Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVED ELECTROLESS GOLD PLATING
FIELD OF TE~E INVENTION
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- The present invention rela-tes to an improved method
for the electroless or autocatalytic deposition of gold
on substrates; and more particularly to the use of a novel
electroless plating bath for depositing gold on metallic
and non-metallic substrates.
BACKGROUND OF THE INVENTION
In recent years a fairly substantial literature has
developed with respect to the electroless method o-f gold
plating on surfaces. U.S. patents of special interest
both as to the electroless yold plating method and the
problems associated with this procedure include 3,300,348
(I,uce); 3,589,916 (McCormack); 3,389,916 (Gos-tir)i
3,69-l,296 (Bellis); 3,700,469 (Okinaka)i 3,798,056
(Okinaka et al.); 3,917,885 (Baker); as well as the
earlier patents and articles cited therein. Relevant
articles include: Rich, D.W., Proc. American Elec-troplatiny
Society, 58 (1971); Y. Okinaka, Plating 57, 914 (1970);
and Y. Okinaka and CO Wolowodink, Plating, 58, 1080
(1971). This body of literature is pertinent to the
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present invention insofar as it discloses alkali metal
cyanides as the source of the gold or related metal in the
electroless bath as well as the use or alkali metal
borohydrides and arnine boranes as reducing agen-ts. Thus,
for example, the 1970 article by Okinaka as well as his
U.S. Patent 3,700,469 describes an electroless gold plating
bath having the following ingredients:
(1) soluble alkali metal gold complexi
(2) excess ~ree cyanide such as potassium cyanidei
(3) a alkaline agent such as potassium hydroxidei and
(4) a borohydride or an amine borane.
The 1971 article by Okinaka et al. as well as Baker's
U.S. Patent 3,917,885 point out the problems associated with
the use of these particular plating baths, particularly
when the cyanide concentrations increased. Other problems
were encountered when bath replenishment was carriecd out
and the baths became unstable when a plating rate of about
2.5 microns was approached. The need to avoid undesirable
gold precipitation from the baths is also noted.
~0 In U.S. Pa-tent 3,917,885 these problems were overcome
by utilizing, as the gold or related metals source, an alkali
metal imide complex formed from certain special imides~
:[n order to maintain the electroless gold plating a-t the
desired pH of about 11 to 14, the Baker paten-t suggests the
addition to the bath of alkali metal buffering salts such
as the citrates, etc. The need to utilize special imides
in preparing the gold imide complex is an obvious commercial
disadvantage.
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It also has been proposed in Canadian Application
No. 398,196 filed on March 12, 1982 to overcome the
prior art problems by utilizing a trivalent yold metal
complex or compound as the source of the gold in the
plating bath~ Unfortunately, under large scale
commercial conditions it has been found that the sub-
stitution of trivalent gold for the rnonovalent gold
complexes or compounds does not entirely overcome the
problems pertaining to erratic bath stability, metal
deposition, and bath replenishment.
OBJECTS OF THE INVENTION
One objec-t of the present invention is to provide
an improved electroless or autocatalytic gold plating
bath which avoids the problems and disadvantages of the
baths heretofore proposed.
Another object of the present invention is to
provide an improved electroless or autocatalytic gold
plating ba-th which will readily deposit gold on gold
as well as on a variety of metallic and non-me-tallic
substra-tes with good adhesion~
A further object of the present invention is to
provide an improved electroless or autocatalytic gold
plating bath which will readily deposit ductile, lemon
yellow pure gold on substrates at very desirable
commercial rates and thicknesses.
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A still further object of the present invention is
to provide an electroless or autocatalytic gold plating bath,
which has enhanced stability and can be effectively
replenished.
These and other objects will become readily apparent
from the following description of the inventi.on.
SU~/LZ~RY OF T~IE INVENTION
In accordance with the present i.nvention it has now
been found that a further improvement in an electroless
or autocatalytic gold plating bath and gold plating
procedure can be achieved by utilizing as the source of the
gold in the plating bath an admixture (a~ a trivalent
gold metal complex or compound such as alkali metal
auricyanides, alkali metal aurates, or alkali meta.l
aurihydroxides; and (b) a mcnovalent gold metal complex
or compound such as alkali metal gold aurocyanicle. More
particularly, the present invention pertains to
autocatalytic baths and procedures, i.e. where the gold
can be plated on gold as well as on other suitably treated
metallic or non-metallic substrates. Thus, the term
"electroless" as used in this specificati.on is intended
to encompass autocatalytic plating.
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The electroless plati.ng baths of this inventi.on will
also contain a sui~able reducing agent such as an amino
borane or an alkali. metal borohydride, cyanoborohydride,
hydrazine or hyposulfite. The baths will be at a p~ of
from about 10 to 13 and may contain additional ingredients
to attain and/or maintai.n this pH, including an alkaline
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agent such as an alkali metal hydroxide, and a buffering
agent such as an alkali metal citrate. An alkali metal
cyanide is a further optional ingredien-t to improve bath
stability.
In most operations the elec-troless plating bath of
the present invention will be operated at a plating
temperature of from about 50C. up to a temperature at
which the bath decomposes. Typically the operating
temperatures will be from about 50C. to 95C., and
preferably from about ~0 to ~5C.
The substrates to be plated in accordance with the
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` teachings of this invention are preferably metals such
.: as yold, copper, etc. No special pretreatments are
required for these metal subs-trates. Additionally,
nle-tallized ceramic, and non-metallic substrates may also
be plated. Such substrates will, of course, be subjected
to appropriate pretreatments, as are known in -the art,
before plating.
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This invention also provides for replenishing the
electroless plating bath with trivalen-~ gold in the form
of an alkali metal aurate or aurihydroxide solution to
maintain the desired gold concentration of the bath.
This is an important feature of the present inven-tion
.. which Gvercomes the prior art problem of erratic metal
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deposition rates associated with the use of monovalent
gold cyanide. Additional alkaline agent and reducing
agent may also be added during replenishment of the bath
without encountering any untoward results.
DETAILED DESCRIPTION OF r~HE INVENTION
. As previously described, the essential feature of the
present invention is to employ, as the source of gold in
the electroless plating bath; an admixture of trivalent
and monovalent water-soluble gold complexes or compounds.
This is in contrast -to the prior art teachings of using
complexes where the gold was either in the monovalent
state such as, for example, potassium aurocyanide, or in the
trivalent state, for e~ample, potassium auricyanide. In
the present invention, the trivalent gold complex or
compound is an alkali metal auri.cyanide, an alkali metal
. aurate or an alkali metal aurihydroxi.de, with -the preferred
materials being the alkali metal auricyanides and alkali
metal auratesi while the monovai.ent gold cornplex is an
alkali metal aurocyanide. For most purposes the alkali
metal is typically ei.ther po-tassi.uM or sodium, and the use
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of potassium as the a~kali metal is especially preferred. Thus, po-
~ tassium auricyanide, KAu(C~ , or potassium aurate, and pc~tassium
aurocyanicle are pre:Eerably utilizecl i.n formulating the electro]ess gold
plating baths of the present invention. It is to be appreeiated hc~-
ever, tha-t other alkali TTetal and/or ammonium trivalent gold and mono-
valent gold compounds or ccmplexes may also be used and that the term
. ~ "alkali m.etal", as used in the p.resent specification and claims, is
intended -to be understood as including ammonium cc,mpc~nds and cc~.plexes.
The reasons why the admi.xture of trivalent gold and monovalent gold
functions better than trivalent gold or monovalent gold alone in these
plating baths and in this autocatalytic plating process are not fully
understocd at this time. Possible explanatic)ns may be that use of the
mixture (1) leads to better depositicn rates through ease of the re-
duction of the mixed gold metals, (2) enhaneed stability of -the bath,
and (3) ease of replenishment of the bath util.izing the trivalent alkali
metal aurate or ~urihydroxide which overccmes the prior art problems
caused by high concentrations of eyanide icns in the plating bath.
Moreover, the bath is exeeptionally stable tcward high concen-trations oE
redueing agen-ts thereby aehieving faster plating rates than heretoEore
at-tained while maintaining bath s-tabili-ty.
It will be understood that the alkali metal gold (mono and trivalent)
e~anides employed in the praetiee of this invention are water-soluble.
~ever, a variety of ccmpounds whieh can provide the gold eonstituent
in the trivalent and monovalent s-tate may be e~ployed in fo~mulati.ng the
baths.
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m e amount of trivalen-t gold used in admixture with the monova1ent
gold will be at least sufficient to provide stabili-ty to the bath and
prevent cyanide build up in the bath as it is replenishecl. Cenerally,
the weight ra-tio of trivalent gold to monovalent gold in the bath will
be at least abou-t 0.2:1, with ratios from about 0.5:1 to abollt 4:1 being
typical and ratios of frcm about 1:1 to abc~lt 3:1 keing preferred. The
maximum amo~mts of trivalen-t gold have not keen found to be cri-tical.
Thus, although weight ratios up to about 4:1 are typically used, weight
ratios of 10 - 15:1, and even higher, can also be used without encounter-
iny adverse effects il~ the cperation of -the process.
The reducing agents employed in cc~nection with the present elec-
troless plating baths include any of the borohydrides, cyanoboro~lydrides
or amine boranes which are soluble and s-table in ac~leous solution.
Thus, alkali me-tal borohydrides, preferably sodium and potassium boro-
hydrides are utilized, although various substituted borohydrides, such
as sodium or potassium trimethoxyborohyclride, (Na(K)B(CC~13)3H, may also
be employed. Plso preferred are the amine boranes suc~ as mc~no- cmd di-
lc~wer alkyl, e.g. up to C6 alkyl amine boranes, preferably isopropyl
amine borane and dimethylamine borane. ~ther reducing agents such as
hyclrazine and hyposulfite may also be employed.
The electroless mixed valent gold plating baths of the present
invention should be maintained at a pl-l of between about 10 and 13, in
order to ac~ieve the desired resul-ts. It is thus preferred -that an
alkali metal hydroxide, such as sodium or potassium hydroxide, be
employed to main-tain the pl-l at -this level. ~lowever, pH control is
considerably easier when alkali metal bufferiny salts are en~loyed in
addition to the alka]i metal hydroxide.
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Suitable alkali me-tal buffering salts include the alkali
metal phosphates, citra-tes, tartrates, borates, metaborates,
etc. Specifically, the alkali metal buffering salts may
include sodium or potassium phosphate, potassium
pyrophosphate, sodium or potassium citrate, sodium
potassium tartrate, sodium or potassium borate, sodium
or potassium metaborate, etc. The preferred alkali metal
buffering salts are sodium or potassium citrate and sodium
or potassi.um tartrate.
In order to further improve the electroless plating
baths of this invention, it is desirable in some instances
to provide further chelating capacity by the addition of
an organic chelating agent such as ethylenediamine
tetraacetic acid, and the di-sodium, tri~sodium and tetra-
sodium and potassium salts of ethylenediamine tetraacetic
acid, di-ethylene triamine pentacetic acid, nitrilotriacetic
acid. The ethylenediamine tetraacetic acid, and its di-,
.`~ tri-, and tetrasodium salts are the preferred chelatiny
agents, with the tri- and tetra-sodium salts being
particularly preferred.
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In addition -to the foreyoing ingredients, the
electroless plating baths of this invention may also contain
alkali metal cyanides, and more particularly the potassium
or sodium cyanides. Such ingredients are added when
greater stability for the autoca-talytic process is
required. When employed, the amount oE alkali metal
cyanide may range from about l to 20 grams per liter,
which is far in excess of the minor critical amounts
employed by McCormack, which at a maximum were 500
milligrams per liter.
In the electroless plating baths of the present
invention, the gold compounds or complexes will be
present in an amount at least sufficient to deposit
gold on the substrate to be plated, up to their maximum
solubility in the plating bath. The reduciny agent is
present in an amount a-t least sufficient to reduce -the
gold, again up to its maximum solubility in the bath.
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The alkaline agent and buffering agen-t are each presen-t in
an amount sufficient to provide and main-tain the desired
bath p~l.
More specifically, the components of the electroless
pla-ting baths of this invention will be present in amounts
within the following ranges:
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Amounts grams/l _er
Components, TypicalPreferred
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(1) Gold (III) as the alkali
metal auricyanide, aurate
or aurihydroxide 0.5-~ 1-4
(2) Gold (I) as the alkali
metal aurocyanide0.5-3 1-2
(3) Reducing agents, as amino
borane, alkali metal
borohydride, cyanoboro-
hydride, etc. 1-15 2~10
: (4) Alkaline agent 10-50 20-40
(5) Buffering agent, as
alkali metal salt10-40 20-30
(6) Alkali metal cyanide
(when present) 1-20 1-10
(7) Organic chelatlng agent
(when present) 2-25 3-15
(8) Water to make one liter
As previously set forth, the pH of the bath is maintained
at a range of about 10 to 13, and in some instances
between about 11 to 13. The typical operational
temperature during plating is from about 50 to 95C.,
preferably from 60 -to 85C. For most purposes, the
plating rates will be up to 8 microns per hour, and
preferably at least about 2 microns per hour.
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Although this invention has been described above
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primarily in conjunction with electroless gold baths,
it should be unders-tood tha-t one or more alloying metals
such as copper, zinc, indium, tin, etc. may be added to the
e]ectroless baths. Where these are employed, they are
A added to the bath as a suitable soluble salt in amounts
suffi.cient to provide up to about 20 percent by weigh-t
of the alloying metal or metals in the gold deposit.
In accordance with the preferred ~eatures of the
present invention the substrates to be plated by the
electroless gold baths are metals such as gold, copper,
copper alloy, electroless copper, nickel, electroless
ni.ckel, nickel alloys, and the like. ~hus, where a
metallic substrate is employed, such surfaces include
all metals which are cata]yti.c to the reduction of the
metal cations dissolved in the described baths. In
some cases it is preferred to further sensitize the
substrate by trea-tments well known to those skilled in
this art. Moreover, it is possible to use ni.ckel, cobalt,
iron, steel, palladium, platinum, copper, brass, manganese,
chromium, molybdenum, -tungsten, titanium, tin, silver,
etc. as the metal subs-trates upon which the gold is to be
plated.
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With the use of non-metallic substrates, however,
these surfaces must be rendered catalytically active by
producing a film of catalytic particles thereon. This
may be done by the method described in U.S. Pa-tent No.
3,5~9,gl6, upon sueh surfaces as glass, ceramics, various
plastics, ete. When a plastic substrate is to be plated
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according to the present invention, i-t is initially etehed,
preferably in a solution or ehromic and sulfuric acid.
AEter rinsing, the substrate is immersed in an acidic
solution of stannous chloride, such as stannous chloride
and hydrochloric acid, rinsed with water and then contacted
with an acid solution of a preeious metal, such as
paladium chloride in hydrochloric acid. Subsequently,
the now catalytically active non-metallic substrate
may be contacted with the eleetroless plating solutions
of this inventi~n.
The method of utilizing the present invention involves
primarily the immersion of the metallie or non-me-tallic
substrates into the electroless pla-tincJ baths. These baths
are maintainecl at the pH deseribed above, while the platincJ
is earried out at the aforemen-tioned temperatures.
Commereially desirable thicknesses of gold metal deposits
have been achievecl without encoun-tering the bath instability
and other prior ar-t problems. The necessary adhesion
characteristics were also readily achieved by the practiee
of the present invention.
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A still further aspect of the present invention is
the ability to replenish the bath without encounteriny
difficulties. It has been found, for example, that aside
from adding additional alkaline acJent, such as potassium
hydroxide, and reducing agent, replenishment of the gold
content may be accomplished by adding an alkali metal
aurihydroxide or alkali metal aurate to the bath. This
replenishment of the bath with water-soluble components
is accomplished without adverse effect on either the
bath plating rate or the bath s-tability.
DESCRIPTION OF_THE PREFERRED EMBODI~ENTS
The invention will be more fully understood by
reference to the following illustrative embodiments.
__A2~1PLE I
An electroless plating bath was formu1ated from the
ingredien-ts set forth below:
Ingredients Amount g/l
Gold, as KAu(CN)L~ 2
Gold, as KAu(CN)2 2
Potassium Hydro~ide 35
Tripotassium Citrate 30
Dimethyl Amino Borane 6
The pH of the resulting bath was about ll to 12.
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The bath was used to plate gold on gold, copper, and
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CGpper alloys (48 square inches per liter) at 80 deyrees
C. The plating rate was 5 micror,s/hour. Deposits from
this bath were ductile, lemon yellow, pore-free pure gold
with excellent adhesion to the substrates.
During a number of runs the bath can be replenished
by the addition of gold as a gold (III) complex with
alkali metal hydroxide or aurate. No cyanide buildup
in the bath was observed and a consistent metal deposition
rate was maintained. As compared to the known baths,
excellent stahility was achieved here as evidence by the
consumption of about 100% of the gold metal content in
the bath, with 5 to 6 replenishments, without encountering
erratic metal deposition or bath instability.
X~MPLE II
An electroless plating bath was formulated as
follows:
Ingredients _Amount g~l_
Gold, as KAu(CN)4 3
Gold, as KAu(CN)2
Tripotassium citrate 30
Potassiurn Hydroxide 35
Dimethylamine Borane 15
Deposits were obtained on copper ancl copper alloys
at a plating rate approaching 2 to 8 microns per hour with
the bath at a pH of 11-12 and a ternperature of 80-85C.
The replenishment procedure of ~xarnple I was employed with
equally good results.
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_A~PLE III
Ano-ther electroless plating bath was formulated
as follows:
Ingredients Amount g/l
5 Gold, as KAuO2 3
Gold, as KAu(CN)2
Potassium Cyanide 10
Potassium Hydroxide 30
Dimethylamine Borane 4
~eposits were obtained on gold at a rate of 3 microns
per hour with the ba-th at 82C. and a-t a pH of 12-13.
The replenishment procedure of Example 1 was fol]owed
using gold (III) hydroxide -to attain a consistent
deposition ra-te and to avoid an undesirable builcl up of
lS cyanide ions.
The above data show that -the improved electroless
bath of this invention leads to superior results and
avoids the problems or the commercial disadvantages
associated with the previously proposed electroless gold
metal baths.
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t will be further understood that the foregoing
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examples are illustra-tive only and that variations and
modifica-tions may be made without depar-ting from the
scope of this invention. Thus, for e~ample, the plating
bath may ~e initially formulated with monovalen-t gold
i.e. an alkali metal gold aurocyanide, and then replenished
with an alkali metal aurate or aurihydroxide whereby a
bath containing both monovalent and trivalent gold
constituents is produced.
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