WO1996006692A1 - Cleaning of printed circuit boards - Google Patents

Cleaning of printed circuit boards Download PDF

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Publication number
WO1996006692A1
WO1996006692A1 PCT/US1995/010928 US9510928W WO9606692A1 WO 1996006692 A1 WO1996006692 A1 WO 1996006692A1 US 9510928 W US9510928 W US 9510928W WO 9606692 A1 WO9606692 A1 WO 9606692A1
Authority
WO
WIPO (PCT)
Prior art keywords
printed circuit
cleaned
radiation
flowing gas
board
Prior art date
Application number
PCT/US1995/010928
Other languages
French (fr)
Inventor
David J. Elliott
Richard F. Hollman
Francis M. Yans
Daniel K. Singer
George D. Whitten
Original Assignee
Uvtech Systems, Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Uvtech Systems, Inc. filed Critical Uvtech Systems, Inc.
Priority to AU34182/95A priority Critical patent/AU3418295A/en
Publication of WO1996006692A1 publication Critical patent/WO1996006692A1/en

Links

Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/14—Working by laser beam, e.g. welding, cutting or boring using a fluid stream, e.g. a jet of gas, in conjunction with the laser beam; Nozzles therefor
    • B23K26/1462—Nozzles; Features related to nozzles
    • B23K26/1464—Supply to, or discharge from, nozzles of media, e.g. gas, powder, wire
    • B23K26/147—Features outside the nozzle for feeding the fluid stream towards the workpiece
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B08—CLEANING
    • B08B—CLEANING IN GENERAL; PREVENTION OF FOULING IN GENERAL
    • B08B7/00—Cleaning by methods not provided for in a single other subclass or a single group in this subclass
    • B08B7/0035—Cleaning by methods not provided for in a single other subclass or a single group in this subclass by radiant energy, e.g. UV, laser, light beam or the like
    • B08B7/0042—Cleaning by methods not provided for in a single other subclass or a single group in this subclass by radiant energy, e.g. UV, laser, light beam or the like by laser
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B08—CLEANING
    • B08B—CLEANING IN GENERAL; PREVENTION OF FOULING IN GENERAL
    • B08B7/00—Cleaning by methods not provided for in a single other subclass or a single group in this subclass
    • B08B7/0035—Cleaning by methods not provided for in a single other subclass or a single group in this subclass by radiant energy, e.g. UV, laser, light beam or the like
    • B08B7/0057—Cleaning by methods not provided for in a single other subclass or a single group in this subclass by radiant energy, e.g. UV, laser, light beam or the like by ultraviolet radiation
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/06—Shaping the laser beam, e.g. by masks or multi-focusing
    • B23K26/064—Shaping the laser beam, e.g. by masks or multi-focusing by means of optical elements, e.g. lenses, mirrors or prisms
    • B23K26/066—Shaping the laser beam, e.g. by masks or multi-focusing by means of optical elements, e.g. lenses, mirrors or prisms by using masks
    • B23K26/0661—Shaping the laser beam, e.g. by masks or multi-focusing by means of optical elements, e.g. lenses, mirrors or prisms by using masks disposed on the workpiece
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/06—Shaping the laser beam, e.g. by masks or multi-focusing
    • B23K26/073—Shaping the laser spot
    • B23K26/0732—Shaping the laser spot into a rectangular shape
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/06—Shaping the laser beam, e.g. by masks or multi-focusing
    • B23K26/073—Shaping the laser spot
    • B23K26/0738—Shaping the laser spot into a linear shape
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/12—Working by laser beam, e.g. welding, cutting or boring in a special atmosphere, e.g. in an enclosure
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/12—Working by laser beam, e.g. welding, cutting or boring in a special atmosphere, e.g. in an enclosure
    • B23K26/1224—Working by laser beam, e.g. welding, cutting or boring in a special atmosphere, e.g. in an enclosure in vacuum
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/12—Working by laser beam, e.g. welding, cutting or boring in a special atmosphere, e.g. in an enclosure
    • B23K26/123—Working by laser beam, e.g. welding, cutting or boring in a special atmosphere, e.g. in an enclosure in an atmosphere of particular gases
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/12—Working by laser beam, e.g. welding, cutting or boring in a special atmosphere, e.g. in an enclosure
    • B23K26/126—Working by laser beam, e.g. welding, cutting or boring in a special atmosphere, e.g. in an enclosure in an atmosphere of gases chemically reacting with the workpiece
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/12—Working by laser beam, e.g. welding, cutting or boring in a special atmosphere, e.g. in an enclosure
    • B23K26/127—Working by laser beam, e.g. welding, cutting or boring in a special atmosphere, e.g. in an enclosure in an enclosure
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/14—Working by laser beam, e.g. welding, cutting or boring using a fluid stream, e.g. a jet of gas, in conjunction with the laser beam; Nozzles therefor
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/14—Working by laser beam, e.g. welding, cutting or boring using a fluid stream, e.g. a jet of gas, in conjunction with the laser beam; Nozzles therefor
    • B23K26/142—Working by laser beam, e.g. welding, cutting or boring using a fluid stream, e.g. a jet of gas, in conjunction with the laser beam; Nozzles therefor for the removal of by-products
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
    • G03F7/26—Processing photosensitive materials; Apparatus therefor
    • G03F7/42—Stripping or agents therefor
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
    • G11B7/24—Record carriers characterised by shape, structure or physical properties, or by the selection of the material
    • G11B7/26—Apparatus or processes specially adapted for the manufacture of record carriers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer
    • H01L21/48—Manufacture or treatment of parts, e.g. containers, prior to assembly of the devices, using processes not provided for in a single one of the subgroups H01L21/06 - H01L21/326
    • H01L21/4814—Conductive parts
    • H01L21/4846—Leads on or in insulating or insulated substrates, e.g. metallisation
    • H01L21/4864—Cleaning, e.g. removing of solder
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K3/00—Apparatus or processes for manufacturing printed circuits
    • H05K3/0011—Working of insulating substrates or insulating layers
    • H05K3/0055—After-treatment, e.g. cleaning or desmearing of holes
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K3/00—Apparatus or processes for manufacturing printed circuits
    • H05K3/22—Secondary treatment of printed circuits
    • H05K3/26—Cleaning or polishing of the conductive pattern
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K2203/00—Indexing scheme relating to apparatus or processes for manufacturing printed circuits covered by H05K3/00
    • H05K2203/08—Treatments involving gases
    • H05K2203/087—Using a reactive gas
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K2203/00—Indexing scheme relating to apparatus or processes for manufacturing printed circuits covered by H05K3/00
    • H05K2203/10—Using electric, magnetic and electromagnetic fields; Using laser light
    • H05K2203/107—Using laser light
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K2203/00—Indexing scheme relating to apparatus or processes for manufacturing printed circuits covered by H05K3/00
    • H05K2203/11—Treatments characterised by their effect, e.g. heating, cooling, roughening
    • H05K2203/1105—Heating or thermal processing not related to soldering, firing, curing or laminating, e.g. for shaping the substrate or during finish plating
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K3/00—Apparatus or processes for manufacturing printed circuits
    • H05K3/22—Secondary treatment of printed circuits
    • H05K3/28—Applying non-metallic protective coatings
    • H05K3/288—Removal of non-metallic coatings, e.g. for repairing

Definitions

  • This invention relates to cleaning printed circuit boards.
  • a printed circuit board typically has an insulating core with plated and/or etched conductor patterns on the surfaces (normally copper).
  • the core 26 may be a laminate of epoxy and woven glass fibers. Holes are drilled through the board for mounting electronic components.
  • the copper foil is etched into a pattern of electrical interconnections 30. It is also plated to provide a conducting path 28 through the drilled holes.
  • An additional photosensitive insulating layer 32 referred to as the "solder mask” is patterned photolithographically to serve as a mask for electroplating solder 34 from the copper conductors. The leads of mounted electronic components will then be connected to the copper conductors by means of the solder pads.
  • the core 26 becomes contaminated when the copper foil is chemically etched.
  • the soldermask 32 absorbs contaminants from the flux used to apply the solder contacts, resulting in a thin contaminated surface layer 36.
  • Another common form of contamination in fabricating boards occurs when holes are drilled through a multilayer board having one or more layers of conducting paths in the interior. Drilling may result in "smear", in which resin softened by the heat of the drilling covers the edges of the inner layer conductors, making contact difficult. This smear is typically removed using a wet chemical etching process.
  • Contamination of these insulating layers is typically measured using an "Omega Meter” , in which the board is immersed for a period of time in an alcohol solution. The solution leaches the contaminants from the board, and the meter monitors the conductivity of the solution as the contaminants accumulate. The results are commonly reported in units of "micrograms NaCl/in2", and a typical maximum specification for high quality boards is 6.5 micrograms NaCl/in2.
  • Contaminants which are absorbed into the laminate structure of the board during plating and etching operations must be removed prior to subsequent processing steps, to prevent unacceptably high levels of electrical conductivity through the buttercoat/laminate matrix or the solder mask layer, which can degrade the quality of the finished product.
  • the invention features cleaning printed circuit boards in the course of fabrication by providing a flowing gas, including a reactant, and delivering pulsed radiation to the surface of the printed circuit board in the presence of the flowing gas, such that the absorbed contamination is reacted and removed.
  • the radiation may be in the wavelength range 150 nm to 405 nm.
  • the radiation source may be an excimer laser, or a solid state laser, such as a frequency-multiplied Nd:YAG laser, or an ultraviolet lamp.
  • the radiation may be optically shaped into a blade, and relative motion may be induced between the optics and the surface to be cleaned so that the blade scans across the entire surface.
  • the intensity of a pulse of radiation at the surface to be cleaned may be between 40 mJ/cm2 and 1000 mJ/cm2.
  • the surface to be cleaned or the flowing gas may be heated.
  • the cleaning may take place in an enclosed chamber, in which pressure or vacuum is applied for the purpose of accelerating to improving the efficiency of the reaction.
  • the surface to be cleaned may be the insulating core of a printed circuit board.
  • the material to be removed may be a thin surface layer containing the contaminant, and the flowing gas may include oxygen or ozone.
  • the surface to be cleaned may be that of the solder mask layer.
  • the material to be removed may be a thin surface layer containing the contaminant, and the flowing gas may include oxygen or ozone.
  • the material to be removed may be a smear of resin on the walls of holes drilled through a printed circuit board, and the gas may be passed through the holes.
  • the radiation initiates a reaction with the gas that consumes a thin layer of the polymer surface containing the contaminants. Cleaning is accomplished by allowing the entire surface of the printed circuit board to be swept by the beam in the presence of the flowing gas.
  • Thin layers of contaminants may be removed from printed circuit boards after etching, plating or soldering, without damaging the boards.
  • This is a "dry" cleaning method which eliminates the need for large volumes of rinse water, and also eliminates the need for treatment of the used rinse water.
  • the process also reduces delamination and other deleterious effects of a wet soak or rinse on the board structure.
  • the method has the potential to improve the quality of printed circuit boards by reducing the contaminant concentrations, and thus the electrical conductivity, of insulating regions of the board below the levels attainable by conventional cleaning methods.
  • the method exhibits a wide process latitude, in that the radiation dose required to perform the cleaning process is a fraction of the dose which would cause damage to the board.
  • Figure 1 is a cross section showing the removal of the contaminated surface layer from the insulating core of a printed circuit board.
  • Figures 2A and 2B illustrate the optical elements used to form a focused blade from the rectangular output beam of an excimer laser.
  • Figure 3 is a cross section of a fragment of a single-layer printed circuit board.
  • Figure 4 shows a top view of a printed circuit board being moved beneath a stationary beam while a flow of reactive gas is provided across the illuminated surface.
  • Figure 5 is a top view of an optical arrangement for scanning both sides of a printed circuit board at the same time, using the beam from a single laser.
  • Figure 6 is a cross section of a hole drilled through a multilayer P.C. board, illustrating the removal of "smear".
  • P.C. board is passed under a high intensity beam of ultraviolet radiation in the presence of a reactive gas.
  • the radiation and gas interact with the board, removing a thin surface layer in which the unwanted chemicals are absorbed.
  • the gas flow aids in the removal of the volatilized material, which is exhausted through a carbon filter.
  • the contaminated surface layer is ablated or otherwise activated by the radiation to cause it to react with the flowing gas, producing gaseous reaction products which are carried away by the gas flow.
  • a contaminated surface layer 4 on an insulating board core 2 is processed to form a reaction product 12, by the combination of providing a directed flow of gas 10, including a reactant, in the vicinity of the contaminated layer, and delivering a beam of radiation 6 to aid the reactant to react with the contaminated surface layer to form the reaction product.
  • the contaminated surface layer which may be in the range of 1 to 5 microns in depth, is completely removed. In the case where the surface to be cleaned is that of the insulating core, the process does not damage the board by etching deeply enough to expose the glass fibers 14.
  • an 80 mil thick epoxy board core with 2 mil etched coper conductors was cleaned using a 248 nm wavelength beam from a KrF excimer laser.
  • the beam was pulsed at a 100 Hz rate, and was focused to a stripe approximately 100 microns thick, with energy per unit length 6 mJ/cm.
  • Oxygen was flowed across the surface at a velocity of approximately 100 cm/sec.
  • the surface was scanned using a single pass at a velocity of 6mm/sec.
  • the cleaned surface measured 4.0 micrograms NaCl/in2 on the "Omega Meter" test, well within the specification limit of 6.5.
  • samples of 125 mil thick board coated with solder mask were cleaned in the same manner as described above.
  • the beam may be deep ultraviolet radiation in the wavelength range from 155 nm to 375 nm.
  • the source of the radiation may be an excimer laser, for example a KrF excimer producing radiation at 248 nm wavelength, or an ArF excimer producing radiation at 193 nm wavelength.
  • the source may be a solid state laser such as a frequency-quadrupled Nd.YAG laser producing radiation at 266 nm wavelength, a tripled Nd.YAG at 355 nm, or a frequency-tripled Alexandrite laser producing radiation with a tunable wavelength in the range 240 nm to 266 nm.
  • a solid state laser such as a frequency-quadrupled Nd.YAG laser producing radiation at 266 nm wavelength, a tripled Nd.YAG at 355 nm, or a frequency-tripled Alexandrite laser producing radiation with a tunable wavelength in the range 240 nm to 266 nm.
  • Other high intensity ultraviolet light sources may be effective in the cleaning process, such as a pulsed xenon lamp, or a high pressure mercury vapor lamp.
  • Other wavelengths of light may be present in addition to the deep ultraviolet light.
  • the beam from a frequency-quadrupled Nd:YAG laser may combine light of the fundamental wavelength 1064 nm and the second harmonic 532 nm along with the fourth harmonic at 266 nm wavelength.
  • the reactant may be a gas flowing at a velocity preferably between 20 mm/sec and 500 mm/sec.
  • the gas may include one or more members of the group of oxidants consisting of oxygen, fluorine and chlorine.
  • the reactant may include an oxidant and the beam may be ultraviolet radiation.
  • the beam may be delivered by receiving a source laser beam 16 and focusing the cross-sectional size of the beam in one dimension using a converging cylindrical lens 22 and broadening the cross-sectional size in the other dimension using a diverging cylindrical lens 18 to form a narrow rectangular beam 26 at the surface to be cleaned.
  • the size of the beam in the direction which is broadened may be at least as great as the width of the board to be cleaned.
  • the size of the beam in the other dimension may be chosen to provide an energy density at the surface to be cleaned sufficient to effect the contamination removal process but insufficient to damage the surface.
  • the entire surface may be scanned by placing the board 38 on a linear translation stage or conveyor (not shown) .
  • the stage or conveyor moves the board under the beam 40 while a flow of gas 44 containing a reactant is provided by a nozzle 42.
  • the entire surface of the board may be scanned by the beam in several passes, for complete cleaning in cases where the contaminated layer is not completely removed by a single pass. Multiple passes of the beam across the surface may be accomplished by several physical scans, or by splitting the beam, using a diffraction grating, beamsplitter or other means, into several focused beams at the surface.
  • An exhaust duct draws off the flowing gas from the area illuminated by the beam, and conveys the gas to a filter (not shown) which removes particles, combustion products and traces of contaminants from the gas stream before releasing or recirculating the gas.
  • Both sides of a printed circuit board may be cleaned. Cleaning both sides may be accomplished by sequentially scanning both sides in two scans.
  • both sides may be scanned at the same time, as shown in Figure 5.
  • the board 38 is moved on a linear translation stage or conveyor (not shown) which holds the board by the edges in order to expose both faces for processing.
  • the output beam 48 from a laser 46 is split into two beams by a beamsplitter 50.
  • Beam steering mirrors 52 direct the two beams to the region where the beams are formed into narrow blades without being blocked by the motion of the board.
  • the blades are formed by diverging cylindrical lenses 18 and converging cylindrical lenses 22, and directed to the two faces of the board by mirrors 54.
  • Figure 6 illustrates the use of photoreactive cleaning to remove "smear" in holes drilled in a P.C. board.
  • the figure shows the cross section of a hole drilled through a multilayer board, which has patterned conductor layers 30 on the outer surfaces, and an inner layer conductor 58.
  • the drilling process has resulted in a smear 60 of resin material over the edges of the inner layer conductor 58.
  • a beam of radiation 62 is directed to the board so that it is transmitted through the hole.
  • a small pressure differential between the two sides of the board will cause a flow 64 of gas through the hole.

Abstract

Printed circuit boards (2) are cleaned in the course of fabrication by providing a flowing gas (10), including a reactant, and delivering pulsed radiation (6) to the surface of the printed circuit board in the presence of the flowing gas (10), such that the absorbed contamination is reacted and removed.

Description

CLEANING OF PRINTED CIRCUIT BOARDS Background This application is a continuation in part of United States Patent Application Serial Number 08/298,023, filed on August 29, 1994.
This invention relates to cleaning printed circuit boards.
Printed circuit boards absorb unwanted chemicals at certain stages of fabrication and must be cleaned before further processing. A printed circuit board typically has an insulating core with plated and/or etched conductor patterns on the surfaces (normally copper). As seen in Figure 3, the core 26 may be a laminate of epoxy and woven glass fibers. Holes are drilled through the board for mounting electronic components. Using a photolithographic process, the copper foil is etched into a pattern of electrical interconnections 30. It is also plated to provide a conducting path 28 through the drilled holes. An additional photosensitive insulating layer 32, referred to as the "solder mask" is patterned photolithographically to serve as a mask for electroplating solder 34 from the copper conductors. The leads of mounted electronic components will then be connected to the copper conductors by means of the solder pads.
There are two surfaces in this structure which can become contaminated with electrolytes. The core 26 becomes contaminated when the copper foil is chemically etched. Similarly, the soldermask 32 absorbs contaminants from the flux used to apply the solder contacts, resulting in a thin contaminated surface layer 36. Another common form of contamination in fabricating boards occurs when holes are drilled through a multilayer board having one or more layers of conducting paths in the interior. Drilling may result in "smear", in which resin softened by the heat of the drilling covers the edges of the inner layer conductors, making contact difficult. This smear is typically removed using a wet chemical etching process.
Contamination of these insulating layers is typically measured using an "Omega Meter" , in which the board is immersed for a period of time in an alcohol solution. The solution leaches the contaminants from the board, and the meter monitors the conductivity of the solution as the contaminants accumulate. The results are commonly reported in units of "micrograms NaCl/in2", and a typical maximum specification for high quality boards is 6.5 micrograms NaCl/in2.
Contaminants which are absorbed into the laminate structure of the board during plating and etching operations must be removed prior to subsequent processing steps, to prevent unacceptably high levels of electrical conductivity through the buttercoat/laminate matrix or the solder mask layer, which can degrade the quality of the finished product.
Summary In general, in one aspect, the invention features cleaning printed circuit boards in the course of fabrication by providing a flowing gas, including a reactant, and delivering pulsed radiation to the surface of the printed circuit board in the presence of the flowing gas, such that the absorbed contamination is reacted and removed.
Implementations of the invention may include one or more of the following features. The radiation may be in the wavelength range 150 nm to 405 nm. The radiation source may be an excimer laser, or a solid state laser, such as a frequency-multiplied Nd:YAG laser, or an ultraviolet lamp. The radiation may be optically shaped into a blade, and relative motion may be induced between the optics and the surface to be cleaned so that the blade scans across the entire surface. The intensity of a pulse of radiation at the surface to be cleaned may be between 40 mJ/cm2 and 1000 mJ/cm2. The surface to be cleaned or the flowing gas may be heated. The cleaning may take place in an enclosed chamber, in which pressure or vacuum is applied for the purpose of accelerating to improving the efficiency of the reaction. The surface to be cleaned may be the insulating core of a printed circuit board. The material to be removed may be a thin surface layer containing the contaminant, and the flowing gas may include oxygen or ozone. The surface to be cleaned may be that of the solder mask layer. The material to be removed may be a thin surface layer containing the contaminant, and the flowing gas may include oxygen or ozone. The material to be removed may be a smear of resin on the walls of holes drilled through a printed circuit board, and the gas may be passed through the holes.
The radiation initiates a reaction with the gas that consumes a thin layer of the polymer surface containing the contaminants. Cleaning is accomplished by allowing the entire surface of the printed circuit board to be swept by the beam in the presence of the flowing gas.
Among the advantages of the invention are the following. Thin layers of contaminants may be removed from printed circuit boards after etching, plating or soldering, without damaging the boards. This is a "dry" cleaning method which eliminates the need for large volumes of rinse water, and also eliminates the need for treatment of the used rinse water. The process also reduces delamination and other deleterious effects of a wet soak or rinse on the board structure. The method has the potential to improve the quality of printed circuit boards by reducing the contaminant concentrations, and thus the electrical conductivity, of insulating regions of the board below the levels attainable by conventional cleaning methods. The method exhibits a wide process latitude, in that the radiation dose required to perform the cleaning process is a fraction of the dose which would cause damage to the board.
Other advantages and features will become apparent from the following description and from the claims.
Description Figure 1 is a cross section showing the removal of the contaminated surface layer from the insulating core of a printed circuit board. Figures 2A and 2B illustrate the optical elements used to form a focused blade from the rectangular output beam of an excimer laser.
Figure 3 is a cross section of a fragment of a single-layer printed circuit board. Figure 4 shows a top view of a printed circuit board being moved beneath a stationary beam while a flow of reactive gas is provided across the illuminated surface.
Figure 5 is a top view of an optical arrangement for scanning both sides of a printed circuit board at the same time, using the beam from a single laser.
Figure 6 is a cross section of a hole drilled through a multilayer P.C. board, illustrating the removal of "smear". In general, in the process of the invention, a
P.C. board is passed under a high intensity beam of ultraviolet radiation in the presence of a reactive gas.
The radiation and gas interact with the board, removing a thin surface layer in which the unwanted chemicals are absorbed. The gas flow aids in the removal of the volatilized material, which is exhausted through a carbon filter. The contaminated surface layer is ablated or otherwise activated by the radiation to cause it to react with the flowing gas, producing gaseous reaction products which are carried away by the gas flow.
Thus, as shown in Figure 1, a contaminated surface layer 4 on an insulating board core 2 is processed to form a reaction product 12, by the combination of providing a directed flow of gas 10, including a reactant, in the vicinity of the contaminated layer, and delivering a beam of radiation 6 to aid the reactant to react with the contaminated surface layer to form the reaction product. The contaminated surface layer, which may be in the range of 1 to 5 microns in depth, is completely removed. In the case where the surface to be cleaned is that of the insulating core, the process does not damage the board by etching deeply enough to expose the glass fibers 14.
In one example, an 80 mil thick epoxy board core with 2 mil etched coper conductors was cleaned using a 248 nm wavelength beam from a KrF excimer laser. The beam was pulsed at a 100 Hz rate, and was focused to a stripe approximately 100 microns thick, with energy per unit length 6 mJ/cm. Oxygen was flowed across the surface at a velocity of approximately 100 cm/sec. The surface was scanned using a single pass at a velocity of 6mm/sec. The cleaned surface measured 4.0 micrograms NaCl/in2 on the "Omega Meter" test, well within the specification limit of 6.5. In another example, samples of 125 mil thick board coated with solder mask were cleaned in the same manner as described above. A sample cleaned with one pass at 6mm/sec gave an "Omega Meter" reading of 4.2 micrograms NaCl/in2, while another cleaned with three passes at 6mm/sec gave a reading of 2.4. The beam may be deep ultraviolet radiation in the wavelength range from 155 nm to 375 nm. The source of the radiation may be an excimer laser, for example a KrF excimer producing radiation at 248 nm wavelength, or an ArF excimer producing radiation at 193 nm wavelength. Alternatively, the source may be a solid state laser such as a frequency-quadrupled Nd.YAG laser producing radiation at 266 nm wavelength, a tripled Nd.YAG at 355 nm, or a frequency-tripled Alexandrite laser producing radiation with a tunable wavelength in the range 240 nm to 266 nm. Other high intensity ultraviolet light sources may be effective in the cleaning process, such as a pulsed xenon lamp, or a high pressure mercury vapor lamp. Other wavelengths of light may be present in addition to the deep ultraviolet light. For example, the beam from a frequency-quadrupled Nd:YAG laser may combine light of the fundamental wavelength 1064 nm and the second harmonic 532 nm along with the fourth harmonic at 266 nm wavelength. The reactant may be a gas flowing at a velocity preferably between 20 mm/sec and 500 mm/sec. The gas may include one or more members of the group of oxidants consisting of oxygen, fluorine and chlorine. When the contaminated layer includes organic material, the reactant may include an oxidant and the beam may be ultraviolet radiation.
As shown in Figures 2A and 2B, the beam may be delivered by receiving a source laser beam 16 and focusing the cross-sectional size of the beam in one dimension using a converging cylindrical lens 22 and broadening the cross-sectional size in the other dimension using a diverging cylindrical lens 18 to form a narrow rectangular beam 26 at the surface to be cleaned. The size of the beam in the direction which is broadened may be at least as great as the width of the board to be cleaned. The size of the beam in the other dimension may be chosen to provide an energy density at the surface to be cleaned sufficient to effect the contamination removal process but insufficient to damage the surface. As shown in Figure 4, the entire surface may be scanned by placing the board 38 on a linear translation stage or conveyor (not shown) . The stage or conveyor moves the board under the beam 40 while a flow of gas 44 containing a reactant is provided by a nozzle 42. The entire surface of the board may be scanned by the beam in several passes, for complete cleaning in cases where the contaminated layer is not completely removed by a single pass. Multiple passes of the beam across the surface may be accomplished by several physical scans, or by splitting the beam, using a diffraction grating, beamsplitter or other means, into several focused beams at the surface. An exhaust duct (not shown) draws off the flowing gas from the area illuminated by the beam, and conveys the gas to a filter (not shown) which removes particles, combustion products and traces of contaminants from the gas stream before releasing or recirculating the gas.
Both sides of a printed circuit board may be cleaned. Cleaning both sides may be accomplished by sequentially scanning both sides in two scans.
Alternatively, both sides may be scanned at the same time, as shown in Figure 5. The board 38 is moved on a linear translation stage or conveyor (not shown) which holds the board by the edges in order to expose both faces for processing. The output beam 48 from a laser 46 is split into two beams by a beamsplitter 50. Beam steering mirrors 52 direct the two beams to the region where the beams are formed into narrow blades without being blocked by the motion of the board. The blades are formed by diverging cylindrical lenses 18 and converging cylindrical lenses 22, and directed to the two faces of the board by mirrors 54.
Figure 6 illustrates the use of photoreactive cleaning to remove "smear" in holes drilled in a P.C. board. The figure shows the cross section of a hole drilled through a multilayer board, which has patterned conductor layers 30 on the outer surfaces, and an inner layer conductor 58. The drilling process has resulted in a smear 60 of resin material over the edges of the inner layer conductor 58. A beam of radiation 62 is directed to the board so that it is transmitted through the hole. A small pressure differential between the two sides of the board will cause a flow 64 of gas through the hole. The combination of high intensity radiation and flowing gas will cause the smeared resin material to react, forming gaseous products 66 which are removed with the gas flow, and leaving the edges of the inner layer conductors exposed for contact by subsequent plating. Other embodiments are within the scope of the following claims.
What is claimed is:

Claims

Claims
1. A system for cleaning printed circuit boards in the course of fabrication comprising providing a flowing gas, including a reactant, and delivering pulsed radiation to the surface of the printed circuit board in the presence of the flowing gas, such that the absorbed contamination is reacted and removed.
2. The system in claim 1 where the radiation is in the wavelength range 150 nm to 405 nm.
3. The system in claim 2 where the radiation source is an excimer laser.
4. The system in claim 2 where the radiation source is a solid state laser, such as a frequency- multiplied Nd.YAG laser.
5. The system in claim 2 where the radiation source is an ultraviolet lamp.
6. The system in claim 1 where the radiation is optically shaped into a blade, and where relative motion is induced between the optics and the surface to be cleaned so that the blade scans across the entire surface.
7. The system in claim 2 where the intensity of a pulse of radiation at the surface to be cleaned is between 40 mJ/cm.2 and 1000 mJ/cm2.
8. The system in claim 1 where the surface to be cleaned or the flowing gas is heated.
9. The system in claim 1 where the cleaning takes place in an enclosed chamber, in which pressure or vacuum is applied for the purpose of accelerating or improving the efficiency of the reaction.
10. The system in claim 1 where the surface to be cleaned is that of the insulating core of a printed circuit board. - lO - ll. The system in claim 9 where the material to be removed is a thin surface layer containing the contaminant, and the flowing gas includes oxygen or ozone. 12. The system in claim 1 where the surface to be cleaned is that of the solder mask layer.
13. The system in claim 12 where the material to be removed is a thin surface layer containing the contaminant, and the flowing gas includes oxygen or ozone.
14. The system in claim 1 where the material to be removed is a smear of resin on the walls of holes drilled through a printed circuit board.
15. The system of claim 14 wherein the gas is passed through the holes.
PCT/US1995/010928 1994-08-29 1995-08-29 Cleaning of printed circuit boards WO1996006692A1 (en)

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
SG99840A1 (en) * 1996-04-09 2003-11-27 Sony Corp Two-way information transmission system, two-way information transmission method, and subscriber terminal device
EP1420620A2 (en) * 2002-10-28 2004-05-19 Shipley Co. L.L.C. Desmear and texturing method
WO2006031270A1 (en) * 2004-06-17 2006-03-23 Uvtech Systems, Inc. System and methods for surface cleaning

Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3890176A (en) * 1972-08-18 1975-06-17 Gen Electric Method for removing photoresist from substrate
US4718974A (en) * 1987-01-09 1988-01-12 Ultraphase Equipment, Inc. Photoresist stripping apparatus using microwave pumped ultraviolet lamp
US5024968A (en) * 1988-07-08 1991-06-18 Engelsberg Audrey C Removal of surface contaminants by irradiation from a high-energy source
US5114834A (en) * 1987-10-23 1992-05-19 Yehuda Nachshon Photoresist removal
US5151134A (en) * 1989-01-17 1992-09-29 Agence Regionale De Developpements Technologiques Method and a device for cleaning a surface with a laser
US5151135A (en) * 1989-09-15 1992-09-29 Amoco Corporation Method for cleaning surfaces using UV lasers
US5236512A (en) * 1991-08-14 1993-08-17 Thiokol Corporation Method and apparatus for cleaning surfaces with plasma

Patent Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3890176A (en) * 1972-08-18 1975-06-17 Gen Electric Method for removing photoresist from substrate
US4718974A (en) * 1987-01-09 1988-01-12 Ultraphase Equipment, Inc. Photoresist stripping apparatus using microwave pumped ultraviolet lamp
US5114834A (en) * 1987-10-23 1992-05-19 Yehuda Nachshon Photoresist removal
US5024968A (en) * 1988-07-08 1991-06-18 Engelsberg Audrey C Removal of surface contaminants by irradiation from a high-energy source
US5151134A (en) * 1989-01-17 1992-09-29 Agence Regionale De Developpements Technologiques Method and a device for cleaning a surface with a laser
US5151135A (en) * 1989-09-15 1992-09-29 Amoco Corporation Method for cleaning surfaces using UV lasers
US5236512A (en) * 1991-08-14 1993-08-17 Thiokol Corporation Method and apparatus for cleaning surfaces with plasma

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
U.S. ARMY ELECTRONIC TECHNOLOGY AND DEVICES LABORATORY, January 1992, JOHN R. VIG, "Ultraviolet - Ozone Cleaning of Semiconductor Surfaces", pages 1, 9, 12-15, 19-23, 27-28. *

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
SG99840A1 (en) * 1996-04-09 2003-11-27 Sony Corp Two-way information transmission system, two-way information transmission method, and subscriber terminal device
EP1420620A2 (en) * 2002-10-28 2004-05-19 Shipley Co. L.L.C. Desmear and texturing method
EP1420620A3 (en) * 2002-10-28 2005-11-23 Shipley Co. L.L.C. Desmear and texturing method
WO2006031270A1 (en) * 2004-06-17 2006-03-23 Uvtech Systems, Inc. System and methods for surface cleaning

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