U.S. patent number RE29,784 [Application Number 05/483,972] was granted by the patent office on 1978-09-26 for thermal dissipating metal core printed circuit board.
This patent grant is currently assigned to International Electronics Research Corp.. Invention is credited to Ruben T. Apodaca, Donald H. Chadwick.
United States Patent |
RE29,784 |
Chadwick , et al. |
September 26, 1978 |
Thermal dissipating metal core printed circuit board
Abstract
A metal core printed circuit board which includes multiple
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 comprising a built up circuit pattern, and wherein areas
intermediate the circuit pattern comprise an exposed surface of the
resin material.
Inventors: |
Chadwick; Donald H.
(Northbridge, CA), Apodaca; Ruben T. (Inglewood, CA) |
Assignee: |
International Electronics Research
Corp. (Burbank, CA)
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Family
ID: |
27047803 |
Appl.
No.: |
05/483,972 |
Filed: |
June 28, 1974 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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370792 |
Jun 18, 1973 |
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131102 |
Apr 5, 1971 |
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Reissue of: |
772672 |
Nov 1, 1968 |
03514538 |
May 26, 1970 |
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Current U.S.
Class: |
174/252; 174/266;
588/404 |
Current CPC
Class: |
H05K
1/056 (20130101); H05K 1/09 (20130101); H05K
3/381 (20130101); C23C 18/1653 (20130101); H05K
3/062 (20130101); H05K 3/181 (20130101); H05K
3/426 (20130101); H05K 3/445 (20130101); H05K
2201/0209 (20130101); H05K 2201/0338 (20130101); H05K
2201/0344 (20130101); H05K 2203/025 (20130101); H05K
2203/0723 (20130101); H05K 2203/1105 (20130101) |
Current International
Class: |
C23C
18/16 (20060101); H05K 3/38 (20060101); H05K
1/05 (20060101); H05K 1/09 (20060101); H05K
3/06 (20060101); H05K 3/42 (20060101); H05K
3/44 (20060101); H05K 3/18 (20060101); H05K
001/00 () |
Field of
Search: |
;174/68.5,52PE,153R
;317/11B,11C,11CC,100 ;204/15 ;156/3,150 ;29/625,626,627 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Grimley; Arthur T.
Parent Case Text
This application is a continuation of application Ser. No. 370,792,
filed June 18, 1973 now abandoned, which was a continuation of
reissue Ser. No. 131,102, filed Apr. 5, 1971 now abandoned.
Claims
Having described the invention, what is claimed as new in support
of Letters Patent is:
1. A metal core printed circuit board comprising
a sheet of metal having a thickness slightly less than the
thickness of a standard complete printed circuit board,
a base film of synthetic plastic resin forming a coating extending
over at least one surface of said sheet, said coating having a
roughened surface texture comprising a multiplicity of keying
depressions,
a circuit pattern on said coating comprising conducting metallic
lines,
said metallic lines comprising a base layer of nickel in keyed
bonded engagement with said coating and the depresions therein, a
film of copper in electroplated engagement with said nickel, an
outer layer of nickel in electroplated engagement with said film of
copper, a layer of pyrophosphate copper in bonded engagement with
said last identified layer of nickel, and an overlying layer of
metal unlike said copper and nickel in adhesive engagement with
said pyrophosphate copper, there being spaces between said
conducting metallic lines wherein said coating is exposed and said
metal core is covered by said coating.
2. A metal core printed circuit board as in claim 1 wherein said
base layer of nickel comprises a nickel deposit having initially
applied portions in said keyed bonded relationship with the coating
and other subsequently applied portions extending between said
initially applied portions.
3. A metal core printed circuit board as in claim 1 wherein holes
extend through the metal core and said coating extends throughout
walls of said holes.
4. A metal core printed circuit board as in claim 3 wherein said
holes communicate with said conducting metallic lines and portions
of the material comprising said conducting metallic lines extend
into the holes and are secured to the walls of said holes.
5. A metal core printed circuit board as in claim 1 wherein said
coating and said circuit pattern is on both sides of said
sheet.
6. A metal core printed circuit board as in claim 5 wherein holes
through the sheet interconnect with the circuit pattern on both
sides of the sheet and portions of the material comprising said
conducting metallic lines extend through the holes and interconnect
said lines.
7. A metal core printed circuit board as in claim 1 wherein said
base film comprises a plurality of layers of successively applied
films of heat cured polyurethane resin .[.separated by a primer.].
.Iadd.and wherein only the layer of resin nearest the metal is
separated from the metal by a primer. .Iaddend.
8. A metal core printed circuit board as in claim 7 wherein there
are not less than six layers of said resin.
Description
Due to the fact that printed circuits are necessarily electrically
conducting metallic lines applied to some appropriate surface, the
surface upon which such lines are placed must be electrically
nonconducting.
Heretofore the practice almost universally pervalent has been to
make use of a board or sheet which itself is of nonconducting
material, the surface of that material being 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 away to leave only the circuit
pattern.
Although circuit boards possessed of a core comprising a sheet of
naturally electrically nonconducting material 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
cooling 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 developments have
materialized in the issue of certain patents 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 coating 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 assuring an adequate bond and hard to prepare in such
fashion that the electric circuit 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 deterring 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
shortcircuiting 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 insulated 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 electrically insulating but thermally
conducting coating of such character that a circuit 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, providing adequate electrically
insulating properties, an which also is thin enough to pass heat,
generated by components in the circuit, readily through the resin
to the metal core to be carried away by conduction as the primary
mode of heat transfer, notwithstanding the benefits of radiation
and convection modes.
Still another object of the invention is to provide a new and
improved metal core printed circuit board wherein 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 combination of the various parts of
the device, whereby the objects 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 subsequent
to 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 showing 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 illustrting the step following that shown in FIG. 9.
FIG. 11 is a perspective view partially in section similar 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 successive steps for producing
the finished circuit pattern which is illustrated in FIG. 13.
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 similar to FIG.
10 but wherein a different method is employed for applying the
circuit pattern.
FIGS. 16 and 17 are perspective views partially in section similar
to FIG. 15 but showing respective successive 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.
In an embodiment of the invention chosen for the purpose of
illustration, there will be described a metal core printed circuit
board which has an electrically conducting 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
sufficient.
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 applied. 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 of
lines on one or both sides which will ultimately determine the
finished thickness. Quite commonly, a finished printed circuit
board is one which is 1/32 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 electrically conductive circuit is
substantially the same.
In the chosen embodiment, where the finished board is to be 1/32
inch thick, the initial metal sheet should be approximately 0.025
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 practice or may be thinner. Board thicknesses of
less than 0.025 inch can be processed. The limiting factor is hole
size to board thickness ratio. Processing has been limited to a
finished hole of 0.020 inch in a 0.25 inch thick substrate. The
nature of the electrically nonconducting coating application is
such that hole diameters greater than 0.020 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 provide 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 precise location of the holes is coded so
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 undertaken. This means deburring the holes 11 previously
referred to and also preparing any other slots, cuts or sundry
configurations, like for example 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
in all 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 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
various chromate conversion films such as Iridite are suitable.
Other metals such as copper, copper alloys, titanium, steel,
magnesium, lithium-magnesium alloys or other base metals or alloys
would require other or similar surface preparations to provide a
receptive surface to promote coating adhesion to the metal
substrate.
The sheet is now ready for application of an electrically
nonconducting coating 15 which, in the present instance, 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 example, 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 surfaces of
the sheet and over the primer are applied multiple successive,
relatively thin coats of a synthetic plastic resin material
containing an appropriate hardener, the consistency of which is
thin enough so that each successive coat will be a very thin coat.
While the actual number of successive coats of the synthetic
plastic resin material 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 conductive
properties which will be needed in the finished printed circuit
board of the quality sought. It will be understood that the same
multiple coats of synthetic plastic 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 advantageous is polyurethane resin and a primer of
desirable characteristics is a catalized primer such as described
in MIL-P-15328B or MIL-P-24504A.
After the multiple layers of resin have been built up, the
composite sheet, coated as described, is stabilized. Stabilization
in the present instance contemplates heat curing at temperatures of
from 150.degree. to 220.degree. 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 recommended produces a coating layer, the
ultimate thickness of which is about 50% to 60% of the thickness
when initially applied.
Since the synthetic plastic resin is depended upon to electrically
insulate the metal core or sheet of metal material 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 concentration 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 alkaline cleaner to remove any
possible oils or greases which may have accumulated on the surface,
followed by a clear water rinse. The next step is to chemically
etch the mechanically etched surface. An acceptable chemical 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 characters 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 material 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 applied 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 subjecting the coated
board to a bath of "noble" metal salts, 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
mechanical etch step followed by the chemical etch step. The effect
of sensitizing as described is to cause tiny seeds 20 or pure metal
to accumulate in the pockets created initially by the mechanical
etch and subsequently enlarged.
A satisfactory "noble" metal is palladium in the form of palladium
chloride. This is a solution having a pH of from 0.01 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 quantity 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 pockets. An acceptable material for this
interconnection has been found to be nickel in the form of a nickel
salt solution using a boron reduction system. Other solutions are
also acceptable, as for example, those described in Pats.
2,532,283, .Badd..[.2,767,723.]..Baddend. .Iadd.2,762,723
.Iaddend.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 different 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.
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
resiults in providing a film of copper over the entire surface, the
surface of the copper is cleaned. In production it has been found
that, if semi-finished raw materials 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 sulfamate bath. Nickel plating
over the copper strike serves the purpose of forming a barrier film
to prevent dissolution of the electroless nickel deposit by the
copper electroplating 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 extent
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-copper strike by
electroplating in a pyrophosphate copper solution long enough to
build up the required thickness. The thicker built up pyrophosphate
copper layer is identified 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, followed 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, means a 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.
In the first described method sequence, the photo-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 means a pattern of lines 32, 33 etc. which will ultimately be
the conducting lines of an electric circuit. In this step the
electric circuit is a positive image. Where the ultraviolet light
has hit the area of the photo-resist, the photo-resist will be
hardened and resistant to plating solutions, clean-up solvents 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 exposure to 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 imperfections.
After such inspection by the operator, excess developer is washed
off as by a spray rinse, the surface then hving the water dried
from it, and subsequently cured in an oven at a temperature of, for
example, 100.degree. C., for up to 1/2 hour in time. The step last
described produces 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
0.0005 to 0.0003 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 copper circuit 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 conventional 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 centain
that no resist remains. Removing the resist lays bare the surface
of pyrophosphate copper 29 over all portions except those where the
overlying unlike metal, such as tin-lead, has been applied.
Throughout all of the preceding 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 solution, an ammonium persulfate
solution, or a chromic-sulphonic acid solution. The selection 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. Although ferric 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 away all 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 away
clears all of the spaces between the lines 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/or salts
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 means of 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 copper build-up followed by the customary
cleaning by physically scrubbing the board with a mild abrasive and
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 conventional silk screen process in such a
manner that the circuit pattern is left bare with the exposed
surface of pyrophosphate copper build-up defining the circuit
pattern whereas the resist, applied by means of the silk screen
process fills the spaces intermediate the lines of the circuit
pattern. A cross-sectional view of the build-up of layers 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 washing off 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 subsequent etching away of the metal layers
initially covered by the resist, down to but not through the
coating of resin.
THIN COPPER PROCESS
In still another form of the invention which is somewhat 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 pyrocopper film or layer applied but the
strike is not followed up at this point by a build-up in thickness
of pyrophosphate 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 ammonium persulfate, or
in other words, cleaning and deoxidizing. 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
circuit pattern on the resist. In the alternative at this point,
the positive circuit pattern may be 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
touched up as previously described.
In this third form of the invention, the material 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 copper build-up is confined to the circuit pattern
and not to the entire surface 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 for illustration.
The resist is then removed by employment of a substantially
conventional resist stripper thereby to bare the surface of the
thin layer 28 of pyrophosphate copper strike which heretofore has
been located beneath the resist. The surface is then cleaned by
spray rinse, for example, to be sure that all resist is completely
removed and the cleaning followed by etching. Although the etching
step for this form of the process is similar to that initially
described, wherein ferric chloride or ammonium persulfate or
chromic sulphuric acid is suggested, depending upon the metal used
for the overplate, the etching requirement is less strenuous in
that only a very thin layer 28 of pyrophosphate copper need be
removed by etching instead of a built up thickness like the layer
29. Thereafter, as etching progresses, the copper strike 26 first
applied is removed and the layer 25 of electroless nickel baring as
previously the surface of the resin coating 15 which is left
intact.
From the foregoing description it will be appreciated that in the
last decribed form of the invention several saving features are
taken advantage of. The pyrophosphate copper is built up only in
the circuit pattern, thereby saving appreciably in the application
of the copper, and in the etching step only a very thin film of
pyrophosphate copper needs to be etched away. Despite these
savings, the circuit pattern itself and all lines of it are built
up to the same desirable degree and structure as in the initially
described form of the process.
While the invention has herein been shown and described in what is
conceived to be a practical and effective embodiment, it is
recognized that departures may be made therefrom within the scope
of the invention.
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