-76.

9 ~25~

Referring to FIG. 2, the cover 38 is provided with an opening 78 therein through which a fluid, such as compressed air, can be applied at a relatively low pressure tless than 15 psi), to force the piston 60 towards the bottom walls 44-44 (see FIG. 3) of the housing 30. The position of the piston 60 within the housing 30 is sensed by two pairs of proximity switches 80-80 and 82-82. Each of the proximity switches 80-80 and 82-82 of each pair is ~ounted to the cover 38 a I0 short distance from one of the ends thereof. As seen in FIG. 3, each proximity switch 80 s fixedly mounted ~o the cover 38 so the bottom of the switch is substantially flush with the lower surface of the cover.
When the piston 60 is at the top of its travel so as to be proximate the cover 38, each of the proximity switches 80-80 will be actuated thereby. Should the piston 60 be tilted so that only one end thereof is proximate the cover 38, then the proximity switch 80 closest to the tilted end of the piston will be actuated thereby.
Each of the proximity switches 82-82 is slidably mounted through the cover 38. The downward travel of each proximity switch 82 is limited by a collar 84 carried thereby. A spring 86 biases each proximity switch 82 downwardly from the cover 38 so that each proximity switch remains in contact with the piston 60 until the piston descends into the housing 30 to a height just above the apertures 76-76~ While the proximity switches 82-82 remain in contact with the piston 60, the proximity switches remain actuated thereby. However, once the piston 60 has descended within housing 30, so as to be just above the apertures 76-76 of FIGS. 4 and 5, the switches 82-82 beco~e de-actuated. If only one end of the piston 60 has 3S desc~nded below the lower point of travel of one of the proximity switches 82-82, then only that proximity switch will become de-actuated. By sensing the - 10 - ~ZS;~3 actuation state of the proximity switches 80-80 and 82-82, the relative position of the piston 60 within the housing 30 can be ascertained.
Referring to FIG. 3, a strip heater 90 runs horizQnt311y alor,g the side wall 42 opposite the one which mounts the manifold 70. the strip heater 90 is held against the side wall 42 by a plate 92 securea to the side wall by bolts 94-94. The strip heater 90 is controlled in response to a signal from a temperature ~ensor 95 ~see ~I~. 3) attached to the plate 92. In this way, the temperature of the solder paste within the housing 30 can be adjusted to control the paste viscosity. Thus, solder paste which normally has a high viscosity which makes the dispensing thereof very difficult, can be heated and made to flow freely to allow for easy dispensing. Once the heated solder paste is forced through the stencil openings 26-26 and onto the ~urface 16 of the circuit board 12, the paste cools, causin~ the viscosity and tackiness thereof to return to their original levels.
In operation, the housing 30 is reciprocated across the stencil 24 of FIG. 2 while at the same time a pressurized fluid is directed against the piston 60.
The fluid pressure causes the piston 60 to descend in the housing 30, thereby forcing the solder paste out through the slot 46 of FIGS. 3-5 and into a working region 96 above stencil 24. The blades 50-50 serve to forse the paste within the region 96 into the openings 26-26 of the stencil 2~ to print the solder paste onto the circuit board 16. Once a sufficient quantity of paste has been printed onto the circuit board 16 ~which is generally achieved after one or two passes of the dispensing head 28 across the stencil 24), the pressure against the piston 30 is released to terminate the dispensing of paste.

258~L3 If during the operation of the dispensing head 28, the piston 60 has descended to a height just above the apertures 76-76, as sensed by the proximity switches 82-~2, then solder paste can be flowed into the opening 74 in the manifold paste can be flowed into the opening 74 in the manifold 70 to uniformly refill the housing 30 with paste. Advantageously, the refilling of the housing 30 with solder paste can be done automatically or manually without the need to remove the piston 60, providing opening 46 is sealed.
In practice, the housing 30 is held by the arms 32-32 of FIGS. 2 and 6 at a predetermined height above the stencil 24 to maintain the edge 51 of each of the blades 50-50 in substantial contact therewith.
thus, the solder paste forced from the slot 46 is confined between the blades 50-50. The confinement of the solder paste between the blades 50-50 during the dispensing thereof from the housing 30 is very advantageous because the exposure of the paste to atmospheric conditions will be reduced, thereby reducing the tendency of the paste to harden and clog before entering the openings 26-26 in the stencil 24.
Because oE variations in the width and thickness of each of the blades 50-50 due to 2S manufacturing tolerances, difficulties sometimes arise in maintainin~ both of the edges 51-51 of the blades in contact with the stencil 2~ during reciprocation of the dispensing head 28 thereacross. One way in which this problem can be mitigated is to pivot the housing 30 about the arms 32-32 to cause a larger pressure to be applied to the trailing one of the blades 50-50 during reciprocation of the dispensing head 28 across the stencil 24. Referring to FIG. 6, there i5 shown an apparatus 97 for pivoting the housing 30. The apparatus 97 is comprised of a cylinder 98 which is pivotally mounted, in spaced relationship above the dispensing head 28, to the same mechanism which reciprocates the - 12 - ~Z~ 3 arms 32-32.
Within the cylinder 98 is a piston 100 which, when no fluid pressure is exerted thereagainst, remains spaced half-way between the top and bottom of the cylinder. The piston 100 has a threaded shaft 102 extending downwardly therefrom through the cylinder towards the housing 30. The shaft 102 passes through an opening 104 in a horizontal bar 106 attached to the mechanism which reciprocates arms 32-32. The opening 104 is of a larger diameter than the shaft 102 to permit the shaft to pass easily therethrough. A pair of nuts 108-108, of a larger diameter than the opening 104, are each threaded onto the shaft 102 above and below the bar 106. The end of the shaft 102 depending below the bar 10~ is pivotally connected to one end of an "L"-shaped bracket 110. The other end of the bracket 110 is secured to the top of the cover 38.
When the nuts 108-108 are spaced apart a distance greater than the thickness of the bar 106, the shaft 102 can move up or down when pressurized fluid is admitted into the cylinder 98 below or above, respectively, the piston 100. The downward movement of the shaft 102 causes the housing 30 to pivot about the arms 32-32 in a counter-clockwise direction through a short arc, causing more pressure to be applied to the lefthand one of the blades 50-50 (not shown).
Conversely, when the shaft 102 moves upwardly, the : housing 30 pivots in a cloekwise direction through a : short arc causing more pressure to be applied to the righthand one of the blades 50-S0.
The arc through which the housing 30 pivots in the counter-clockwise and clockwise directions is determined by the spacing o~ the upper and lower one of the nuts 108-108, respectively, from the top and bottom, : 35 respectively, of the bar 106. The greater the spacing of the upper and lower one of the nuts 108-108 from the bar 106, the greater the arc ~hrough which the hou.qing - 13 ~ 8~3 30 pivots in the counter-clockwise and clockwise directions, respectively. Note that the housing 30 can be prevented from pivoting altogether by threading each of the nuts 108-108 tightly against the bar 106.
The dispensing head 28 is capable of dispensing other types of viscous material in addition to solder paste. For example, the dispensing head 28 can be used to dispense viscous inks, or even RTV
silicones~ The sealing fluid, which is admitted into the channel 66 to obtain the liquid fluid seal between the piston 60 and the interior s~rface of the side and end walls ~2-42 and ~3~43, is choosen so as to be compatible with the type of material being dispensed.
Note that while the dispensing head 28 has been described as incorporating a piston 60, the dispensing head could be constructed withowt the piston.
Instead, pressure, such as provided by a stream of compressed air, could be applied to the viscous material in the housing 30 to force it therefrom. However, the advantage in using the piston 60 is that the likelihood of the air being blown through the viscous material is avoided.

Claims

1. A method for dispensing viscous material through apertures in a substrate comprising the steps of:
directing a viscous material into a housing;
simultaneously compensating for fluctuations in the pressure of the material during the flow thereof into the housing;
applying pressure against the viscous material to force it from the housing through an opening therein;
confining the viscous material forced from the housing between a pair of flexible members extending from the housing on opposite sides of the opening so as to be in contact with the substrate; and imparting a relative motion between the substrate and the housing so that the members urge the viscous material forced from the housing through the substrate apertures.

2. The method according to claim 1 further including the step of refilling the housing with viscous material once the material reaches a predetermined level.

3. The method according to claim 2 wherein the level of the viscous material within the housing is sensed automatically,

4. The method according to claim 1 further including the step of pivoting the housing in opposite directions as the relative motion is imparted between the housing and the substrate in opposite directions to apply a greater pressure to a trailing one of the elastomeric members.

5. The method according to claim 1 further including the step of heating the viscous material admitted within said housing to control the viscosity thereof.

6. A method for dispensing viscous solder paste onto an apertured stencil in intimate contact with a printed circuit board so that the solder paste will be deposited on areas of the circuit board exposed through the apertures in the stencil comprising the steps of:
directing solder into a housing beneath a piston slidably mounted therein;
simultaneously compensating for fluctuations in the pressure of the material during the flow thereof into the housing;
applying a pressure against the piston to force the solder paste out from the housing through a slot therein;
confining the solder paste forced from the slot in the housing between a pair of flexible blades mounted to the housing on opposite sides of the opening, the blades extending from the housing toward the stencil so as to be in contact therewith; and moving the housing across the stencil so that the blades urge the solder paste into the apertures in the stencil and onto the areas on the circuit board in registration therewith.

7. The method according to claim 6 further including the step of refilling the housing with solder paste once the piston has descended a predetermined distance.

8. The method according to claim 7 wherein the height of the piston within the housing is sensed automatically to determine whether the piston has descended said predetermined distance.

9. The method according to claim 6 further including the step of pivoting the housing in one of two opposite directions as the housing is moved in one of two opposite directions across the substrate to apply a greater pressure to the trailing one of the blades.

10. The method according to claim 6 further including the step of heating the solder paste admitted within said housing to control the viscosity thereof.

11. An apparatus for dispensing viscous material into apertures in a substrate comprising:
a housing having a first opening therein and a plurality of spaced second openings of successively increasing size;
a piston slidably mounted within said housing for movement to and from said first opening;
conduit means, having a successively increasing cross sectional area in communication with said second openings in said housing for carrying viscous material into said housing while simultaneously compensating for fluctuations in the pressure of the viscous material during the flow thereof;
means for applying pressure against said piston to cause said piston to force the viscous material out from said first opening in said housing;
and a pair of flexible members depending from the housing on opposite sides of the first opening for confining the viscous material forced from the first opening therebetween.

12. The invention according to claim 11 further including means for pivoting the housing to cause a greater pressure to be applied to one of said pair of flexible members.

13. The invention according to claim 11 further including means for heating the viscous material within the housing.

14. The invention according to claim 11 wherein each of said flexible members is prismatic in shape and has an edge thereof in contact with the substrate.