U.S. patent number RE29,015 [Application Number 05/637,504] was granted by the patent office on 1976-10-26 for method of generating precious metal-reducing patterns.
This patent grant is currently assigned to Western Electric Company, Inc.. Invention is credited to Michael Anthony De Angelo, Donald Jex Sharp.
United States Patent |
RE29,015 |
De Angelo , et al. |
October 26, 1976 |
**Please see images for:
( Certificate of Correction ) ** |
Method of generating precious metal-reducing patterns
Abstract
A pattern capable of reducing thereon a precious metal is
generated on a suitable substrate by first coating selected
portions of the substrate with a solution, called a photopromoter,
which contains a metal salt. The metal salt possesses two
characteristics: (a) the oxidation state of the salt (i.e., of the
metal ion) is alterable (either increasable or decreasable) by
exposure of the salt to radiation of the proper wavelength; and (b)
in either the original or the altered oxidation state (but not in
both states) the salt is capable of reducing a precious metal,
e.g., a metal of the platinum group such as palladium, platinum or
rhodium, from a salt thereof. The photopromoter-coated substrate is
next selectively exposed to the proper wavelength radiation to
produce a pattern of the photopromoter salt capable of reducing the
precious metal. The remainder of the salt is incapable of reducing
the precious metal. The pattern may then be exposed to a solution
containing a salt of the precious metal to reduce thereon the
precious metal. Ultimately, the precious metal pattern may be used
to reduce electrolessly a metal, such as copper, to produce a
metallic pattern, such as an electric circuit pattern.
Inventors: |
De Angelo; Michael Anthony
(Ewing Township, Mercer County, NJ), Sharp; Donald Jex
(Albuquerque, NM) |
Assignee: |
Western Electric Company, Inc.
(New York, NY)
|
Family
ID: |
27092859 |
Appl.
No.: |
05/637,504 |
Filed: |
December 4, 1975 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
719976 |
Apr 9, 1968 |
03562005 |
Feb 9, 1971 |
|
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Current U.S.
Class: |
205/78;
427/383.1; 430/406; 430/315; 427/97.4; 427/98.5; 427/99.5;
427/99.1; 430/319; 205/126; 427/304; 428/54; 430/401; 430/414 |
Current CPC
Class: |
G03C
5/58 (20130101); H05K 3/185 (20130101); C23C
18/1608 (20130101); C23C 18/1612 (20130101); C23C
18/208 (20130101); C23C 18/30 (20130101); Y10T
428/18 (20150115) |
Current International
Class: |
C23C
18/20 (20060101); C23C 18/16 (20060101); C23C
18/28 (20060101); G03C 5/58 (20060101); H05K
3/18 (20060101); B44D 001/50 (); H05K 003/12 ();
H05K 003/18 () |
Field of
Search: |
;427/54,98,383,304
;96/48R,49,88,92 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Kosar, Light Sensitive Systems, John Wiley & Sons, 1965,
"Inorganic Compounds," paragraph 1.3b, pp. 37-39..
|
Primary Examiner: Kendall; Ralph S.
Attorney, Agent or Firm: Rosenstock; J.
Claims
What is claimed is:
1. A method of rendering an article selectively capable of reducing
thereon a precious metal from a precious metal salt which method
comprises the steps of:
coating said article with a solution containing a salt of a metal
selected from the group consisting of tin, titanium and lead, the
oxidation state of the metal ion thereof being alterable by
exposure to short wavelength ultraviolet light; and then
producing a pattern capable of reducing said precious metal from
said precious metal salt by selectively exposing portions of said
coated article to short wavelength ultraviolet light.
2. The method of claim 1 wherein said metal ion is capable of
reducing said precious metal.
3. The method of claim 2 wherein said metal salt in said solution
is a halide.
4. The method according to claim 3 wherein said halide in said
solution is selected from the group consisting of tin chloride,
titanium chloride and lead chloride.
5. The method according to claim 4 wherein said ultraviolet light
has a wavelength of less than 3000 A.
6. The method according to claim 5 wherein said ultraviolet light
has a wavelength within the approximate range of about 1800 A. to
about 2700 A.
7. The method of claim 6 wherein exposing selectively said coated
article portions to said short wavelength ultraviolet light renders
said metal ion at said coated article portions incapable of
reducing said precious metal.
8. The method of claim 7 wherein about 75% of said short wavelength
ultraviolet light is at approximately the 2537 A. line and said
exposure is continued until said coated article portions receive
energy amounting to about 30 milliwatt-seconds/cm..sup.2 to about
1000 milliwatt-seconds/cm..sup.2.
9. The method of claim 8 wherein said precious metal salts are
selected from the group consisting of iridium, osmium, palladium,
platinum, rhodium, ruthenium, gold and silver. .[.10. A method of
producing a metallic pattern on a substrate comprising the steps
of:
coating the substrate with a first solution containing a metal salt
selected from the group consisting of ferric oxalate, ferric
citrate, ferric tartrate mercuric oxalate mercuric citrate and
mercuric tartrate, the oxidation state of the metal ion thereof
being alterable by exposure to short wavelength ultraviolet
light;
producing a pattern capable of reducing a precious metal from a
precious metal salt by selectively exposing portions of said coated
substrate to short wavelength ultraviolet light;
immersing the substrate in a precious metal salt solution to reduce
on said pattern said precious metal; and then
placing said precious metal pattern in an electroless plating bath
which is catalyzed by said reduced precious metal to produce the
metallic pattern..]. .[.11. The method according to claim 10
wherein said ultraviolet light has a wavelength of less than 3000
A..]. .[.12. The method according to claim 11 wherein said
ultraviolet light has a wavelength within the approximate range of
about 1800 A. to about 2700 A..]. .[.13. The method of claim 12
wherein exposing selectively said coated substrate to said short
wavelength ultraviolet light renders said metal ion capable of
reducing said precious metal..]. .[.14. The method of claim 13
wherein about 75% of said short wavelength ultraviolet light is at
approximately the 2537 A. line and said exposure is continued until
said coated substrate receives energy amounting to approximately 25
milliwatt-seconds/cm..sup.2..]. .[.15. The method of claim 14
wherein said precious metal salts are salts of metals selected from
the group consisting of iridium, osmium, palladium, platinum,
rhodium, ruthenium, gold and silver..]. .[.16. The method of claim
15 wherein said coating and immersing steps are effected
simultaneously by admixing said precious
metal salts and said metal salts in said first solution..]. 17. A
method of producing a precious metal pattern on a substrate, the
pattern being usable to catalyze and reduce thereon metal from an
electroless bath, which method comprises:
effecting the retention on selected areas of said substrate of a
first solution containing a salt of a metal selected from the group
consisting of tin, titanium and lead, the oxidation number of the
metal ion of said first solution being alterable by exposure
thereof to radiation deep within the ultraviolet spectrum;
selectively exposing said selected areas to said radiation to
generate a pattern of said metal ion capable of reducing said
precious metal; and then
immersing said pattern in a second solution containing a salt of
said
precious metal to produce said precious metal pattern. 18. The
method of
claim 17 wherein said salt is a metal halide. 19. The method of
claim 18 wherein said metal halide is selected from the group
consisting of tin
chloride, lead chloride and titanium chloride. 20. The method of
claim 19 wherein said retention of said first solution is primarily
by adsorption.
1. The method of claim 20 wherein said radiation is within the
range of about 1800 A. to about 2700 A., with approximately 75% of
said radiation
being at about the 2537 A. line. 22. The method of claim 21 wherein
said selective exposure continues until said substrate areas
receive energy therefrom in the approximate range of about 30
milliwatt-seconds/cm..sup.2 to about 1000
milliwatt-seconds/cm..sup.2, the amount of said energy being
approximately inversely proportional to the degree to which said
first
solution is adsorbed on said substrate. 23. A method of producing a
metallic pattern on a nonconductive substrate comprising the steps
of:
adsorbing onto selected surfaces of said substrate a metal halide
solution, the metal being selected from the group consisting of
tin, lead and titanium, the metal ion thereof being capable of
reducing a precious metal from a precious metal salt;
exposing selected portions of said selected surfaces to ultraviolet
light until said metal ion thereat is rendered incapable of
reducing said precious metal from said precious metal salt, said
selected portions conforming to a negative of said pattern and said
ultraviolet light having a wavelength in the range of about 1800 A.
to about 2700 A.;
immersing said substrate in said precious metal salt solution
whereby said ion reduces said precious metal therefrom onto said
surfaces in said pattern; and then
immersing said substrate in an electroless plating bath which is
catalyzed
by said reduced precious metal to produce said metallic pattern.
24. The method of claim 23 which further includes an initial step
of: forming
holes through said substrate outside of said selected portions. 25.
The method of claim 24 wherein said precious metal salt is a salt
of a metal selected from the group consisting of iridium, osmium,
palladium,
platinum, rhodium, ruthenium, gold and silver. 26. A method of
increasing the bond strength between a substrate and a metal
pattern produced thereon by the method of claim 23 which comprises
the steps of:
selecting a substrate having a predetermined service temperature
and which is permeable to an atmosphere conducive to the formation
of an oxide of said metal;
depositing said metal pattern on said substrate, said metal being
oxidizable in said atmosphere at a second temperature less than
said service temperature;
placing said substrate with said pattern thereon in said
atmosphere; and then
heating said pattern to said second temperature to form said oxide
at the
substrate-pattern interface until said bond strength is increased.
27. A method of increasing the bond strength between a metallic
pattern electrolessly deposited on a polyimide substrate and the
substrate by the method of claim 23, the substrate having a
predetermined service temperature, which method comprises:
placing said substrate with said metal pattern thereon in an
atmosphere conductive to the formation of an oxide of said metal,
and then
heating said metal pattern to a temperature less than said service
temperature and sufficient to oxidize said metal until sufficient
oxide formation takes place at the substrate-pattern interface to
increase said
bond strength. 28. A method of producing a metallic pattern on a
substrate surface comprising the steps of:
a. selecting a suitable substrate;
b. immersing a surface of said substrate in a metal halide
solution, said metal being selected from the group consisting of
lead, tin and titanium, said metal halide solution having a molar
concentration in the approximate range of about .045 to about .3,
said solution being retained on said substrate surface;
c. exposing selectively said surface to ultraviolet light in a
negative of said pattern, said light having approximately 75% of
its radiation at about the 2537 A. line, said radiation supplying
energy to said surface within the approximate range of from about
30 milliwatt-seconds/cm..sup.2 to about 1000
milliwatt-seconds/cm..sup.2.
d. immersing said exposed surface in a solution containing a salt
of a precious metal selected from the group consisting of iridium,
osmium, palladium, platinum, rhodium, ruthenium, gold and silver to
reduce said metal in a positive of said pattern; and then
e. immersing said surface in an electroless plating bath to
electrolessly
plate said precious metal positive to produce said metallic
pattern. 29. The method of claim 28 in which step (a) further
includes: selecting a substrate so that the retention of said
solution on said substrate surface
in step (b) is effected primarily by adsorption. 30. The method of
claim 29 wherein in step (c) the amount of said energy is inversely
proportional to the degree to which said solution is adsorbed on
said substrate
surface. 31. The method of claim 28 wherein step (b) further
comprises: maintaining said metal halide solution at a pH within
the approximate
range of from about .8 to about 3.5 during said immersion. 32. The
method of claim 31 wherein step (a) further comprises: selecting a
substrate at
least said surface of which is a polyimide. 33. The method of claim
32 wherein step (a) further comprises: cleaning said polyimide
surface in
NaOH, and then rinsing said polyimide surface in clean water. 34.
The method of claim 33 wherein step (b) further comprises, after
said immersion: rinsing said substrate surface in clean water, and
then drying
said substrate surface. 35. The method of claim 28 wherein step (a)
further comprises: rendering said substrate surface nascent so that
said solution in which said surface is immersed to adsorptively
retained
thereon. 36. A method of producing an electrical circuit pattern on
a non-conductive substrate which comprises the steps of:
selecting a metal salt selected from the group consisting of tin
halides, titanium halides, lead halides.[., ferric oxalate, ferric
citrate, ferric tartrate, mercuric oxalate, mercuric citrate and
mercuric tartrate.].;
coating said substrate with a solution containing said metal
salt;
drying said coated substrate;
selectively exposing said dried substrate to radiation deep within
the ultraviolet spectrum to generate a first pattern and a second
pattern, said radiation being within the approximate range of 1800
A. to 2700 A. with approximately 75% of said radiation being at
about the 2537 A. line, said radiation continuing until energy
within the approximate range of 25 to 1000
milliwatt-seconds/cm..sup.2 has impinged on said dried substrate,
said first pattern being capable of reducing a precious metal from
a salt solution thereof, said second pattern being incapable of
reducing a precious metal;
selecting a precious metal salt from the group of precious metals
consisting of iridium, osmium, palladium, platinum, rhodium,
ruthenium, gold and silver, said precious metal being catalytic to
an electroless plating bath;
immersing said exposed substrate in a solution containing said
precious metal salt, said first pattern reducing said precious
metal thereon to generate a precious metal pattern; and then
immersing said precious metal pattern in an electroless plating
bath to
produce said electric circuit pattern. 37. The method of claim 36
which comprises the additional step of: electroplating metal onto
said
electrical circuit pattern. 38. The method of claim 37 which
comprises the additional step of: removing said substrate from said
electroplated circuit pattern.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to a method of generating
precious-metal-reducing patterns; and particularly, to a
photographic-like method of generating, on a substrate, a pattern
capable of reducing thereon a precious metal, which reduced
precious metal is usable, for example, as a reduction catalyst in
an autocatalytic electroless plating process.
Thus, in a more specific sense, the present invention relates to
the production, by photographic-like techniques, of a starting
product usable in an electroless plating process to produce a
metallic pattern, such as an electric circuit pattern.
Discussion of the Prior Art
The frequency of use of so-called circuit boards has in recent
times increased greatly. The advantages of such boards need not be
enumerated, because they are well-known. Various methods for
producing metallic patterns on substrates to produce the circuit
boards are similarly well-known. These methods include, alone or in
various combinations, positive and negative printing processes,
positive and negative silk screening processes, positive and
negative etching techniques, electroplating and electroless
plating.
Electroless plating has found great favor with many workers in the
art and has, in fact, been known in at least rudimentary form since
before 1845 (see Symposium of Electroless Nickel Plating, published
by the American Society for Testing Materials as ASTM Special
Technical Publication No. 265 in November of 1959).
Generally speaking, electroless plating requires a so-called
sensitization or catalization step during which a substrate surface
to be electrolessly plated with a metal has placed thereon a
material, usually a metal salt. This metal salt is capable of
reducing the plated metal from an electroless bath without the use
of an electrical current. Sensitization or catalization by such a
material (called a "catalyst" or "sensitizer" ) is referred to as
such because the materials used, usually the salts of the precious
metals (palladium, platinum, gold, silver, iridium, osmium,
ruthenium, and rhodium), serve as reduction catalysts in the
autocatalytic electroless plating process. Often, sensitization or
catalization is characterized as providing "nucleating sites" onto
which the plated metal is "brought down" by a chemical reduction,
or more generally, by a redox reaction. See, for example, U.S. Pat.
Nos. 3,119,709 and 3,011,920.
Refinements of the basic electroless plating technique are
necessary when the plated metal is electrolessly plated onto
selected portions of a substrate surface in a pattern, rather than
on the entire surface, to produce a circuit board.
The prior art embraces three broad categories of such refinements
for producing electrolessly plated metallic patterns, namely, (I)
masking, (II) selective, particulated catalyst placement, and (III)
the use of photographic emulsions.
(I) MASKING
Masking, as it applies to metallic pattern and circuit board
production by electroless plating, involves one of three
techniques, all of which may utilize a photoresist or other type of
mask.
First, after sensitization or catalization of the entire surface of
a nonconductive substrate, a negative mask, having open areas which
conform to the positive of the metallic pattern, is coated or
placed onto the sensitized substrate surface. Subsequently, when
the sensitized substrate is placed in an electroless bath, metal is
reduced onto the substrate only where the mask is open.
This type of masking is inexpeditious for numerous reasons, among
which are waste (the mask is not reusable when it comprises a
photoresist) and registration difficulties. Moreover, such masking
may result in poor edge definition of the metallic pattern by
virtue of some of the electrolessly plated metal adhering to the
mask as that metal is built up by the electroless plating.
Subsequent removal of the mask removes some of the adherent metal
from the edge of the metallic pattern.
Second, the substrate surface may be positively masked before
sensitization. Sensitization, therefore catalyzes both the exposed
surfaces of the substrate and the mask. The mask is either removed
prior to, or is left in place during, electroless plating and is
then removed.
Removal of the sensitized mask prior to electroless plating leaves
portions of the substrate (conforming to a negative of the circuit
pattern unsensitized, but is undesirable because, inter alia,
on-line process time is involved and many solvents for the mask
have been observed to have a deleterious effect on the
catalyst.
Removal of the sensitized mask after electroless plating not only
wastes metal which has been plated thereon, but also may result in
poor edge definition of the electrolessly plated metallic pattern,
as described above.
It should be noted that some workers in the art have observed that
certain masking materials apparently reject the sensitizer (see
Western Electric Technical Digest, No. 9, January 1968, p. 35).
Such rejection is evidenced by the ability to remove, by a
prolonged rinsing step, the sensitizer from the surface of the
mask. However, this characteristic is not held by all masking
materials, is not well-understood, and may be unpredictably
manifested.
The third technique which may be referred to as masking comprises
sensitizing and then electrolessly plating the entire surface of a
substrate followed by selective etching of the plated metal to
produce a metallic pattern. The selective etching is preceded by
the application of a positive mask to the plated metal which
prevents an etchant from attacking the metal thereunder.
Masking and selective etching of an electrolessly plated metal are
undersirable for a number of reasons. Among these reasons are the
extra time, money, and material required for the masking and
etching steps. Additionally, etching may undercut the plated metal
and the mask must be removed before electrical connections can be
made to the metallic pattern where such is intended as a circuit
pattern. Moreover, the plated metal which is etched away is
wasted.
(II) SELECTIVE, PARTICULATED CATALYST PLACEMENT
Selective particulated catalyst placement generally comprises
mixing a metal, such as palladium, in particulated form together
with a resin binder or adhesive. This mixture is applied
selectively to a substrate using standard printing techniques, for
example, silk screening. Immersion of the substrate into an
electroless bath is followed by the deposition of electroless metal
onto the palladium particles, which serve as nucleating sites for
reducing the electroless metal from the bath. See U.S. Pat. No.
3,259,559.
Selective catalyst placement is most disadvantageous. First, it is
noted that a volatile component of the resin binder or adhesive
must usually be driven off by heat or other energy to "cure" the
binder or adhesive prior to immersion of the substrate into the
electroless bath. This heat may have a deleterious effect on the
substrate. Moreover, even after such volatile component has been
driven off, the binder or adhesive may be subject to degradation
(e.g., melting or thermal decomposition) upon the application of
concentrated heat generated by soldering conductors to the metallic
pattern when the pattern is used as a circuit pattern.
Additionally, such an electroless coating usually exhibits surface
roughness or "pinholes" which may sometimes be partially obviated
by prolonged electroless plating. However, prolonged plating
necessitates an undesirable additional expenditure of time, money
and materials.
(III) USE OF EMULSIONS
The use of photographic emulsions generally includes an initial
step of exposing a silver-containing, gelatinous emulsion to light
where a metallic pattern is desired. Photographic development fixes
the pattern and washing removes the emulsion which is not exposed.
Electroless plating, the silver providing nucleating sites,
produces a metallic pattern.
The use of emulsions is disadvantageous for several reasons, among
which is a requirement for "safe light" conditions. That is, the
emulsions must be protected from ambient light to prevent
undesirable, nonselective fixing of the nucleating sites. In
addition, photographic emulsions are often degraded by subsequent
manufacturing operations. For example, concentrated heat due to a
soldering operation may melt the developed emulsion causing the
metallic pattern to "swim"; or the heat may thermally decompose the
emulsion.
The most severe problem residing in the use of emulsions is the
surface roughness of the metallic patterns produced thereby. This
surface roughness is produced by the initial physical separation
between the silver nucleating sites after the emulsion is fixed.
Thus, rather long electroless plating times are required to ensure
a smooth plated coating having no pinholes or craters. Insufficient
electroless plating time leads to a rather rough-surfaced metallic
pattern having poor electrical characteristics. Moreover, when
electroplating on such a metallic pattern, the high currents used
may cause "flashover" or arcing which may burn or render
electrically discontinuous the pattern.
In summation, it may be said that masking (I), selective,
particulated catalyst placement (II), and the use of emulsions
(III) as they apply to electroless plating, are far from being
optimum techniques. In addition, to the disadvantages noted above,
none of these prior art techniques conveniently lends itself to the
production of a sensitized or catalyzed substrate followed by
storage of the substrate, electroless plating being effected at a
later time. It should also be observed that none of the prior art
techniques described above are continuous manufacturing processes,
but rather are one-at-a-time, noncontinuous processes.
Moreover, categories (II) and (III) are observed to entail
techniques for providing a bed or reservoir (i.e., the binder or
the photographic emulsion) which "holds" nucleating sites to the
substrate. This bed or reservoir, as noted above, may be subject to
various types of degradation. Further, the bed adds thickness to
the metal-patterned substrate which is undesirable in many cases,
for example, where the substrate must be very flexible. Flexing a
substrate with such a bed or reservoir thereon may ruin the
overlying metallic pattern by cracking the underlying bed.
Lastly, all three of the above broad categories are incapable of
generating satisfactory metallic patterns wherein individual
pattern portions have dimensions or widths on the order of 1-2.mu.
or a fraction of 1.mu., or wherein individual pattern portions are
separated by distances on the order of 1.mu.. Integrated
microcircuits, with which metallic circuit patterns are used,
require such dimensions and separations.
SUMMARY OF THE INVENTION
Accordingly, an object of this invention is to provide a new and
improved method of generating precious-metal-reducing patterns.
Another object of this invention resides in the provision of a new
and improved method of generating, on a substrate, a pattern
capable of reducing thereon a precious metal, which reduced
precious metal is usable as a reduction catalyst in an
autocatalytic electroless plating process.
Further, another object of the present invention is the provision
of a novel method for producing, by photographic-like techniques, a
starting product usable in an electroless plating process to
produce a metallic pattern, such as a circuit pattern.
Yet another object of this invention resides in a novel method of
producing a pattern on a nonconductive substrate, such pattern
being usable as a catalyst in the electroless reduction of a
conductive metal thereon to produce a circuit-pattern wherein prior
art masking, etching, mask removal, curing, silk screening,
binders, adhesives, emulsions, photoresists, and other inefficient
operations and materials and undue material waste are completely
eliminated.
A still further object of the present invention is a
photographic-like method of producing a precious-metal-reducing
pattern on a nonconductive substrate by a continuous series of
steps to produce a starting product for electroless plating, which
product may be stored prior to its use in the electroless
plating.
Another object of this invention is a method of generating metallic
patterns on a suitable substrate, the method being capable of
producing patterns having a definition of 1.mu. or less.
Another object of this invention is a new and improved method of
generating a pattern onto which a metal may be electrolessly
plated, the plated metal exhibiting improved edge definition over
prior art methods.
An additional object of this invention is a novel method of
producing a starting material for an electroless plating process
whereby a smooth, even, pinhole-free electroless plate is
efficiently deposited because of the enhanced properties of the
starting material.
Yet another object of this invention is a method of generating, on
a nonconductive substrate, a pattern capable of reducing thereon a
precious metal, which generation does not entail the use of
adhesives, binders, emulsions or other catalyst beds or reservoirs,
the precious metal pattern being usable as a reduction catalyst in
an electroless plating bath to produce a metallic pattern having
improved characteristics as a consequence.
With these and other objects in view, the present invention
contemplates a new and improved method of generating
precious-metal-reducing patterns. A suitable substrate is first
coated with a solution which contains a metal salt. Such solution
is hereafter called a "photopromoter." The photopromoter possesses
two characteristics:
(a) The oxidation state of the salt (i.e., of the metal ion)
therein is alterable (that is, either increasable or decreasable)
by exposure thereof to high energy such as actinic radiation or
ultraviolet light quanta of short (less than 3000 A.) wavelength;
and
(b) In either the original or the altered oxidation state (but not
in both states), the salt is capable of reducing a precious metal,
for example, metals of the platinum group, such as palladium,
platinum or rhodium, from a solution containing a salt of the
precious metal.
The photopromoter-coated substrate is next selectively exposed to
the ultraviolet light of short wavelength to produce a pattern of
the salt capable of reducing the precious metal. The remainder of
the salt is incapable of reducing the precious metal.
The actinically exposed substrate may then be immersed in a
solution containing a salt of the precious metal to generate, by
chemical reduction, a pattern of such precious metal. Ultimately,
the precious metal pattern may be used as a catalyst to reduce
thereon a metal, such as copper, for producing a metallic pattern
in an autocatalytic electroless plating bath. The metallic pattern
may be used as a circuit pattern of a circuit board.
Two broad categories of photopromoter solutions are
contemplated.
The first category contains metal salts which in their original
oxidation state are capable of reducing a precious metal. When such
a photopromoter is used, the selective photopromoter-coated
substrate is exposed selectively to the actinic radiation where the
ultimate metallic pattern will not reside. Such exposure places the
salt thereat in a higher oxidation state (i.e., in the altered
oxidation state) wherein the exposed salt is rendered incapable of
reducing the precious metal.
The second category contains metal salts which in their original
oxidation state are incapable of reducing a precious metal. When
this type of photopromoter is utilized, the photopromoter-coated
substrate is selectively exposed to the actinic radiation where the
ultimate metallic pattern will reside. Such exposure renders the
salt thereat capable of reducing the precious metal by placing the
exposed salt in a lower (i.e., in the altered) oxidation state.
Some of the photopromoters utilized in the instant invention have
been recognized as useful chemical entities for some time. However,
even though the phenomenon of electroless plating has been known
for over 120 years, as pointed out above, none of these
photopromoters have been incorporated into an electroless plating
process in the novel manner of this invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and advantages of the present invention will appear
upon consideration of the following detailed description in
conjunction with the accompanying drawings wherein:
FIG. 1 is a generalized flow chart depicting, step-by-step, the
present new and improved method of generating
precious-metal-reducing patterns and including the optional steps
of producing a precious metal pattern and a metallic pattern
electrolessly deposited thereon and reduced thereby;
FIG. 2 is a side elevational view of apparatus which may be used to
carry out the method depicted in FIG. 1;
FIG. 3 is a perspective view of a portion of the apparatus of FIG.
2 showing an actinic radiation source which effects the generation
of precious-metal-reducing patterns in accordance with the method
of FIG. 1;
FIG. 4 is a stylized view of some of the alternatives to the
apparatus shown in FIG. 3; and
FIG. 5 is a perspective view of a few of the various products which
can be made by the method shown in FIG. 1.
DETAILED DESCRIPTION
Preliminarily, definitional bounds will be placed on certain
language contained herein.
The terms "negative" or "negative mask" refer to a process
employing a mask to produce a pattern which is opaque where the
mask is not opaque. That is, the solid portion of the mask is a
negative representation of (or does not replicate) the pattern.
The terms "positive" or "positive mask" refer to a process
employing a mask to produce a pattern which is opaque where the
mask is opaque. That is, the solid portion of the mask is a
positive representation of (or replicates) the pattern.
"Catalyst" refers to a substance or material which initiates or
accelerates a chemical reaction but which itself remains chemically
unchanged, or is at least not permanently changed at the end of the
reaction.
"Promoter" in the present context refers to a substance other than
a "catalyst" which promotes or encourages a chemical reaction. A
promoter differs from a catalyst in that the promoter does undergo
a chemical change in performing its function.
"Photopromoter," a term used previously, defines substances which,
upon being exposed to appropriate radiation, either (a) dissipate
chemical energy already possessed thereby or (b) store chemical
energy not previously possessed thereby. When these substances
possess or have stored chemical energy, they are capable of acting
as promoters, a term defined immediately above. The dissipation or
storage may be either a primary reaction caused by the radiation or
a secondary reaction brought about by unspecified primary reactions
to the radiation. Typically, the dissipation of chemical energy is
manifested by the transition of the oxidation state or number of a
metal salt (or metal ion) from a chemically active low number to a
less chemically active high number. The storage of chemical energy
is manifested by a reverse process.
Specifically, the first type [(a)] of photopromoter is rendered
incapable of acting as a promoter if and where such photopromoter
is exposed to the radiation. A photopromoter of this first type is
inherently capable of acting as a promoter if and where it is not
exposed to the radiation. The second type [(b)] of photopromoter is
rendered capable of acting as a promoter only if and where exposed
to the radiation. A photopromoter of this second type is inherently
incapable of acting as a promoter if and where it is not exposed to
the radiation.
"Precious metals" includes metals of the "platinum group" (iridium,
osmium, palladium, platinum, rhodium and ruthenium) and gold and
silver.
"Adsorption" is defined as a surface phenomenon exhibited by solids
which is evidenced by adhesion in an extremely thin layer of the
molecules of a liquid to the solid. Adsorption may be due to
physical or chemical forces exerted on the molecules of a liquid by
the surface of a solid with which the liquid is in contact.
"Actinic radiation" in this context is ultraviolet radiation deep
within the ultraviolet spectrum. Specifically, such radiation
possesses a short wavelength of less than 3000 A. and within the
approximate range of from about 1800 A. to about 2700 A.
A wide variety of substrate materials are suitable for use in the
present invention. Electrically nonconductive polyimide films,
semipolymerized silicone films, "B stage" epoxy films or unfired
ceramics are preferred, however, when this invention is used to
produce electrical circuit patterns. Nevertheless, other types of
substrate materials such as polyamides, paper, cloth, fiberglass,
other plastics, glass, and fired ceramics may be used.
Referring now to FIG. 1, the initial step of the subject method may
comprise subjecting a surface of a substrate 11 to a cleaner, as
shown at 12, which renders the substrate surface "nascent." A
nascent surface is a surface on which a solution is retained after
the surface is immersed therein. Such retention may be caused by
any one or combination of the phenomena known as adsorption,
physical adsorption, chemisorption, wetting, or absorption. In any
event, the surface must be capable of retaining a thin layer of the
solution. Thus, the cleaner may be unnecessary and is described
here only as a possible expedient in the event the surface is not
already nascent. As an example, when a polyimide film is used as a
substrate, it has been found convenient to clean the surface
thereof by momentary immersion in NaOH. This immersion actually
removes a very minute amount of the film, ensuring that the film's
surfaces are nascent. On the other hand, a ceramic substrate may be
rendered nascent by momentary immersion in a mixture of HNO.sub.3
and HF. Other expedients, such as sandblasting, may be used to
render the substrate surface nascent.
The nascent substrate surface may then be bathed in a rinse 13 of
deionized, clean water or of zeolite-exchanged water. All
subsequent water rinsings are of a similar character.
Next the nascent surface is immersed in a photopromotor solution,
represented at 14. The photopromoter, as previously described and
defined, is a solution which contains a metal salt having specific
properties. The photopromoter may be nonaqueous but is typically
aqueous, and, in any event, is retained as a thin layer on the
nascent substrate surface.
Worthy of mention is the fact that such retention depends on
phenomena previously discussed and not on the adhesives, binders,
emulsions or beds of the prior art. For example, an aqueous
stannous chloride solution has been observed to adsorb on a nascent
polyimide surface while an aqueous ferric oxalate solution has been
observed to be retained on a nascent ceramic surface by an
uncharacterized mechanism which may be a combination of the
phenomena of absorption, adsorption and wetting.
The metal salt in the photopromoter 14 possesses two
characteristics, namely:
(a) The oxidation state or number of the salt is alterable (either
increasable or decreasable) by exposure of the salt to radiation of
the proper wavelength, that is, actinic radiation which is
typically short ultraviolet light of less than 3000 A.; and
(b) In either the original or the altered oxidation state (but not
in both states) the salt is capable of reducing a precious metal,
for example palladium, from a solution containing a salt of the
precious metal.
Two general classes of photopromoter solutions have been
discovered.
The first group of such solutions is denoted as the positive group.
The positive group includes solutions which contain a metal salt
capable in its original state of reducing the precious metal.
Proper exposure of this group of solutions to the actinic radiation
places the salt in a higher, less chemically active oxidation state
wherein the salt is incapable of acting as a promoter by reducing
the precious metal. Positive photopromoters are type (a), discussed
above.
Accordingly, when a positive photopromoter solution has been
retained on the nascent substrate surface, the salt is subsequently
exposed to the actinic radiation only where a conductive metal
pattern is not ultimately desired, that is, through a positive
mask.
The second group of photopromoter solutions is referred to as the
negative group. The negative group includes solutions which contain
a metal salt having an originally higher and less chemically active
oxidation state which renders the salt incapable of reducing the
precious metal. Proper exposure of the negative group of solutions
to the actinic radiation reduces the oxidation state or number of
the salt rendering the salt more chemically active and capable of
reducing the precious metal. Negative photopromoters are type (b),
discussed above.
Accordingly, when a negative photopromoter solution is retained on
the nascent substrate surface, the salt is exposed to actinic
radiation only where a conductive metal pattern is ultimately
desired, that is, through a negative mask.
After the immersion of the nascent substrate surface in and the
retention of the photopromoter solution thereon, the substrate is
dried as shown at 15. Such drying may be preceded by bathing in a
clean water rinse 16 if the photopromoter 14 is of a type (e.g.,
stannous chloride) which absorbs on the nascent substrate surface.
Otherwise the rinse 16 is not used.
It has been found that drying of the retained photopromoter 14
leaves behind on the substrate surface a residue which includes the
metal salt, termed a photopromoter salt, formerly in solution. This
residue is retained by the surface. If a positive photopromoter has
been used, such retained salt is in a low oxidation, chemically
active state capable of reducing a precious metal. If a negative
photopromoter has been used, the retained salt of the residue is in
a high, chemically inactive state incapable of reducing a precious
metal.
The mechanism by which the salt is left behind on and retained by
the nascent substrate surface is not clear. Such retention may be
due, again, to physical forces, adsorption, chemisorptive forces,
or the like. In any event, such retention is an accomplished fact
at this point in the process and requires no adhesives or the
like.
The drying (15) of the substrate 11 is a desirable expedient. As
noted previously, and as more fully described below, the retained
salt of the photopromoter 14 is subsequently exposed to short
(about 1800 A. to about 2700 A.) wavelength, ultraviolet light or
actinic radiation as at 17. If the photopromoter is not dried, a
liquid layer remains on the substrate surface. Such layer has been
observed to attenuate and even prevent the chemical action which
the actinic radiation is intended to effect in the photopromoter
salt. This attenuation is especially noticeable when the
photopromoter is an aqueous solution.
Next, the retained salt of the photopromoter is exposed selectively
to the actinic, ultraviolet radiation (17), e.g., through a mask
represented at 18. This exposure produces a pattern of the retained
salt on the substrate surface which is capable of reducing a
precious metal. The remainder of the salt is incapable of reducing
the precious metal. Of course, the type of mask (negative or
positive) used is dictated by the character of the photopromoter
solution and salt. Specifically, if a positive photopromoter
solution is used, the mask is a positive mask which permits the
radiation to strike the retained salt only where a metallic pattern
is not ultimately desired. If a negative photopromoter solution is
used, the mask is a negative mask which permits the radiation to
strike the retained salt only where a metallic pattern is
desired.
Obviously, instrumentalities other than the mask 18 could be used
to selectively expose the retained salt to the proper radiation.
For example, the salt residue may be swept by a fine beam of the
actinic radiation which may be controlled by a computer or by
rotating mirrors.
It should be noted that the use of a positive photopromoter 14
permits the efficient reduction of the precious metal on the walls
of holes in the substrate 11. Specifically, if holes of small
diameter are formed in the substrate 11 prior to immersion thereof
in the positive photopromoter solution 14, the selective exposure
to the actinic radiation does not impinge upon the salt retained by
the hole walls after drying. Thus, the hole walls are rendered
capable of reducing the precious metal.
The actinic radiation-exposed substrate is next immersed in a
catalyst or sensitizer solution 19 containing a salt of a precious
metal. The pattern of the photopromoter salt (whether of the
positive or negative variety) which is capable of reducing the
precious metal does in fact now reduce such precious metal onto the
pattern. This reduction may be said to effect selective
sensitization or catalization of the substrate 11 by rendering the
pattern catalytic to an electroless plating bath 20.
It should be noted, that in some instances, it is possible to mix
together in a single bath, the photopromoter 14 and the precious
metal salt solution 19. The nascent substrate surface is immersed
in this mixture and then exposed to the actinic radiation 17.
Obviously, such a mixture may be used only in the case of a
negative photopromoter.
Following the immersion in the precious metal salt solution 19, the
selectively catalyzed substrate 11 may be water bathed in a rinse
21, subjected to the electroless plating bath 20, subjected to an
electroplating bath 22, bathed in a water rinse 23, and finally
dried as at 24. The electroless and electroplating baths 20 and 22
may contain salts of a variety of metals including copper, nickel,
cobalt, palladium, platinum, silver or gold.
In the electroless plating bath 20, the precious metal pattern
provides nucleating sites for the metal contained therein. The
occurrence of electroless plating, sometimes referred to as
"physical development," is well-known. The electroless metal is
built up on the pattern for a sufficient time dictated by the
ultimate use of the metallic pattern. If it is desired to
subsequently electroplate, the electrolessly plated pattern at 22,
sufficient metal thickness (e.g., 1500 to 3000 A.) must be built up
in the electroless bath 20 to render the metallic pattern
sufficiently thick to withstand both the electroplating current and
the acidity of the electroplating bath 22 without being rendered
discontinuous by arcing.
It is worthy of mention that all of the photopromoter solutions 14,
whether negative or positive, do not require safe light conditions
during any step of the process. That is to say, photopromoter
solutions 14 have been discovered which are responsive to the
radiation of the selected wavelength, which radiation is
substantially absent from the ambient environment. The
photopromoter solutions 14 utilized herein respond to the actinic
ultraviolet radiation 17 having a wavelength in the approximate
rage of 1800 to 2700 A., or, more generally, below 3000 A.
The substrate 11 may be conveniently stored or shipped after the
electroless bath, as indicated at 27. The substrate 11 may then be
transmitted to the ultimate metallic pattern user or producer who
may then himself electroplate metallic patterns on the film. In any
event, after electroplating, at 22, the metallic pattern may be
subjected to one of two steps, namely, removal, depicted at 28 or a
bond enhancement treatment, shown at 29.
The adherence of the metallic pattern to the substrate 11 has been
observed to be quite good and comparable to prior art methods.
However, the pattern may, if care is used, be removed (delaminated)
from the substrate, or the substrate may be dissolved by a
substance which is inert to the metallic pattern. In either event,
both of which are represented at 28, disassociation of the metallic
pattern from the substrate 11 may be the final step in producing an
electroformed metal article, such as a lead frame for use in the
manufacture of integrated semiconductor devices. Thus the thrust of
the present method may be the production of a metal article by
electroforming using the substrate 11 as a temporary carrier.
Obviously, articles of substantial thickness may be produced by
lengthening the time the electrolessly plated pattern dwells in the
electroplating bath 22.
If a substrate 11 is used which is made of one of the preferred
materials, the thrust of the present method may be the production
of a metallic pattern permanently residing on the substrate, for
example, as an electric circuit board pattern. In this event, it
may be desirable to enhance the bond between the metallic pattern
and the substrate as depicted at 29.
Such enhancement may be effected by completing the polymerization
of the silicone resin, completing the curing of the epoxy film, or
firing the ceramic, depending on the type of substrate used, which
results in bond strengths of 5-15 lbs./linear inch.
When a polyimide film is used, the bond enhancement entails heating
the substrate and metallic pattern in an atmosphere conducive to
the formation of oxides of the metal of the pattern and to a
temperature below the service temperature of the polyimide. Such
heating has been found to induce the growth of a metal oxide on the
metallic pattern at the pattern-polyimide interface. Oxide growth
in this manner drives the oxide into the polyimide to increase the
pattern-polyimide bond to about 15 lbs./linear inch. Such oxide
growth may be due to the permeability of polyimides to the oxide
formation-conducive atmosphere.
The wavelength range of the actinic radiation 17 utilized in this
invention may explain why the method of this invention has remained
undiscovered for over 145 years. Actinic radiation deep in the
ultraviolet spectrum and in the 1800 to 2700 A. wavelength range is
not only seldom used in research and in commercial industrial
processes, but is also attenuated by air to such a great extent
that it is not normally present under ambient conditions. In
addition, substances other than air, for example, common glass,
translucent or transparent plastics and thin liquid layers
similarly attenuate such radiation. As a matter of fact, it has
been found that only certain materials, such as quartz,
quartz-related glasses and boro-silicates, efficiently transmit
without substantial attenuation, the actinic radiation used in this
invention. Use is made of this characteristic as described
below.
The positive photopromoter solutions and the negative photopromoter
solutions are now described in detail.
A number of negative photopromoters [type(b)] have been discovered
with effect efficient selective reduction of a precious metal
following selective exposure to the actinic radiation 17. Six of
these solutions are: ferric oxalate, ferric citrate, ferric
tartrate, mercuric oxalate, mercuric citrate, and mercuric
tartrate.
The ferric ion or salt in any of the first three photopromoter
solutions is configurated as follows:
while the mercuric ion is configurated as:
Exposure of the ferric ion in the photopromoters to the actinic
radiation alters that ion to the ferrous ion:
while exposure of the mercuric ion alters such ion to the mercurous
ion:
Both of the lower oxidation state and more chemically active ions
(ferrous [Fe.sup.+.sup.+] and mercurous [Hg.sup.+]) are capable of
reducing a precious metal. The reaction for the actinically
rediated ferric ion, when a palladium chloride precious metal
sensitizer solution 19 is used, is: ##EQU1##
The reaction for the mercuric ion, when the same sensitizer is
used, is: ##EQU2## In both cases, hv is the quanta of energy
carried by the actinic radiation 17 and applied to the higher
oxidation state photopromoter salts to render such salts more
chemically active.
The reacton for the ferric and mercuric ions not exposed to the
actinic radiation is: ##EQU3##
It should be noted that the expressions: ##EQU4## may not define
primary reactions. Such expressions may represent partial secondary
reactions and illustrate only what ultimately happens to the ferric
and mercuric ions.
For example, the complete reaction for the ferric oxalate
photopromoter may be: ##EQU5## where the two electrons (2e) broken
away from the oxalate by the actinic radiation, make a ferrous ion
from a ferric ion.
To ensure that the negative photopromotor solutions contain only
the higher oxidation state metallic ions prior to actinic radiation
exposure, a trace (1/2% or less) of an oxidizer may be placed in
the photopromoter solution 14. For example, it is known that
commercial solutions of ferric oxalate generally contain a certain
percentage of the Fe.sup.+.sup.+ (ferrous) ion. To ensure that all
ions in the solution are in the Fe.sup.+.sup.+.sup.+ (ferric) state
prior to the actinic exposure, a trace amount of an oxidizer, such
as nitric acid, may be added to the solution 14. The addition of
the oxidizer prevents the precious metal from being reduced by
Fe.sup.+.sup.+ ions on the substrate where such reduction is not
desired, this undesirable effect being referred to as "fogging."
Only trace amounts of nitric acid are added because such amounts of
the relatively volatile acid are easily driven off when the
substrate is dried at 15. Greater amounts of nitric acid would
reoxidize the actinically exposed, lower oxidation state ion (i.e.,
Fe.sup.+.sup.+) back up to the higher oxidation state (i.e.,
Fe.sup.+.sup.+.sup.+). Thus, too much oxidizer in the photopromoter
solution negates the actinic radiation exposure.
At least three classes of positive photopromoter solutions are
known. Such classes are the tin halides, the titanium halides and
the lead halides. The positive photopromoters include, inter alia,
tin chloride, tin bromide, titanium chloride, titanium bromide,
lead chloride, and lead bromide.
The .[.negative.]. .Iadd.positive .Iaddend.photopromoter solutions
contain metal ions which in their normal state are capable of
reducing a precious metal. The three ions are, respectively:
Exposure of such negative photopromoter solutions to the actinic
radiation places these ions in a higher, less chemically active
oxidation state incapable of reducing the precious metal. The
reactions for the three photopromoter solutions when palladium
chloride is used as a sensitizer 19 are given below: ##EQU6##
It should be noted that the first two lines of each of the three
groups of reactions refer to those portions of the substrate
exposed to actinic light whereat a conductive pattern is not
desired. The first line represents a reaction whereby ions are
produced by the actinic radiation 17 which are incapable of
reducing the precious metal as shown by the second line. The third
line of each of the three reactions refers to chemical action
taking place at those portions of the substrate which are not
exposed to the actinic radiation 17.
PREFERRED APPARATUS EMBODIMENTS
Referring now to FIG. 2, a typical embodiment of one form of
apparatus for effecting the previously described inventive method
is shown.
On a support member 30 are rotatably mounted a plurality of upper
rollers 32. One of these rollers 32a may be made of a conductive
material such as copper, while the remainder are made of a
nonconductor, such as plastic. The support member 30 rests on top
of a network 34 of serial tanks each of which has rotatably mounted
therein one or more of a plurality of nonconductive lower rollers
35. The substrate 11, made of a suitable flexible material, is
pulled into and out of the tanks of the network 34, over the
rollers 32 and under the rollers 35 from a supply reel 38 by a
takeup reel 40 driven by a motor (not shown).
The substrate 11 first enters a tank 41 containing the cleaner 12
where the substrate surface is rendered nascent, as previously
described. Should the surface of the substrate 11 be nascent as it
comes from the supply 38, the tank 41 is not used.
The substrate 11 next moves into a tank 42 containing the deionized
or zeolite-exchanged water rinse 13. After being rinsed in the tank
42 the substrate 11 moves into a tank 44 which contains one of the
photopromoters solutions 14 described above.
In the tank 44 the photopromoter solution contacts the nascent
substrate surface enabling the surface to retain the photopromoter
14 as described previously. Specifically, the positive
photopromoters adsorb onto the nascent surface, while other
photopromoters are retained on the surface by other mechanisms such
as wetting, chemisorption, etc.
As the substrate 11 moves out of the tank 44, an air blast from a
pair of air knives 46 may be directed against the substrate 11.
Such air blast dries (15 in FIG. 1) the substrate 11, for example,
by evaporating the water from an aqueous photopromoter or the
volatile components from a nonaqueous photopromoter.
A following tank 47 contains the deionized water rinse 16. The tank
47 is used only where the photopromoter 14 is one which adsorbs on
the nascent substrate surface, such as a tin halide solution.
Adsorption makes it virtually impossible to rinse away the metal
salts adsorbed on the substrate surface, and the rinsing may be
desirable to ensure that the adsorbed photopromoter layer is as
thin as possible. This thinness enables the actinic radiation 17 to
be aberrated as little as possible by refraction or diffusion which
might otherwise be caused.
The path taken by the substrate 11 in the case of a photopromoter
which adsorbs is shown in phantom in FIG. 2. Similarly, in this
case air knives 46a direct drying air against the substrate 11.
In any event, the air knives 46 (or 46a) by removing a liquid
portion of either type (adsorbed or nonadsorbed) of the retained
photopromoter 14 ensure that the actinic radiation does not pass
through an appreciable liquid layer. This assurance makes it more
likely that the actinic radiation will more faithfully expose the
photopromoter only where desired by substantially eliminating
refraction and diffusion.
The substrate 11 is next exposed to the actinic radiation 17 from
one or more of the ultraviolet sources 48 and 49 by exposure
facilities 50 to be described in greater detail subsequently. Such
exposure renders the photopromoter-coated, nascent surface of the
substrate 11 selectively capable of reducing thereon a pattern of a
precious metal, the salt solution 19 of which is contained in a
tank 52.
The substrate 11 next moves into the tank 52 containing the
precious metal solution 19. In the tank 52 the precious metal is
reduced onto the photopromoter pattern defined by the actinic
radiation 17.
Following the reduction of the precious metal, the substrate may
proceed through a tank 54 containing the water rinse 21, an
electroless plating tank 56 containing the electroless plating bath
20, a tank 58 containing the electroplating bath 22, and a tank 60
containing the final water rinse 23. From the final rinsing tank 60
the substrate now containing a metallic pattern moves onto the
take-up reel 40, after being contacted by a stream of drying air
emitted by a blower 62 (at 24 in FIG. 1).
The electroless plating bath 20 in the tank 56 may be any one of a
commercial variety, but is preferably a high speed bath so that
dwell time of the substrate 11 in the tank 56 need not be overlong.
Moreover, the electroless bath 20 may be capable of depositing
copper, nickel, cobalt, palladium, platinum, silver or gold on the
precious metal pattern. Typically, electroless plating occurs at
rates of from 400 to 2000 A. per minute and autocatalytic.
Specifically, after the nucleating sites provided by the precious
metal pattern have been covered by electroless metal, such
electroless metal provides further nucleating sites for itself. It
has been observed that electroless plating action is initiated by
precious metal patterns which are present in mere trace amounts.
That is, precious metal patterns only a few atom layers thick
"bring down" metal from an electroless bath.
A reducing agent may be placed in the electroless bath 20. Such an
agent must be sufficiently "gentle" so that spontaneous reduction
of metal from the electroless bath 20 is not induced except onto
the precious metal pattern. Suitable reducing agents are hydrazine,
formaldehyde, sodium hypophosphite, and certain borane compounds.
The electroless bath 20 may be stabilized and controlled by the
addition thereto of a complexing agent such as EDTA, cyanides or
Rochelle salts.
Long term stability of electroless baths may be nonexistent due to
spontaneous decomposition. Stability may be greatly improved by
continuous filtration (by apparatus not shown) of the electroless
bath 20 and by the addition thereto of stabilizing additives such
as trace amounts of the cyanide (CN.sup.-) ion in the amount of
about .05 gram/liter. Such electroless baths may be used for
weeks.
The electroplating bath 22 in the tank 58 may be any one of a
commercial variety, such as a standard copper sulphate solution.
Preferably, the bath 22 is high speed, such as one containing a
copper fluoborate electrolyte capable of effecting electroplating
rates as high as .5 mil per minute.
In the tank 58, the electrolessly plated conductive metal pattern
is cathodically connected to the negative side of a plating current
source 70 through the conductive roller 32a. This connection
requires that the metallic pattern be electrically continuous. Such
continuity is accomplished in one of two ways.
Referring to FIG. 5(a), the desired metallic pattern may be
inherently continuous, such as when it comprises a straight cable
pattern 72 including a plurality of parallel conductors 73.
Alternatively, the desired metallic pattern may be inherently
discontinuous such as the patterns 74 shown in FIG. 5(b). These
latter patterns 74 may be produced continuously on the substrate 11
on serial portions 11a--11a thereof.
To accommodate such continuous production, part of the pattern
generated by the exposure of the photopromoter 14 to the actinic
radiation 17 and later electrolessly plated in the tank 56 is one
or more common buses 76 which interconnect individual paths 78 of
the patterns 74 and also interconnect the respective patterns 74 on
the serial substrate portions 11a--11a. These common buses 76
contact the roller 32a to effect the desired electrical
continuity.
After electroplating in the tank 58, the patterns 74 may be first
severed along lines 80 to separate them and then the buses 76 are
shorn off along lines 82 to electrically isolate the paths 78 as
shown in FIG. 5. Alternatively, holes 84 may be punched in the
separated patterns 74 to isolate the paths 78 from the buses 76.
This alternative effects the retention of the bus 76 on the
substrate portion 11a which may, in some cases, be useful.
Referring again to FIG. 2, in the tank 58 a plating anode 86 is
also connected to the plating current source 70. The plating anode
86 may comprise two parts, namely a lower elongated portion 88,
generally parallel to the substrate 11 in the tank 58, and an upper
portion 90 which angles away from the substrate 11 below the point
of entry of such substrate 11 into the electroplating bath 22. The
angling away of the portion 90 prevents the electrolessly plated
metal from exposure to plating currents of high density until a
sufficient amount of electroplated metal is thereon.
Specifically, the thin electroless deposit is subject to
"flashover" or "arc over" upon exposure thereof to the strong
electroplating currents and high acidity in the bath 22.
Accordingly, the anode portion 90 is angled away to decrease the
current density during the initial entry of the electroless deposit
into the tank 58. Such decreased current density does decrease the
rate of buildup of electroplated metal, but this rate is sufficient
so that when the substrate 11 reaches the anode portion 88, the
deposit is able to withstand the higher current densities.
Referring now to FIG. 3, the facilities 50 are shown for
selectively exposing the photopromoted substrate 11 to the actinic
radiation (17 in FIG. 1).
The substrate 11 is wound sinuously about a pair of tension rollers
100 and an exposure cylinder or drum 101. The rollers are rotatably
mounted under the cylinder 101 to a pair of supports 102 between
which the cylinder 101 is positioned.
It has been observed that the actinic radiation of this invention
is attenuated, and in fact nearly completely blocked by common
glass, plastics, water, air, etc. Accordingly, the cylinder 101 is
made of quartz, quartz-related glasses, or borosilicates which, it
has been found, do not appreciably attenuate the radiation.
A mask pattern 103 is placed on either surface of the quartz
cylinder 101 to produce the mask 18 of FIG. 1. Whether the mask is
a negative or a positive of the final metallic pattern depends on
the type of photopromoter 14, negative or positive, which is used,
as previously discussed.
The mask pattern 103 may be any substance opaque to the actinic
radiation 17 but is preferably a black ink, Ruby Lith, or a
photoresist with or without a dye. Typically, the mask pattern 103
should be a material capable both of defining lines of less than
one .mu., the present process being able to generate precious
metallic patterns of such a dimension, and of completely blocking
the actinic radiation 17.
It is noted that the cylinder 101 is nowhere attached to the
supports 102 or to the rollers 100. Rather, tension in the
substrate 11 due to the operation of the takeup 40 holds the
cylinder 101 against the top of such rollers 100. Such tension also
rotates the cylinder 101 as the substrate 11 moves, the cylinder
and the substrate moving at the same speed in a 1:1 ratio. That is,
any given point on the substrate 11 which initially contacts the
cylinder 101 remains in the same position with respect to the
cylinder and to the mask pattern 103 thereon until the substrate 11
moves away and toward the tank 52.
The ultraviolet sources 48 and 49 have an actinic radiation output
substantially in the 1700-2800 A. range. One of the sources 48 is
mounted within the cylinder 101. As the cylinder 101 and the
substrate 11 move together, the source 48 constantly irradiates all
portions of the photopromoted substrate surface next to the
cylinder and which are not overlain by the mask pattern 103. If a
negative photopromoter is used, nothing more need be done; the
negative mask pattern allows the actinic radiation to pass through
selected portions of the quartz cylinder 101 to render nonmasked
substrate areas in contact with the cylinder capable of reducing
the precious metal. The substrate surface not touching the cylinder
is not later capable of reducing the precious metal inasmuch as the
actinic radiation is unable to pass through the substrate 11.
If a positive photopromoter is used two courses are available. In
the first, the photopromoted substrate surface in contact with the
cylinder 101 is exposed to the actinic radiation from the source 48
where a metallic pattern is not desired by virtue of the positive
mask pattern 103. If the source 49 is not operated, the reverse
side of the substrate, being already capable of reducing the
precious metal and being unaffected by the radiation from the
source 48 due to the attenuation thereof by the substrate,
ultimately reduces the precious metal. This course produces a
"ground plane" on the substrate separated from the metallic
pattern.
The second course involves irradiating the reverse side of the
substrate 11 with the other actinic source 49 to render all of the
photopromoter therein incapable of reducing the precious metal.
This second actinic source 49, of course, does not affect the
substrate surface contacting the cylinder 101, again due to the
attenuating effect of the substrate 11.
It should be apparent, in view of this description, that other
actinic exposure expedients may be used. For example, a second
cylinder 104 and a second set of tension rollers 105 immediately
after and similar to the first cylinder 101 and rollers 100 may be
used. The second cylinder 104 may contain a second different mask
pattern through which the reverse side of the substrate 11 is
selectively exposed to the actinic radiation 17 from a source 106
as shown in FIG. 4(a).
Numerous other schemes are possible for continuously generating
multisided circuit patterns.
Moreover, the present method may also be realized in semicontinuous
apparatus. Specifically, prior art step and repeat techniques are
feasible. In this instance, the mask (not shown) is a planar quartz
sheet having a mask pattern thereon. The photopromoted substrate is
selectively exposed through the sheet to an actinic source
appropriately close to the mask to prevent atmospheric attenuation
of the actinic radiation. Also, it has been demonstrated that a
projection system is feasible. Specifically, and referring to FIG.
4(b), an actinic source 109 passes radiation 110 through a mask 111
and then through a quartz lens 112 which focuses the mask image and
the actinic radiation on the photopromoted substrate 11.
EXAMPLES OF POSITIVE PHOTOPROMOTERS
(I) Polyimides served as substrates 11 in producing metallic
patterns 72, 74 of 1/2-1.mu. definition which were bonded strongly
to the substrate 11.
Two general types of polyimides were used, namely, (a) commercial
polyimide films, such as those sold as Kapton in thickness varying
from 1 mil to 5 mils and up and (b) polyimide resins, such as those
sold as Pyr M.L., which were coated on various miscellaneous bases.
The resins were heat cured at 375-400.degree. F. for about 1/2 hour
after being coated onto the bases in thickness of 1 mil or more.
Such heating produced polyimide surfaces substantially like that of
Kapton film.
When polyimide films served as the substrate 11, the surfaces
thereof were immersed in a cleaner 12 for periods of 1 minute or
more, but preferably, for 4-5 minutes when the cleaner 12 comprised
10 N NaOH. Such immersion removed a small amount of the film,
ensuring that the surfaces thereof were nascent. The films were
then bathed in the clean water rinse 13 with agitation for from 1
to 4 minutes.
The polyimide resins coated onto miscellaneous bases to provide the
substrate 11, were found, after curing, to be nascent. Accordingly,
neither the cleaner 12 nor the rinse 13 was used with such cured
resins.
The nascent character of both types of polyimide substrate is
indicated by their ability to retain on surfaces thereof a thin
layer of solutions into which the substrate surfaces are
immersed.
Specifically, both types of substrates 11 having nascent surfaces
were immersed in the following positive photopromoter solutions 14
under the following conditions:
__________________________________________________________________________
Time of Photopromoter immersion, Substrate 11 14 pH min.
__________________________________________________________________________
.1 m. SnCl.sub.2 1.0 1/4-4 Polyimide films and polyimide .045 m.
SnCl.sub.2 3.5 1/4-2 coated miscellaneous bases .045-.1 m.
SnCl.sub.2 1-3.5 2-4 .1 m. PbCl.sub.2 1-2 1/4-4 .1 m. TiCl.sub.2
<1 1/4-4
__________________________________________________________________________
All of these photopromoters 14 were observed to adsorb onto the
nascent polyimide substrate surface in thin layers (about 10-20 A.)
and were thus retained by the substrate 11.
Both types of substrates 11 were then bathed in the clean water
rinse for from 1-2 minutes. This bathing did not remove the
adsorbed photopromoter 14. Rather, such bathing ensured that the
photopromoter was adsorptively retained on the substrate 11 in the
thinnest of possible layers.
Next, the photopromoted substrate 11 was dried by the air knives
46a, a stream of air being directed thereagainst for about 1/4
minute.
Then the photopromoted substrates 11 were exposed to the actinic
radiation 17 from the ultraviolet source 48 through the positive
mask 18. Such exposure was effected, variously, by one or more 4-8
watt mercury lamps placed at varying distances from the quartz
cylinder 101.
The amount of energy supplied to the substrate surfaces by the
sources 48 and 49 was not found to be critical. Nevertheless, it
was found desirable to irradiate the substrate surface at energy
levels ranging from about 30 milliwatt-seconds per CM.sup.2 to
about 250 milliwatt-seconds per CM.sup.2. Moreover, it was found
that such energy levels were best supplied by sources 48 and 49
which emitted approximately 75% of their radiation at the 2537 A.
line. It should be noted that sources were used which emitted
radiation of less than 3000 A. Typically, such radiation ranged
from about 1800 A. to about 2700 A.
The actinic exposure, as described above, generated a positive
pattern on the substrate 11 capable of reducing a precious metal
from a salt solution thereof. Specifically, the actinic exposure
was found to render irradiated portions of the photopromoted
substrate 11 incapable of reducing the precious metal by bringing
about the transition of the metal ions of the photopromoter 14 from
a low oxidation number, chemically active state to a higher
oxidation number, chemically inactive state.
The exact mechanism by which the precious metal reducing pattern is
generated is not clear. It is theorized, without dependence
thereon, that immersion of the nascent substrate 11 into the
photopromoter 14, followed by the rinse 16 and the drying, leaves
the metal ions (derived from the metal salts in the photopromoter)
adsorbed tenaciously on the polyimide substrate 11. These metal
ions, in the case of tin, may ultimately be in one of the forms:
SnO.H.sub.2 O or Sn(OH).sub.2. Thus, the actinic radiation 17 may
effect the precious metal reducing pattern in conjunction with
adsorbed oxygen normally present on surface of the substrate.
One further observation was made regarding the mode of actinic
exposure. Specifically, it was found that the actinic sensitivity
of the photopromoted substrate increased with increasing humidity.
That is, the higher humidity, the easier it was to generate the
precious-metal-reducing pattern with lower energy output from the
source 48.
After actinic exposure, the polyimide substrates were immersed in
precious metal salt solutions 19. Some of these solutions and the
parameters connected therewith are listed below:
______________________________________ Time of immersion
(approximate), pH min. ______________________________________
Precious metal salt solution: .01 m. PdCl.sub.2 1.2 2 .01 m.
PtCl.sub.2 2 2 .01 m. PdCl.sub.2 1.0-1.5 1/2 .0057 m. PdCl.sub.2
1.0-1.5 1/2 ______________________________________
Other salt solutions such as those of rhodium, silver or the other
precious metals were used as well.
In any event, after immersion into the solutions 19, the
precious-metal-reducing pattern reduced the precious metal from
such solution 19. The precious metal was reduced in a thin layer,
barely observable under polarized light, in the positive of the
ultimately desired metallic pattern.
Next, the polyimide substrates 11 were bathed in the water rinse 21
for 1 or 2 minutes and then immersed into the electroless plating
bath 20.
The bath 20 may be any type of autocatalytic electroless bath.
Three different types of commercially available baths were used to
electrolessly plate copper onto the precious metal positive pattern
to a thickness of about 1500-200 A. as follows:
______________________________________ Copper Immersion thick-
Time, ness, Bath min. A. ______________________________________
Cuposit (Shipley) .6 1,500.sup.+ Enthone 400 4 1,500.sup.+ Enthone
402 1 1,500.sup.+ ______________________________________
Metals other than copper were deposited. Specifically, nickel and
palladium were electrolessly plated onto the precious metal. In all
cases, the precious metal provided nucleating sites for and
catalyzed the electroless bath 20 and were present only in trace
amounts. Compounds of metals, for example nickel-phosphide, were
also electrolessly deposited.
The substrates 11 with the electroless metallic patterns thereon
were then utilized as shown in FIG. 5. In many cases, however,
thicker metallic patterns are desired. Accordingly, the substrates
11 were then immersed in the electroplating bath 22 to deposit
copper thereon as follows:
______________________________________ Immer- Thickness sion
electro- time, plated, Type of bath 22 pH min. mils
______________________________________ Acid Cu(SO.sub.4).sub.2 1.0
10 2 Copper fluorborate (Allied Chem. Co.) 1.0 4 2
______________________________________
Obviously, other baths, from which metals other than copper can be
electroplated, are usable.
After bathing in the water rinse 23, the substrates 11 with the
metal patterns 72, 74 were dried by the blower 62.
At this point, such patterns 72, 74 were subjected to either the
pattern removal 28 or to the bond enhancement 29.
Some of the metallic patterns 72, 74 were carefully peeled off the
substrate 11. This was able to be done even though the bond between
the patterns and the polyimide was at least as strong as the
pattern-substrate bonds of the prior art. The peeled-off patterns
were subjected to additional electroplating to produce a thick,
electroformed metallic article. Preferably, however, such
additional electroplating was best performed while the metallic
pattern remained on the substrate. This expedient minimized the
possibility of damaging, as by bending, the rather fragile metallic
pattern which, prior to the additional electroplating, is only
slightly in excess of .002 inch thick.
Bond enhancement entailed heating the polyimide substrate 11
patterns 72, 74 structure in an atmosphere conducive to growing an
oxide on the pattern. This technique was especially useful when the
patterns 72, 74 were composed primarily of copper or nickel. The
parameters of such bond enhancement follow:
__________________________________________________________________________
Patterns Tempera- Time, Substrate 11 72, 74 ture, .degree. F. hr.
Atmosphere
__________________________________________________________________________
Polyimide film 165 mil 2 mil Cu 400 1/2 Ambient air. " 2 mil Ni 500
1/2 " Polyimide film 2 mil 2 mil Cu 437 1-5 " Pyr M.L. about 1 mil
2 mil Cu 400 2-5 " " 2 mil Ni 400 2-5 "
__________________________________________________________________________
It is theorized, no dependence on the theory being intended, that
the polyimide substrates 11 are permeable, or somehow previous, to
ambient air. Thus, heat plus the ambient air reaching the
substrate-pattern interface through the substrate, effects the
growth at such interface of an oxide of the metal composing the
pattern. Such growth "drives" the oxide into the substrate, thereby
greatly increasing the bond strength.
Patterns on polyimides produced in the above manner have been
successfully used as circuit boards with electrical components
attached thereto by soldering, T.C. bonding, and other high
temperature, high force bonding techniques without adversely
affecting the pattern, the substrate or the bond between the two.
Moreover, pattern definition of less than 1-2.mu., in both pattern
thickness and separation, was achieved. The only factor seriously
affecting the attainment of such definition appeared to be the
accuracy to which the mask pattern 103 can be put on the cylinder
101 (or 104).
In the above example, the apparatus in FIGS. 2 and 3 was utilized.
Also, the source 49 was continuously energized so that the patterns
72, 74 were produced only on one of the major surfaces of the
polyamide substrate 11.
(II) Example (I) was repeated using only polyimide films, the
initial cleaning step being preceded by the punching of holes
through the substrate 11. The holes were punched within substrate
portions on which the patterns 72, 74 were to ultimately
reside.
The final product was a circuit board made of the substrate 11 and
the pattern 74 with holes 113 having a metallic coating residing
therein. The metallic coating in the holes 113 was contiguous with
the pattern 74. In other words, the method of this invention was
successfully used to produce circuit patterns with plated
through-holes.
(III) Examples (I) and (II) were repeated without energizing the
source 49. Produced were metallic patterns 72, 74 on one side of
the substrate 11 and a metallic coating, or ground plate,
completely covering the opposite side thereof both with (as in
Example II) and without (as in Example I) the plated through-holes
113. When the through-holes were produced, the metal plated on the
walls thereof was contiguous both with the metallic patterns 72, 74
and with the ground plane.
(IV) A substrate 11 of B stage epoxy tape, 8 mils thick was used in
this example. The term "B stage" indicates that the epoxy was not
fully cured. Full curing was effected by heating. An exemplary
material is Fibermat 5 made by 3M Co. The following table shows the
manner in which the subject method was applied to this
substrate:
__________________________________________________________________________
Substance Time pH Other NaOH 1 min. or more or Cleaner 12 Alconox 1
min. or more or Preheating Sufficient to render nascent Water rinse
13 D.I. H.sub.2 O 1-4 min. .3m. SnCl.sub.2 1-4 min. <1
Photopromoter 14 .3m. PbCl.sub.2 1-4 min <1 Sonic agitation and
heat may .3m. TiCl.sub.2 1-4 min. <1 aid in photopromotion.
Water rinse 16 D.I. H.sub.2 O 1-2 min. Drying 15 1/4min. Actinic
radiation 17 30-250 milliwatt-sec./cm..sub.2 at 2,537 A. Sensitizer
19 See Example (I) Electroless bath 20 See Example (I) Water rinse
21 See Example (I) Electroplating bath 22 See Example (I) Water
rinse 23 See Example (I) Drying 24 See Example (I) Pattern removal
May be used. Bond enhancement, 325 .degree. F. 1 hr.
__________________________________________________________________________
Cleaners 12 of NaOH or Alconox rendered the substrate 11 nascent so
that the photopromoter 14 was adsorbed thereon. Preheating also
rendered the substrate nascent by some uncertain mechanism, which
may have been volatilization or evaporation of oily or other
substances which prevent adsorption.
The method was then carried out as set forth previously in
connection with the description of FIGS. 1, 2, and 3, and as
described under Examples I-III. The molar concentration and the
acidity of the photopromoter 14 was increased somewhat over the
general range of values in Example I because it was slightly harder
to effect the adsorption of adequate metallic ion from the
photopromoter onto the substrate surface.
Metallic patterns 72, 74 were produced having definition 1-2.mu..
These patterns could be used to form an electroformed article, or a
circuit pattern after bond enhancement. In the latter event, bond
enhancement 29 is effected by completely curing the B stage epoxy
by heating to about 325.degree. F. for about 1 hour. Heating and
curing firmly bonds the patterns 72, 74 to the substrate 11.
(V) A substrate 11 was produced by coating various bases with a B
stage, two-part, high temperature epoxy resin. An example of
suitable material was a 2-5 mil coating of Scotchcast No. YR5155.
As in the case of the polyimide resin, the surface of the epoxy
resin was found to be nascent. Accordingly, the cleaner 12 and
rinse 13 were not used.
All of the parameters, except bond enhancement, were then the same
as the parameters given for Example IV. Bond enhancement was at
375.degree. F. for about 1 hour. Again, a superior metallic pattern
was produced.
(VI) A fired ceramic material was next used as a substrate 11. The
surface thereof was rendered nascent by cleaning with
trichloroethylene, acetone or heating. The rinse 13 was not used.
The photopromoters 14 listed in Example IV were also used in this
example.
Adsorption of the photopromoters onto the substrate 11 was noted,
but the porous, rough nature of the surface apparently retained the
photopromoter in greater than adsorbed quantities by other
mechanisms. Accordingly even after the drying 15, the actinic
radiation less easily generated the precious metal reducing
pattern, because such radiation had to pass through a layer thicker
than the usual adsorbed layer. Consequently, the range of energy of
the actinic radiation was increased to about 150 to 1000
milliwatt-sec./cm..sup.2.
Such increased radiation produced a good metallic pattern 72, 74
well-adhered to the substrate 11. The remainder of this example was
as in Example I, except that no bond enhancement 29 was used.
(VII) Example I was repeated using a polymerized
tetrafluoroethylene, such as Teflon, as the substrate 11. After
immersion in a cleaner 12 of hot, concentrated NaOH, the production
of the metallic patterns 72, 74 proceeded as in Example I except
for the bond enhancement.
The pattern-substrate bond was found to be considerably less than
the materials of Examples I-VI and no expeditious bond enhancement
29 was found. Accordingly, such a substrate 11 was deemed ideal as
a temporary substrate in producing an electroformed article.
(VIII) Example I was repeated using Mylar, as the substrate 11.
After immersion in a cleaner 12 of dilute H.sub.2 SO.sub.4, the
metallic pattern generation proceeded as in Example I except for
the bond enhancement.
The pattern-substrate bond was found to be similar to that of
Example VII and, therefore, this substrate was deemed ideal as a
temporary substrate in producing electroformed articles.
EXAMPLES OF NEGATIVE PHOTOPROMOTERS
(IX) Fired ceramic was used as the substrate 11. After cleaning
with either trichloroethylene and acetone or alternatively for 5
seconds in a solution of 3 parts HNO.sub.3 and 2 parts HF to render
the substrate nascent, the substrate was bathed in the rinse
13.
A number of negative photopromoters 14 were used, mercuric oxalate
(Hg.sub.2 C.sub.2 O.sub.4) or ferric oxalate
solutions being preferred. The two preferred photopromoter
solutions are either .1 M to .5 M Hg.sub.2 C.sub.2 O.sub.4 or .1 M
to .5 M Fe.sub.2 (C.sub.2 O.sub.4).sub.3, both with a wetting
agent, such as Leconal or Photo Flo plus a trace (.mu. 1/2%) of
HNO.sub.3 added thereto. The wetting agent is used because the
negative photopromoters were not observed to adsorb on the
substrate surface as were the positive. The purpose of the
HNO.sub.3 as an easily volatilized oxidizer has been previously
described.
When the negative photopromotoer was used, two alternate modes of
operation were followed. First, as with the positive
photopromoters, the precious metal solution 19 was kept in the
separate tank 52. Second, the photopromoter 14 and the solution 19
were both put in the tank 44, thus eliminating the tank 52. The
second alternative is possible, of course, because the negative
photopromoters do not reduce the precious metal until exposed to
the actinic radiation.
The precious metal solution 19 comprised a salt solution of any of
the precious metals, .5 M AgNO.sub.3, .5 M K.sub.2 PtClO.sub.3 or
.5 M PdCl.sub.2 being preferred. When the solutions 14 and 19 were
mixed together in the tank 44, their ratio was 4 or 5 parts
photopromoter 14 to one part precious metal salt solution 19. Some
of the photopromoter-precious metal mixtures had added thereto one
of the following solutions which were found to accelerate the
effect of the actinic radiation on the photopromoter. The
accelerators are:
.25 N K.sub.2 PtCl.sub.6
.25 N Na.sub.2 PdCl.sub.4
.5 N AgNO.sub.3
.5 n agBr
.25 N Na.sub.2 PtCl.sub.6
.5 N Hg(NO.sub.3).sub.2
.5 n hgCl (dissolved in Aqua Regia)
The rinse 16 was not used. The table below lists the parameters of
the two process alternatives.
__________________________________________________________________________
Material-solutions 14 and 19, Material-solutions 14 and 19 not
mixed Time mixed Time
__________________________________________________________________________
Cleaner 12 3 parts HNO.sub.3 plus 2 parts HF or Until nascent 3
parts HNO.sub.3 plus 2 parts HF or Until nascent trichloroethylene
followed by and clean. trichloroethylene followed and clean.
acetone. tone. Rinse 13 Not used Not used Photopromoter 14 Hg.sub.2
C.sub.2 O.sub.4 or Fe.sub.2 (C.sub.2 O.sub.4).sub. 3 plus wett- 1-4
minutes 4 or 5 parts .1-.5 m. (Hg.sub.2 C.sub.2 O.sub.4, 1-4
minutes. ing agent plus trace HNO.sub.3. Fe.sub.2 (C.sub.2
O.sub.4).sub.3) plus 1 part .1-.5 m. (PdCl.sub.2, PtClO.sub.3,
AgNO.sub.3) Rinse 16 Not used Not used Dry 15 5-10 minutes 5-10
minutes. Actinic radiation 17 .about.25 milliwatt-sec./cm..sup.2
Precious metal salt solution 19 PdCl.sub.2, K.sub.2 PtCl.sub.6,
AgNO.sub.3 1-4 minutes Not used separately Rinse 21 3-5 minutes 3-5
minutes. Electroless bath 20 Electroplating bath 22 Rinse 23 See
Example (I) See Example (I) Dry 24
__________________________________________________________________________
(X) Example IX was repeated using other substrate materials
including fiberboard, Fiberglas, filled paper, and other
miscellaneous materials. Results like those of Example IX were
achieved.
It is to be understood that the above-described embodiments are
simply illustrative of the principles of the invention. Numerous
other arrangements and modifications may be devised by one skilled
in the art without departing from the spirit and scope of the
invention.
* * * * *