3,514,538
`D. H. CHADWICK ET AL
`May 26, 1970
`-THERMAL DISSIPATING METAL CORE PRINTED CIRCUIT BOARD
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`3,514,538
`DD. HW. CHADWICK ETAL
`May26, 1970
`THERMAL DISSIPATING METAL CORE PRINTED CIRCUIT BOARD
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`D. H. CHADWICK ETAL
`May 26, 1970
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`3,514,538
`D. H. CHADWICK ET AL
`May 26, 1970
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`THERMAL DISSIPATING METAL CORE PRINTED CIRCUIT BOARD
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`Filed Nov. 1, 1968
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`

`United States Patent Office
`3,514,538
`Patented May 26, 1970
`
`1
`
`2
`
`3,514,538
`THERMAL DISSIPATING METAL CORE PRINTED
`CIRCUIT BOARD
`Donald H. Chadwick, Northridge, and Ruben T. Apodaca,
`Inglewood, Calif., assignors to International Electronics
`Research Corporation, Burbank, Calif., a corporation
`of California
`Filed Nov. 1, 1968, Ser. No. 772,672
`Int, Cl. HO5k 1/02
`
`US. Ch. 174—68.5
`
`
`8 Claims
`
`ABSTRACT OF THE DISCLOSURE
`
`A metal core printed circuit board which includes mul-
`tiple layers of synthetic plastic resin material on a sheet
`of metal, and wherein the surface of the plastic material
`is of such character that it provides an acceptable bond
`on which are built up sundry layers of different metals,
`the innermost layer on the plastic surface and the other
`layers positioned one upon another, ultimately compris-
`ing a built up circuit pattern, and wherein areas inter-
`mediate the circuit pattern comprise an exposed surface
`of the resin material.
`
`
`Due to the fact that printed circuits are necessarily
`electrically conducting metallic lines applied to some ap-
`propriate surface, the surface upon which such lines are
`placed must be electrically nonconducting.
`Heretofore the practice almost universally prevalent has
`been to make use of a board or sheet which itself is of
`nonconducting material, the surface of that material be-
`ing one on which metal lines have been printed and built
`up to a sufficient thickness throughout the circuit pattern
`to provide a mechanically stable circuit, and wherein those
`portions intermediate the circuit pattern have been etched
`awayto leave only the circuit pattern.
`Although circuit boards possessed of a core compris-
`ing a sheet of naturally electrically nonconducting mate-
`tial have been widely used and have been highly effective,
`they lack the desirable property of being capable of
`quickly and effectively dissipating heat which is generated
`by components in the circuit when the apparatus in which
`they are used is operated. This situation has progressively
`become more critical as circuits and components have
`become smaller, especially those of micro-miniature size,
`in that compaction of the components and circuits into
`increasingly smaller spaces diminishes the amount of
`space available around them for the circulation of cool-
`ing air whereby to keep the temperature of the electrical
`apparatus when operating at a desirable minimum.
`In recent years some developers have undertaken to
`make use of metal cores for circuit boards. Typical de-
`velopments have materialized in the issue of certain pat-
`ents among which are: Eisler, 2,706,697; Gellert, 3,165,-
`672; Dinella, 3,296,099.
`Although the developments mentioned have undertaken
`to make use of some form of dielectric material for coat-
`ing the surfaces of the metallic sheet or core, dielectric
`materials which heretofore have been made use of have
`‘been hard to handle, difficult to apply in a manner assur-
`ing an adequate bond and hard to prepare in such fashion
`that the electric circnit pattern, once applied to them,
`will be durable as well as precisely dependable, to the
`degree required by complex electronic circuitry. The high
`expense of adequately treating a metallic board to accept
`a satisfactory circuit pattern has been an additional de-
`terring factor. Other difficulties have been experienced
`when the metallic sheet has been drilled and fabricated,
`as for example, insulating the walls of holes drilled
`through the metallic sheet sufficient
`to avoid short-
`
`circuiting of electric leads from electric components
`passed through the board.
`A still further obstacle to the design of a metal core
`printed circuit ‘board has been the difficulty of having
`components in close enough contact with the circuit board
`so that heat generated in the components can pass readily
`to the metal core, serving in such instances as a heat sink,
`and at the same time have the components adequately in-
`sulated electrically from the electrically conducting metal
`core.
`It is therefore an object of the invention to provide a
`new and improved metal core printed circuit board which
`is provided with an especially adequate layer of electri-
`cally insulating but thermally conducting coating of such
`character that a cicuit pattern is applied to the coating
`in a dependable. fashion whereby to result in a finished
`circuit board of precision character and capable of long
`life.
`Still another object of the invention is to provide a
`new and improved metal core printed circuit board to
`the metal surface of which are applied multiple films of
`a synthetic plastic resin material wherein the resin is such
`that it will be tough and durable where left exposed, pro-
`viding adequate electrically insulating properties, and
`which also is thin enough to pass heat, generated by com-
`ponents in the circuit, readily through the resin to the
`metal core to be carried away by conduction as the pri-
`mary mode of heat transfer, notwithstanding the bene-
`fits of radiation and convection modes.
`Still another object of the invention is to provide a
`new and improved metal core printed circuit board where-
`in the resin surface is in a special condition providing a
`keying bond between an initial metallic layer and the resin
`surface so that a hard, fast, durable and permanent bond
`will be achieved.
`With these and other objects in view, the invention
`consists in the construction, arrangement, and combina-
`tion of the various parts of the device, whereby the ob-
`jects contemplated are attained, as hereinafter set forth
`and illustrated in the accompanying drawings.
`In the drawings:
`FIG. 1 is a fragmentary perspective view of a metal
`core subsequentto drilling and machining.
`FIG. 2 is a fragmentary perspective view partially
`broken away showing the metal core after application
`thereto of an insulating coating, on line 2—2 of FIG. 1.
`FIG. 3 is a fragmentary perspective view on the line
`3—3 of FIG. 2, after the step of mechanical etching.
`FIG. 4 is a fragmentary perspective view on the line
`4—4 of FIG. 3 showing the condition of the insulating
`coating after the chemical etch.
`.
`FIG. 5 is a fragmentary cross-sectional view of the
`coating in a condition of the step following FIG.4.
`FIG. 6 is a fragmentary view of the insulating coating
`after a nucleating step.
`FIG. 7 is a cross-sectional view showing the material
`in the same condition as in FIG. 6.
`FIG, 8 is a fragmentary cross-sectional view showing
`the insulating coating after application of the first nickel
`layer is complete.
`FIG. 9 is a perspective view partially in section show-
`ing the condition of the board after initial build-up of
`all of the layers of material.
`FIG. 10 is a perspective view partially in section
`similar to FIG, 9 illustrating the step following that shown
`in FIG.9.
`FIG. 11 is a perspective view partially in section simi-
`lar to FIG. 10 wherein the build-up of the line of the.
`circuit pattern has-been completed.
`FIGS. 12 and 13 show fragmentary perspective views
`partially broken away similar to FIG. 11 illustrating suc-
`
`10
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`60
`
`65
`
`6
`
`

`

`~ 8,514,588
`
`
`cessive steps for producingthe finished circuit pattern
`whichis illustrated in FIG. 13.0”
`—
`FIG. 14 is a cross-sectional view on the line 14—14
`of FIG. 13 showing the build-up of materials in one of
`the holes" .
`_ FIG. 15 is a perspective view partially in section simi-
`lar to FIG, 10 but wherein a different method is em-
`ployed for applying the circuit pattern.
`FIGS. 16 and 17-are perspective views partially in
`section similar to FIG. 15 but showingrespective suc-
`cessive steps in the production of the circuit pattern and
`removal. of materials.therebetween..
`,
`FIG. 18 is a fragmentary perspective view of a finished
`circuit board. a
`.
`the
`_
`In’ an embodiment of the invention chosen for.
`purpose ofillustration, there will be described a metal
`core printed circuit board which has an electrically con-
`ducting printed circuit pattern on both sides. of the board,
`the circuit pattern being interconnected by means of
`conducting metal extending through holes in the board.
`It will be understood, however, that the technique which
`produces the product is readily applicable. to a single
`surface where a single circuit pattern on one side is suf-
`ficient.
`Customarily, the thickness of a printed circuit board
`is assumed to be the over-all finished thickness of the
`composite board, after the circuit pateern has been ap-
`plied. For that reason the sheet of material, which in this
`instance .is a metal sheet, is made slightly smaller than
`the expected finished thickness to allow a build-up oflines
`on one or both sides which will ultimately determine
`the finished thickness. Quite commonly,a finished printed
`circuit board is one which is 142 of an inch thick. Other
`thicknesses are prevalent, however, but irrespective of
`the relative thickness of the finished board,
`the process
`herein described of preparing it and applying to it an elec-
`trically conductive circuit is substantially the same.
`In the chosen embodiment, where the finished board is
`to. be 142 inch thick,
`the initial metal sheet should be
`approximately .025 inch thick to allow for the build-up
`of the sundry layers of material. Other sheets may be
`double, triple or even four times as thick in actual prac-
`tice or may be thinner. Board thicknesses of less than
`025 inch can be processed. The limiting factor is hole
`size to board thickness ratio. Processing has been limited
`‘to a finished hole of .020 inch in a 0.25 inch thick sub-
`strate. The nature of the electrically nonconducting coat-
`ing application is such that hole diameters greater than
`020 inch would allow thinner substrates to be used.
`The metal sheet is preferably of aluminum because of
`its toughness,
`its thermal conducting ability, and other
`physical attributes which make it readily workable. Other
`kinds of metal however will also serve. A metal sheet 10
`is initially trimmed to size and then drilled so as to pro-
`vide. the holes 11 which will be needed to interconnect
`circuit patterns on opposite sides of the sheet and also to
`permit the wire leads from electric components mounted
`on one side of the board to be extended through the
`board and electrically connected to a circuit pattern on
`the opposite side. In the sheet 10 only some of the holes
`11 are. shown and it should be understood that the pre-
`cise location of the holes is coded sc that when the printed
`circuit pattern is ultimately applied, it will encompass the
`holes in their initially drilled position.
`It is also desirable to fabricate the sheet before any
`succeeding step is uridertaken. This means deburring the
`holes 11 previously referred to and also preparing any
`other slots, cuts or sundry configurations,
`like for ex-
`ample the slot 12, the cutout portion 13, and the cutoff
`corner 14. These cutout portions are referred to merely
`by way of example, since each different circuit board will
`inall expectation be individually tailored to fit the cabinet
`in, which it will be ultimately used.
`;
`Following fabrication, the sheet is etched in a caustic
`solution and then anodized. Anodizing amounts to a
`
`10
`
`.
`
`25
`
`30
`
`35
`
`40
`
`20
`
`4
`chemical surface treatment, the object being to make use
`of a treatment which will chemically clean the surface
`upon which subsequent applications of materials are to
`be made. Anodizing is a suitable surface preparation for
`aluminum, chemical conversion coatings such as the vari-
`ous chromate conversion films such as Iridite are suit-
`able. Other metals such as.copper, copperalloys, titanium,
`steel, magnesium, lithium-magnesium alloys or other base
`metals or alloys would require other or similar surface
`prepartions to provide a receptive surface to promote
`coating adhesion to the metal substrate.
`The sheet is now ready for application of an electri-
`cally nonconducting coating 15 which, in the present in-
`stance, is a coating of such character as to be capable
`of offering relatively a minimum amount of resistance
`to the transfer of heat to the sheet: In the chosen exam-
`ple, both sides of the-sheet 10 are coated whereby to
`provide for the application of a circuit pattern to both
`sides. Initially, a primer is applied to both sides or sur-
`faces of the sheet and over the primer are applied multi-
`ple successive, relatively thin coats of a synthetic plas-
`tic resin material containing an appropriate hardener, the
`consistency of which is thin enough so that each succes-
`sive coat will be a very thin coat. While the actual num-
`ber of successive coats of the synthetic plastic resin mate-
`rial is not critical, it has been found in practice that there
`should not be less than three coats and that as many as
`ten coats may be found desirable to achieve the needed
`physical, electrically nonconductive and thermally con-
`ductive properties which will be needed in the finished
`printed circuit board of the quality sought. It will be un-
`derstood that the same multiple coats of synthetic plas-
`tic resin material will also be applied to the walls of the
`holes 11 which have been drilled through the board. A
`synthetic plastic resin material which is especially ad-
`vantageous is polyurethane resin and a primer of desira-
`ble characteristics is a catalized primer such as described
`in MIL-P-15328B or MIL—P-14504A.
`After the multiple layers of resin have been built up,
`the composite sheet, coated as described, is stabilized. Sta-
`bilization in the present instance contemplates heat cur-
`ing at temperatures of from ‘150° to 220° C. for a period
`of about 72 hours. Curing as described stabilizes the
`resin and also makes it appreciably dense. In practice,
`it has been found that a curing such as that herein rec-
`ommended produces a coating layer, the ultimate thick-
`ness of which is about 50% to 60% of the thickness
`wheninitially applied.
`Since the synthetic plastic resin is depended upon to
`electrically insulate the metal core or sheet of metal ma-
`terial from the metallic lines of the circuit pattern and
`also to provide a base upon which the circuit pattern is
`to be built, it will be appreciated that the coating of the
`resin material must be durable and must. also be one
`which will be compatible to a build-up of materials on
`it in such a manner that the materials when built upon
`it will be mechanically stable and not readily damaged
`or removed.
`A multiple step procedure is found advantageous to
`prepare the surface of the synthetic plastic resin for the
`process, Initially, the surface of the coating 15, which in
`the present
`instance means the surface on both sides
`of the sheet,
`is sandblasted, preferably with No. 220
`garnet particles and at a pressure of 50 to 100 pounds
`per square inch. Sandblasting mechanically creates a
`multiplicity of pockets 16, 17, 18 etc.
`throughout the
`surface,
`the- pockets being of various shapes and sizes
`depending in part upon the size of the garnet particles, in
`part upon the pressure, and in part upon the concentra-
`tion of particles when the sandblasting takes place.
`After the sandblasting has been completed, the board is
`thoroughly cleaned, as for example, by a spray rinse or
`’ mechanical scrubbing, followed by application of an alka-
`line cleaner to remove any possible oils or greases which
`7om
`may have accumulated on the surface, followed by a
`
`45
`
`50
`
`55
`
`60
`
`65
`
`-
`
`70
`
`7
`
`7
`
`

`

`5
`clear water rinse. The next step is to chemically etch
`the mechanically etched surface. An acceptable chemi-
`cal etch is a chromic type mixture in solution which is
`capable of eating into the resin material. The purpose
`of the chemical etching step is to form smaller pits in
`the bottoms of the pockets 16, 17, 18 etc. formed by the
`mechanical etching step as shown by the reference char-
`acters 16’, 17’, 18’ etc. so that they are more capable
`of retaining materials which may be deposited into them
`and so that they will provide a keying effect for a ma-
`terial buildup. In practice,
`the surface of the resin is
`normally nonwettable and the successive etching steps
`hereinabove described are for the purpose of making it
`temporarily wettable for application of subsequently ap-
`plied materials.
`An acceptable chromic type mixture solution capable
`of chemically etching the mechanically etched surface
`of resin to a desirable degree consists of the following:
`Niklad #230 Etchant,
`Following the successive etching steps, the coating is
`sensitized. This in the present disclosure comprises sub-
`jecting the coated board to a bath of “noble” metalsalts,
`namely metallic salts in which agents are present to cause
`the metal from the salts, that is to say pure metal, to
`deposit on the surface and especially to deposit in the
`pockets 16’, 17’, 18’ etc. which were created by the me-
`chanical etch step followed by the chemical etch step.
`Theeffect of sensitizing as described is to cause tiny seeds
`20 of pure metal to accumulate in the pockets created
`initially by the mechanical etch and subsequently en-
`larged.
`A satisfactory “noble” metal is palladium in the form
`of palladium chloride. This is a solution having a pH
`- of from .01 to 5 for example. Palladium is one of the
`more stable and long lasting of the noble metal salts.
`Although in fact expensive, such a relatively small quan-
`tity is needed to sinsitize a composite coated sheet of
`the kind described that the relatively high cost of metal
`is not a determining factor.
`Following the deposit of the tiny metallic seeds 20 in
`the pockets, build-up of layers or films of materials on
`the surface of the resin commences. An initial step is to
`nucleate the surface prepared in the manner heretofore
`described. This means to interconnect the metal seeds
`20 of palladium, which have been deposited in the pock-
`ets. An acceptable material for this interconnection has
`been found to be nickel in the form of a nickel salt solu-
`tion using a boron reduction system. Other solutions are
`also acceptable, as for example, those described in Pats.
`2,532,283, 2,767,723 and 2,935,425. What is accomplished
`by the foregoing step is to commence a growth 21 of
`nickel upon the seeds 20 left by the sensitizing step so
`that the nickel growing as described fills the pockets and
`expands over the outside edges of the pockets over the
`surface of the resin material.
`In practice it is a growth in patches 22 within which
`are appreciable bare spots 23. Hence to nucleate alone
`will not provide a dependable nickel surface over the
`entire resin material. Consequently,
`the nucleating step
`is immediately followed by an electroless nickel deposit.
`This means subjecting the previously nucleated surface
`to an electroless nickel -bath of a more rapid plating rate
`to build up thickness sufficient for electrical conductivity,
`namely a layer 25.
`The layer of nickel 25 is from about 10 to about 50
`millionths of an inch thick. The nickel covered board
`is then dipped in a weak acid for cleaning purposes. Such
`a weak acid being, for example, 2 to 10% sulfuric acid
`solution. Following this treatment
`the board is again
`rinsed,
`Different types of markets demand ultimately differ-
`ent types of printed circuit boards. One type of market
`can be met by providing a board the circuit pattern of
`which is formed, built up, and cleared in accordance with
`the following procedure.
`
`_
`
`3,514,538
`
`The layer of nickel 25 formed, as previously described,
`is subjected to a copper strike. This consists of building
`up a film .26 of copper upon the nickel to a depth of
`20 to 100 millionths of an inch by making use of a
`copper pyrophosphate bath or other suitable strike bath.
`Such a bath results in the deposit of only a very small
`amount of copper but does not produce a copper film
`wherein there is good adhesion. After the copper strike
`which results in providing a film of copper over the entire
`surface, the surface of the copper is cleaned. In produc-
`tion it has been found that, if semi-finished raw mate-
`rials are to be inventoried in quantity, the semi-finished
`material can best be handled by carrying the process
`through to the end of the copper strike, after which the
`boards may be stored. If there is no need for storage,
`then a cleaning step will follow the application. of the
`copper strike immediately rather than at some future
`date when the inventoried boards are to be used.
`The succeeding step is an electroplating step wherein
`a second layer 27 of nickel is electroplated to the copper
`strike, as for example, by employment of a nickel sul-
`famate bath, Nickel plating over the copper strike serves
`the purpose of forming a barrier film to prevent dissolu-
`tion of the electroless nickel deposit by the copperelectro-
`plating bath.
`From here on, if the board is to be shifted from one
`tank to another,
`the next step will be a 2% to 10%
`sulfuric acid rinse which, however, may be omitted when
`the process is to be carried on continuously in the same
`tank. The exposed surface of the second nickel layer
`27 is then subjected to a pyro-copper strike, this being
`accomplished by immersing the board, coated to the ex-
`tent that it has now become, in a pyrophosphate copper
`solution for about 30 to 90 seconds, to build up a layer
`28 of thickness of about 10 to 50 millionths of one inch of
`copper of the type referred to.
`Pyrophosphate copper is then plated on the pyro-cop-
`per strike by electroplating in a pyrophosphate copper
`solution long enough to build up the required thickness.
`The thicker built up pyrophosphate copperlayeris identi-
`fied by the reference character 29. Following the copper
`build-up the board is cleaned with pure water and by
`physically scrubbing the board with a mild abrasive, fol-
`lowed then by a spray rinse. After cleaning, the surface
`of the pyrophosphate copper is subjected to a mild etch
`of ammonium persulfate,
`The built up multiple metal layers are now ready for
`application of a resist 30 which, in terms of the trade,
`meansa light-sensitive or photo-sensitive emulsion. After
`the emulsion is coated on, it is cured, using care not to
`expose the coating to ultraviolet light.
`the photo-
`In the first described method sequence,
`resist or light-sensitive emulsion is next covered by a
`photographic negative (not shown) and the surface of
`the photo-resist exposed to ultraviolet light. This creates
`a circuit pattern 31 (FIG. 18) which meansa pattern of
`lines 32, 33 etc. which will ultimately be the conducting
`lines of an electric circuit. In this step. the electric cir-
`cuit is a positive image. Where the ultraviolet light has
`hit the area of the photo-resist, the photo-resist will be
`hardened andresistant to plating solutions, clean-up sol-
`vents and solvents in general. The lines 32, 33, however,
`which are created by the positive of the image, which will
`be the lines where the circuit is to be traced, are not
`subjected to the ultraviolet light and will remain soft.
`Following exposureto create the circuit pattern 31, the
`surface is dipped in a developing solvent. The developer
`dissolves the lines which constitute the surface pattern,
`the photo-resist in that line pattern being washed away
`and exposing the pyrophosphate copper 29 beneath it. The
`remaining coating is dyed so that the operator will have
`something which can be visually inspected for imperfec-
`tions. After such inspection by the operator, excess de-
`veloper is washed off as by a spray rinse, the surface then
`having the water dried from it, and subsequently cured
`in an oven at a temperature of, for example, 100° C.,
`
`10
`
`15
`
`20
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`70°
`
`75
`
`8
`
`

`

`8,514,538
`
`7
`for up to % hour in time. The step last described pro-
`duces a hard surface on the board which can be handled.
`It is now time for touching up pin holes which may exist
`in the conducting circuit pattern, physical imperfections,
`damage, defects in the negative, dust particles falling upon
`the pattern, and perhaps other defects. The touch-up is
`done by use of a paint brush to paint on a compatible
`material such as an asphalt or vinyl paint.
`Now that the circuit pattern consists of recessed lines
`32’ etc, which reveal bare pyrophosphate copper, they are
`in condition to have applied thereto another unlike or
`different metal. Commonly, an acceptable unlike metal
`is a tin-lead mixture which is applied in layers 35 to the
`exposed pyrophosphate copper to a thickness of .0005 to
`.003 inch. Another acceptable metal is gold, except that
`when gold is used, applied to the exposed pyrophosphate
`copper,
`the thickness will be built up only 80 to 100
`millionths of an inch.
`_ Once the exposed pyrophosphate coppercircuit pattern
`31 has been covered with the unlike metal 35, the resist
`is then removed from the spaces intermediate the lines
`of the circuit pattern. This is accomplished in a conven-
`tional manner by use of what
`is commonly called a
`“resist stripper.” After the resist has been removed as
`described, the surface is cleaned by a spray rinse to be
`certain that no resist remains. Removing the resist lays
`bare the surface of pyrophosphate copper 29 over all por-
`tions except those where the overlying unlike metal, such
`as tin-lead, has been applied. Throughoutall of the pre-
`ceding steps it should be borne in mind that the metallic
`layers are being built up on the walls of the holes which
`go through the sheet as well as on the surface or surfaces
`of the sheet, Where there are circuit pattern lines on both
`sides, the multiple layers of metal build-up will coat the
`wall of each hole 11 and form a bridge or connection
`between the lines of the surface pattern on one surface of
`the sheet and lines of the surface pattern on the other
`surface of the sheet, as shown in FIG. 14.
`With the resist having been removed from intermediate
`areas 37 of pyrophosphate copper, the composite sheet is
`then ready for etching. Etching may take place in an
`appropriate bath, as for example, a ferric chloride solu-
`tion, an ammonium persulfate solution, or a chromic-
`sulphuric acid solution. Theselection of the solution will
`depend upon what the overplating or overlying unlike
`metal is on the board. For example, if the unlike metal
`were tin-lead, then a chromic-sulphuric solution would be
`used. If gold were the unlike metal, then a ferric chloride
`solution would be used. Althoughferric chloride solution
`is cheaper, such a ferric chloride solution would not be
`used where the unlike material is tin-lead because ferric
`chloride would affect the lead and destroy the overplating.
`Etching as described takes awayall of the copper and the
`nickel layers and leaves the lines 32, 33 etc. of the circuit
`pattern 31 on the surface by themselves. The etching
`awayclears all of the spaces between thelines 32, 33 etc.
`of all metals leaving only the bare surface of the synthetic
`plastic resin coating 15.
`The composite printed circuit board is then cleaned to
`the extent of cleaning of the entire surface so that all
`acids and/orsalts have been neutralized and removed, and
`the product. is then ready for use by having appropriate
`electronic components (not shown) applied thereto, and
`leads. (not shown) extended through the holes 11 and
`soldered to the lines of the circuit pattern on the opposite
`side of the sheet.
`
`SILK SCREEN PROCESS
`In a second form of the invention the circuit pattern
`may be applied by meansof a silk screen process. In this
`form of the invention, the steps of the process already
`described are followed partially through,to and including
`the pyrophosphate copper strike and pyrophosphate cop-
`per build-up followed by the customary cleaning by
`physically scrubbing the board with a mild abrasive and
`
`8
`spray rinse followed by a mild etch using a material such
`as ammonium persulfate. At this point the process changes
`in that resist is applied by a conventionalsilk screen proc-
`ess in such a manner that the circuit pattern is left bare
`with the exposedsurfaceof ‘pyrophosphate copper build-
`up defining the circuit pattern whereas the resist, applied
`by means of the’silk screen process fills the spaces inter-
`mediate the lines of the circuit pattern. A cross-sectional
`view of the build-up oflayers at this stage will be similar
`to that of FIG. 9 ‘except for the build-up having been
`arrived at without the step of printing from a photographic
`negative and washingoff the resist from the circuit pattern.
`Thereafter the overplating or application of unlike metal
`such ‘as tin-lead or gold to the exposed pyrophosphate
`copper is carried on in the same manner as previously
`described, followed by removal of the resist and subse-
`quent etching away of the metal layers initially covered
`by theresist, down to-but not through the coating ofresin.
`
`THIN COPPER PROCESS
`
`In still another form of the invention which is some-
`what more economical of materials and. process time, the
`initially described steps of the process are repeated up
`to and through the pyrophosphate copper strike over the
`nickel plating. By this third form there is in fact a pyro-
`copper film or layer applied but the strike is not followed
`up at this point by a build-up in thickness of pyrophos-
`phate copper.
`Thereafter the board is cleaned as previously described
`by scrubbing the board with a mild abrasive, then spray
`rinsing followed by a mild etch using, for example, am-
`monium persulfate, or in other words, cleaning and de-
`oxidizing. ‘The photo-resist is then applied to the thin
`layer of pyrophosphate copper strike, the emulsion cured
`as heretofore described, and then exposed to ultraviolet
`light through a negative, thereby to create a positive cir-
`cuit pattern on the resist. In the alternative at this point,
`the positive circuit pattern maybe created by the silk
`screen process, previously described, wherein the areas
`intermediate the circuit pattern are filled with a resist
`leaving the pyrophosphate copper exposed in the circuit
`pattern. Again the process throughout all of the steps
`heretofore defined takes place inside of the holes on the
`walls of the holes, as well as on the surfaces.
`Here again the resist is dried, cured and the circuit
`pattern touchedup as previously described.
`In this third form of the invention, the exposed mate-
`rial is cleaned in a mild alkaline solution, as for example,
`to remove fingerprints and comparable blemishes, and
`activated, as for example, by means of a deoxidizing step
`with ammonium persulfate solution. In either of the
`alternatives, last made reference to, the pyrophosphate
`copper material is laid bare in a receptive condition in
`the circuit pattern so that the next step which is the build-
`up step for the pyrophosphate copper can take place only
`in the circuit pattern. In other words, the copperbuild-up
`is confined to the circuit pattern and notto the entire sur-
`face of the board,
`Following the build-up the circuit pattern is overplated
`much as previously described with another unlike metal,
`tin-lead or gold, in the example chosen forillustration.
`The resist is then removed by employment of a sub-
`stantially conventional resist stripper thereby to bare the
`surface of the thin layer 28 of pyrophosphate copperstrike
`which heretofore has bee