Field of the invention
[0001] The present invention relates to a liquor for use as an electroless gold deposition
bath and to a process for depositing gold on a substrate using such a liquor.
Background to the invention
[0002] An electroless (autocatalytic) deposition system has two main advantages over conventional
electroplating. It can deposit metal on electrically isolated or non-conducting substrates
and it can deposit an even thickness of metal over a component, regardless of geometry.
For some time the electronics industry has been searching for a truly autocatalytic
gold deposition process. Most of those previously developed have proved unsuitable
due to the instability of the solutions. A survey of previously proposed electroless
gold deposition systems has been provided by H.O. Ali and I.R.A. Christie, "A review
of electroless deposition processes", Gold Bull., 1984,
17(4), pages 118-127 (the teaching in which article is incorporated herein by reference).
[0003] The term "autocatalytic deposition" means that the metal already deposited on the
workpiece acts as a catalyst for further deposition of the same metal from the solution
onto the workpiece. In galvanic deposition, for example copper deposition from an
acidic solution onto an iron substrate, the more noble metal (Cu) replaces the less
noble metal (Fe) on the surface. However, once the surface is covered with copper
the reaction stops. In galvanic (immersion) gold deposition baths, the more noble
metal (Au) replaces less noble metal, in particular either copper or nickel, on the
surface of the component, until it is covered with gold, whereupon the reaction ceases.
The maximum thickness of gold deposited by this method is 0.1-0.2 µm. An autocatalytic
bath, however, will deposit more metal on the same metal substrate and, in theory,
assuming that all the operating parameters are within their limits, will continue
to deposit the desired metal to an unlimited thickness.
[0004] In an autocatalytic bath metal ions are reduced to metal atoms by electrons provided
by the oxidation of a reducing agent. For electroless gold deposition wherein the
reducing agent is potassium borohydride, the oxidation consists of two steps:
(1) BH₄⁻ + H₂O → BH₃OH⁻ + H₂
(2) BH₃OH⁻ + H₂O → BO₂⁻ + H₂
[0005] At optimum agitation, that is when the reaction is not diffusion controlled, the
formation of BH₃OH⁻ is the rate determining step. The reduction of Au(CN)₂⁻ to gold
metal by BH₃OH⁻ in a bath containing KAu(CN)₂, KCN, KOH and KBH₄ is investigated by
Y. Okinaka in "An electrochemical study of electroless gold-deposition reaction",
J. Electrochem. Soc., Vol. 120, No. 6 (June 1973), pages 739-744 (the teaching in
which article is incorporated herein by reference).
[0006] Electroless deposition baths are prone to spontaneous decomposition, which is the
sudden precipitation of metal ions from the solution as metal particles (sometimes
known as "plating up"). In an attempt to prevent this all commercial electroless baths
have hitherto contained stabilisers, in particular alkali metal cyanides such as KCN.
Metallic impurities in the bath may also cause this decomposition and so chelating
agents are added to complex any dissolved metals. Accelerators are often added to
increase deposition rates.
[0007] For successful electroless (autocatalytic) deposition it is considered important
that all operating parameters should be closely controlled. It is therefore accepted
that the tank used to contain the solution must be properly constructed from a suitable
material and must be kept clean. The solution should be continuously filtered (up
to ten solution turnovers per hour is common for electroless nickel deposition). The
temperature must be controlled to within plus or minus 5% of the optimum value, agitation
must be sufficient but not too vigorous and bath loading should be maintained at the
optimum level. In electroless nickel systems low or high bath loadings can cause spontaneous
decomposition, the optimum being 0.5-1 dm²/l. Baths must be analysed frequently and
kept at their optimum by small, regular additions of the necessary chemicals. Automatic
dosing meters are often used for this purpose. Thus, the operation of an autocatalytic
(electroless) deposition system has required a conscientious and skilled person.
[0008] Despite such measures, electroless gold deposition systems have hitherto still suffered
from the problems of instability, in particular a variable rate of deposition (typically
varying from 0 to 3 µm per hour) and a tendency to precipitate gold in bulk suddenly.
Furthermore, such systems cannot be replenished and, therefore, are usually operated
to exhaustion or until precipitation occurs. Clearly, then, there is a need in the
art for an electroless system which is stable, replenishable and capable of depositing
gold of high purity at an acceptable rate. Desirably, such a system should be suitable
for wire bonding and should have a robustness approaching that of the electroless
copper or nickel systems already available, in order to reduce the demand on operator
skill.
Summary of the invention
[0009] The present invention provides an aqueous liquor for use as an electroless gold deposition
bath, comprising a source of gold and a reducing agent, which liquor also contains
a reduction-stabilising agent selected from (a) a mixture of an alkali metal or ammonium
ferrocyanide and an alkali metal or ammonium ferricyanide, (b) 1-H-tetrazole, (c)
redox mediators and (d) mixtures of any of these.
[0010] The present invention also provides a method for the electroless deposition of gold,
onto a surface of a substrate, from an aqueous liquor comprising a source of gold
and a reducing agent, wherein the liquor also contains a reduction-stabilising agent
selected from (a) a mixture of an alkali metal or ammonium ferrocyanide and an alkali
metal or ammonium ferricyanide, (b) 1-H-tetrazole, (c) redox mediators and (d) mixtures
of any of these.
[0011] The present invention also provides an article having at least one surface onto which
gold has been deposited by such a method.
Description of preferred embodiments
[0012] The aqueous liquor according to the present invention contains a source of gold in
solution. In principle, any of the gold compounds, including salts and complexes,
that have been used or proposed previously for use in electroless gold deposition
(plating) solutions come into consideration, these including such compounds as MAu(CN)₂,
MAu(CN)₄, MAu0₂ and MAu(0H)₄ wherein M is ammonium or alkali metal, in particular
potassium; M₃Au(S0₃)₂ wherein M is ammonium or alkali metal, in particular sodium;
AuCN; and alkali metal gold imides, in particular potassium gold succinimide or potassium
gold phthalimide. However, the alkali metal gold cyanides are particularly preferred,
especially potassium gold cyanide, which may be represented by the formula KAu(CN)₂.
[0013] It will be understood that the gold compound may be formed
in situ. Thus, for example, with the addition of a sufficient (at least stoichiometric) amount
of cyanide ions, any gold in the solution will, in effect, be present as a cyanide
complex. Also, due to the presence of the reducing agent, Au
III will tend to be converted into Au
I in the solution.
[0014] Normally, the liquor will have a gold concentration (expressed as elemental gold)
of up to 10 g/l and it has been found that the concentration is desirably at least
0.25 g/l in order to achieve an acceptable deposition rate. Although gold concentrations
as high as 3.5 g/l have been tested satisfactorily, the gold concentration is preferably
maintained at a level of between 0.8 and 1 g/l in order to minimise the effects of
dragout and to ensure a good distribution of the deposit.
[0015] The aqueous liquor according to this invention also contains a reducing agent in
solution. Various reducing agents have been proposed in the prior art for use in electroless
gold plating solutions, including hypophosphites, formaldehyde, hydrazine and boron-based
compounds such as borohydrides and amine boranes (eg. isopropyl amine borane, di-
or tri-ethylamine borane and di- or tri-methylamine borane), although the amine boranes
can be unpleasant to use and are expensive. However, the alkali metal borohydrides,
especially potassium borohydride, are preferred, these being particularly effective
in conjunction with the alkali metal gold cyanides.
[0016] The reducing agent will usually be present in an amount of 1 to 25 g/l, preferably
5 to 15 g/l. Potassium borohydride, for example, may be used typically at a level
of 5 to 21.6 g/l.
[0017] In accordance with the present invention, the aqueous liquor also contains a reduction-stabilising
agent, that is to say an agent that is intended to stabilise the reaction(s) whereby
the gold is reduced in order to reduce or inhibit the tendency to random deposition
varying from the very slow to the very sudden or even to the spontaneous precipitation
of the gold. Depending upon its nature, the reduction-stabilising agent is generally
employed in an amount of from 0.25 to 100 g/l, more usually 1 to 10 g/l.
[0018] For this purpose, the aqueous liquor in certain preferred embodiments may contain
in solution an alkali metal or ammonium ferricyanide and an alkali metal or ammonium
ferrocyanide; the potassium compounds, K₃Fe
III(CN)₆ and K₄Fe
II(CN)₆, are preferred. Preferably, 0.25 to 3 parts by weight of alkali metal or ammonium
ferricyanide, more preferably 0.4 to 2.5 parts by weight thereof, will be used per
part by weight of alkali metal or ammonium ferrocyanide. Subject to such ratios, potassium
ferrocyanide and potassium ferricyanide, for example, may be used typically at levels
of, respectively, 1 to 50 g/l and 0.5-20 g/l.
[0019] In certain other preferred embodiments, the aqueous liquor contains 1-H-tetrazole
as a reduction-stabilising agent. Although 1-H-tetrazole - which may be used typically
at levels of 1 to 10 g/l - may be used as the sole such agent, useful results have
also been obtained using it in the presence of an alkali metal ferricyanide, especially
K₃Fe
III(CN)₆.
[0020] Other reduction-stabilising agents may be selected from the class of compounds known
as redox mediators (oxidation-reduction mediators). Redox mediators are known in biochemistry
and cell biology as chemicals that promote transfer of electrons essential to the
analysis of enzymes and tissues and have recently been proposed as additives in microbial
fuel cells as they couple rich sources of electrons within micro organisms to an electrode
(see J. Bennetto, "Microbes come to power", New Scientist, 16 April 1987, pages 36-39).
Redox mediators which come into consideration are, for example, thionine (at a level,
for example, of 1 to 5 g/l), resorufin (at a level, for example, of 1 to 5 g/l) and
1,10-phenanthroline (at a level, for example, of 0.25 to 1 g/l).
[0021] It will, of course, be understood that only those redox mediators that are compatible
with the hot alkaline liquor used in the plating process can be used. For example,
tetrazolium violet, 2-hydroxy-1,4-naphthoquinone and 2,3-epoxy-2,3-dihydro-1,4-naphthoquinone
have been found to cause bulk precipitation of gold from the plating liquor.
[0022] It is known that electron transfer between [Fe(CN)₆]⁴⁻ and [Fe(CN)₆]³⁻ is very fast
(see A.G. Sharpe, The Chemistry of Cyano Complexes of the Transition Metals, Academic
Press (1976), pages 115-120) and, although the Applicants do not wish to be bound
by any theory, it is thought that the ferrocyanide/ferricyanide mixtures may act as
electron reservoirs thereby reducing or eliminating the tendency to spontaneous decomposition
and thus may, like 1-H-tetrazole, act in a similar manner to the redox mediators in
stabilising the gold-reduction (Au
I → Au°) process.
[0023] The pH of the aqueous liquor may be adjusted, as appropriate, for the deposition
to proceed properly. When employing an alkali metal borohydride as the reducing agent,
the pH of the liquor has not been found to be a critical parameter: as long as there
are sufficient hydroxide ions for the formation of BH₃OH⁻ ions, the deposition reaction
will proceed. Thus, an alkaline pH, usually at least 11 and conveniently between 11
and 11.5, will in general be maintained when using such a borohydride, and may also
be appropriate if a different reducing agent is employed. The liquor may contain alkali
metal hydroxide, eg. sodium or potassium hydroxide (which latter, for example, may
be used typically at a level of 5 to 22.4 g/l), to achieve the alkaline pH and/or
a buffer salt may be used, such a buffer salt being selected, for example, from the
alkali metal (especially sodium or potassium) orthophosphates, pyrophosphates, citrates,
tartrates, borates and metaborates.
[0024] In order to improve the performance of the aqueous liquor of this invention as an
electroless gold deposition bath, it is preferred to add an organic chelating agent,
which serves to complex or otherwise combine with metal ions present as impurities
in the plating solution and thereby prevent their interference with the deposition
or plating process. Such chelating agents are generally employed in an amount of from
0.1 to 100 g/l, preferably 1 to 20 g/l, and include ethylenediaminetetraacetic acid
(EDTA) and the alkali metal salts thereof, diethylenetriaminepentaacetic acid and
the alkali metal salts thereof, nitrilotriacetic acid and the alkali metal salts thereof,
ethanolamine and triethanolamine. EDTA has been found to be particularly effective
in complexing nickel in solution and may be used typically at a level of 1 to 10 g/l.
Ethanolamine and triethanolamine may be used typically at levels of, respectively,
25-75 ml/l and 5-50 ml/l.
[0025] Normally, the aqueous liquor according to the present invention will contain an alkali
metal cyanide, such as sodium, potassium or lithium cyanide, in order to improve the
stability of the source of gold, in particular when a gold cyanide complex is used.
In general, such cyanides are employed in an amount of 0.1 to 50 g/l, preferably 1
to 25 g/l. Amongst such auxiliary stabilisers, potassium cyanide is particularly preferred
and may be used typically at a level of 5 to 22 g/l.
[0026] It is also possible to include one or more other auxiliary, stabilisers. Alpha-hydroxynitriles
have been proposed as stabilisers in electroless deposition baths (see US-A-3,589,916)
but for the purposes of the present invention, the use of glycine has been found to
be beneficial. It is possible that the glycine, which may typically be used at a level
of 1 to 10 g/l, acts as a chelating agent within the aqueous liquor.
[0027] The aqueous liquor of the present invention may also contain an accelerator in order
to improve the rate of deposition. As taught in US-A-4,307,136, water-soluble salts
of semi-metals and metals of Groups IIIB, IVB and VB of the Periodic Table (especially
those elements in the 4th, 5th and 6th periods), such as thallium, lead or arsenic,
may be used for this purpose, although in certain embodiments the presence of such
an accelerator has increased the sensitivity of the bath to the presence of nickel
contaminants.
[0028] The concentration of the accelerator (expressed as elemental metal or semi-metal)
will generally be from 0.001 to 500 mg/l, more usually 0.001 to 100 mg/l. Preferred
accelerators are thallium (eg. as thallium sulfate), which can be used typically at
a level of 0.001 to 7 mg/l (calculated on the elemental thallium), and lead (eg. as
lead sulfate), which can be used typically at a level of 2 to 50 mg/l (calculated
on the elemental lead).
[0029] It will be understood, of course, that any of the specified components (gold source,
reducing agent, reduction-stabilising agent, chelating agent, auxiliary stabiliser,
and accelerator) may be constituted by a mixture of two or more compounds of the appropriate
description. When alkali metal compounds have been mentioned above as possible components,
the corresponding ammonium compounds may also come into consideration; however, the
potassium compounds are usually the least expensive and the most readily available
in high purity.
[0030] The aqueous liquor of the present invention may be employed in conventional manner
as an electroless gold deposition bath. Thus, the substrate to be plated will be immersed
in the bath for a period of time sufficient to achieve a deposit of gold of the desired
thickness.
[0031] Since the electroless deposition of the gold is an autocatalytic process, the substrate
should present a catalytically active surface, especially a surface of a metal such
as nickel, cobalt, iron, steel, palladium, platinum, copper, brass, manganese, chromium,
molybdenum, tungsten, titanium, tin, silver, kovar and permalloy. However, and especially
in cases where contamination of the bath by dissolution of the substrate or base metal
will adversely affect the purity of the gold deposit and the stability of the plating
bath, it would be possible to carry out a pre-plating step in order to provide a thin
deposit of gold on the substrate surface, for example using a galvanic gold deposition
process, for instance the "Atomex" process (as described in US-A-3,230,098).
[0032] If a nickel undercoat is deemed necessary, it is recommended that nickel-boron type
(ENi-B) electroless nickel deposits be used as the undercoat in preference to nickel-phosphorus
type (ENi-P) electroless nickel deposits (since the present formulation will not deposit
gold on the latter). If an ENi-P undercoat has to be used, it should be flashed with
ENi-B prior to the electroless plating with gold.
[0033] A pre-dip containing potassium borohydride and potassium hydroxide (typically 5 g/l
of each) has proven useful in accelerating the initiation of the gold deposition,
protecting the gold bath from contamination and reducing the heat-sink effect of large
components. Immersion of the substrate in the pre-dip for 1 minute at 80°C has been
found to be suitable.
[0034] Non-metallic substrates may be prepared for gold-plating in accordance with this
invention by first rendering the surfaces thereof catalytically active, for example
by the method described in US-A-3,589,916.
[0035] Where the substrate is provided by an article that also comprises surfaces which
do not require to be plated, it is possible to mask such surfaces in known manner,
the masking material being removed after the plating step.
[0036] Articles which may be gold-plated or gold-metallised in accordance with this invention
include electronics components, especially those which have electrically isolated
islands, pads and tracks, for example microwave components, chip carriers, printed
circuit boards, integrated circuits and transistor headers.
[0037] The gold deposition bath will normally be operated at a temperature of 80°C ± 5°C.
If the temperature exceeds 85°C, there may be a significant risk of spontaneous decomposition
whereas below 75°C there is a significant reduction in the rate of gold deposition.
During operation, the bath should be agitated, for example by magnetic stirring. Preferably,
the rate of stirring is from 200 to 600 rpm; higher rates of stirring may cause spontaneous
decomposition, while lower rates of stirring will reduce the rate of gold deposition
on the substrate.
[0038] Surface loadings as low as 0.5 dm²/l and as high as 10 dm²/l have been tested satisfactorily;
the tolerance to loadings as high as 10 dm²/l is surprising, since prior-art autocatalytic
systems would be expected to undergo spontaneous decomposition at loadings as high
as this. However, the recommended surface loading for practical applications is from
1 to 3 dm²/l, depending upon the bath volume.
[0039] As the deposition or plating reaction proceeds, the components in the bath liquor
may be replenished as appropriate. Thus, the source of gold will need to be replenished
so as to maintain the concentration of gold at the required level. In principle, the
gold compound added by way of replenishment need not be the same as the gold compound
used in making up the initial liquor.
[0040] Potassium borohydride decomposes rapidly at 80°C. Accordingly, when this reducing
agent is used, it is desirable to analyse the bath every thirty minutes during operation
and to replenish the potassium borohydride (the concentration of which typically decreases
by 30% for each thirty minutes that the bath is at the working temperature), as appropriate.
The accelerator, if used, normally also requires to be replenished frequently. The
ferrocyanide and ferricyanide if used, have also been found to require frequent replenishment.
Examples
[0041] The present invention is illustrated in and by the following examples.
[0042] The aqueous solutions were made up in a new, or at least scratch-free, beaker with
carbon-treated demineralised water (the carbon treatment being effected in order to
remove any colloidal polymer from the ion-exchange resin). Distilled water could have
been used instead. A solution of the accelerator (if used), the reducing agent and
a solution of the gold source were added, in that order and with stirring, to an aqueous
liquor already containing the reduction-stabilising agent, chelating agent and auxiliary
stabiliser(s). The solution was heated to 80°C and filtered if any particulate matter
was visible therein.
[0043] The exemplary formulations were investigated as electroless gold deposition baths
using life tests on substrates provided (unless otherwise stated) as copper panels
electroplated with pure gold to 2.5 µm and at a surface loading of 0.5 dm²/l, since
at such a low loading any weaknesses in a system under test tend to be quickly revealed.
In these tests, the baths were operated at a temperature of 80°C ± 4°C, with magnetic
stirring with a PTFE coated stirrer at 400 rpm.
[0044] The concentration of the reducing agent was analysed every 30 minutes and replenishment
was effected with the required amount dissolved in the minimum quantity of carbon-treated
demineralised water. The gold and other constituents were replenished after each 0.25
g of gold had been removed from solution. Filtration, through two glass-fibre filter
papers under vacuum, was effected if particles of gold appeared on the bottom of the
beaker or if the solution (initially an orange/yellow colour, turning to very pale
yellow upon heating) turned light brown.
Example 1
[0045] An aqueous liquor was prepared with the following formulation:
gold (present as KAu(CN)₂) |
1 g/l |
potassium orthophosphate, K₃PO₄ |
12 g/l |
potassium cyanide, KCN |
11 g/l |
potassium borohydride, KBH₄ |
10.8 g/l |
monoethanolamine |
50 ml/l |
potassium ferrocyanide |
5 g/l |
potassium ferricyanide |
2 g/l |
thallium (present as the sulfate) |
2 mg/l |
[0046] The liquor was tested as an electroless gold-deposition bath and it was found that
an acceptable rate of deposition (2.5-3.0 µm/h) could be achieved without precipitation
(plating up) of gold into the liquor.
Example 2
[0047] An aqueous liquor was prepared with the following formulation:
gold (present as KAu(CN)₂) |
1 g/l |
potassium cyanide |
11.0 g/l |
potassium hydroxide |
11.2 g/l |
EDTA |
5.0 g/l |
potassium borohydride |
10.8 g/l |
potassium ferrocyanide |
5.0 g/l |
potassium ferricyanide |
2.0 g/l |
monoethanolamine |
50 ml/l |
[0048] When tested, this liquor was found to provide a robust bath, which was resistant
to nickel contamination. The deposition rates were, in general, about 1 µm/h. Initiation
of deposition on nickel was found to take, in general, from 5 to 15 minutes; the contact
with the nickel surface in this test did not cause spontaneous decomposition of the
liquor.
Example 3
[0049] An aqueous liquor was prepared with the following formulation:
gold (present as KAu(CN)₂) |
1 g/l |
potassium cyanide |
11.0 g/l |
potassium hydroxide |
11.2 g/l |
EDTA |
5.0 g/l |
potassium borohydride |
10.8 g/l |
potassium ferrocyanide |
5.0 g/l |
potassium ferricyanide |
2.0 g/l |
triethanolamine |
10.0 ml/l |
glycine |
4.5 g/l |
[0050] When tested, this liquor provided a robust bath, resistant to nickel contamination.
The gold turnover was between 150 and 200%, with a mean deposition rate of 2.5 µm/h.
Initiation of the deposition on nickel was immediate; the contact with the nickel
surface in this test did not cause spontaneous decomposition of the liquor.
Example 4
[0051] An aqueous liquor was prepared with the following formulation:
gold (present as KAu(CN)₂) |
1 g/l |
potassium cyanide |
11 g/l |
potassium hydroxide |
11.2 g/l |
potassium borohydride |
10.8 g/l |
monoethanolamine |
50.0 ml/l |
EDTA |
5.0 g/l |
potassium ferrocyanide |
5.0 g/l |
potassium ferricyanide |
2.0 g/l |
thallium (present as the sulfate) |
2.0 mg/l |
[0052] When tested, this aqueous liquor provided a robust, general purpose bath which proved
particularly suitable for depositing gold on copper and copper alloys. The bath could
normally be operated to 200-250% gold turnover, with a mean deposition rate of 2-2.5
µm/h, at a low loading of 0.5 dm²/l, the gold being replenished with further KAu(CN)₂.
Such replenishment was found to have no adverse effect on the free cyanide concentration.
[0053] The potassium ferrocyanide, potassium ferricyanide, thallium and monoethanolamine
were replenished as the bath was used, at rates of 1.25 g, 0.5 g, 0.5 mg and 2.5 ml
respectively, per 0.25 g gold removed by deposition.
[0054] The bath liquor (unlike certain prior-art formulations) did not spontaneously decompose
when presented with a nickel surface. Initiation of the deposition on nickel was immediate.
The bath liquor was, however, sensitive to soluble nickel contamination, it being
found that a concentration of nickel of 10 mg/l could cause spontaneous decomposition.
This sensitivity could be overcome by omitting the thallium from the solution.
[0055] The deposits obtained from the baths of Examples 2, 3 and 4 were 99.9% pure gold
(with 0.1% K) having a density of 19.3 g/cm³ and a hardness of between 90 and 95 HV.
The deposits which were up to 30 µm in thickness were matt and lemon yellow (Examples
2 and 4) or orange-yellow (Example 3) in colour.
Example 5
[0056] An aqueous liquor was prepared with the following formulation:
gold (present as KAu(CN)₂) |
1.0 g/l |
potassium cyanide |
11.0 g/l |
potassium hydroxide |
11.2 g/l |
EDTA |
5.0 g/l |
potassium borohydride |
10.8 g/l |
monoethanolamine |
50.0 ml/l |
thallium (present as the sulfate) |
2.0 mg/l |
1-H-tetrazole |
1.0 g/l |
[0057] When tested as a gold plating bath, the liquor reached 188% gold turnover, with a
mean deposition rate of 2.07 µm/h. The gold deposit was comparable in properties to
those of the preceding Examples.
Example 6
[0058] An aqueous liquor was prepared with the following formulation:
gold (present as KAu(CN)₂) |
1.0 g/l |
potassium cyanide |
11.0 g/l |
potassium hydroxide |
11.2 g/l |
EDTA |
5.0 g/l |
potassium borohydride |
10.8 g/l |
monoethanolamine |
50.0 ml/l |
potassium ferricyanide |
2.0 g/l |
thallium (present as the sulfate) |
2.0 mg/l |
1-H-tetrazole |
1.0 g/l |
[0059] When tested as a gold plating bath, the liquor reached 225% gold turnover, with a
mean deposition rate of 2.05 µm/h. The gold deposit was comparable in properties to
those of the preceding Examples.
Comparison Example A
[0060] An aqueous liquor was prepared with the following formulation:
gold (present as KAu(CN)₂) |
1.0 g/l |
potassium cyanide |
11.0 g/l |
potassium hydroxide |
11.2 g/l |
potassium borohydride |
10.8 g/l |
[0061] When tested, the above solution gave deposition rates of about 1 µm/h and proved
to be prone to spontaneous decomposition, especially when in contact with nickel.
[0062] It will be seen from the foregoing description that in accordance with the present
invention it is possible to formulate aqueous liquors for electroless gold deposition
baths that are replenishable, that are stable (in particular resistant to precipitation)
and that give acceptable and reasonably constant rates of gold deposition with good
distribution.
[0063] It will of course be understood that the present invention has been described above
purely by way of example, and modifications of detail can be made within the scope
of the invention.
1. An aqueous liquor for use as an electroless gold deposition bath, comprising a
source of gold and a reducing agent, characterised in that it also contains a reduction-stabilising
agent selected from (a) a mixture of an alkali metal or ammonium ferrocyanide and
an alkali metal or ammonium ferricyanide, (b) 1-H-tetrazole, (c) redox mediators and
(d) mixtures of any of these.
2. A liquor according to claim 1, characterised in that it contains potassium ferrocyanide
and potassium ferricyanide.
3. A liquor according to claim 1 or 2, characterised in that the source of gold is
an alkali metal gold cyanide.
4. A liquor according to claim 1, 2 or 3, characterised in that the reducing agent
is an alkali metal borohydride.
5. A liquor according to any of claims 1 to 4, characterised in that it also contains
a chelating agent.
6. A liquor according to claim 5, characterised in that the chelating agent is EDTA
or a salt thereof, monoethanolamine, triethanolamine or a mixture of any of these.
7. A liquor according to any of claims 1 to 6, characterised in that it also contains,
as an auxiliary stabilizer, glycine, an alkali metal cyanide or a mixture thereof.
8. A liquor according to claim 7, characterised in that it contains, as an auxiliary
stabilizer, potassium cyanide.
9. A liquor according to any of claim 1 to 8, characterised in that it contains thallium
and/or lead.
10. A liquor according to any of claims 1 to 9, characterised in that it contains
an alkali metal hydroxide and/or a buffering agent to maintain the liquor at a pH
at which oxidation of the reducing agent takes place.
11. A method for the electroless deposition of gold, onto a surface of a substrate,
from an aqueous liquor comprising a source of gold and a reducing agent, characterised
in that the said liquor also contains a reduction-stabilising agent selected from
(a) a mixture of an alkali metal or ammonium ferrocyanide and an alkali metal or ammonium
ferricyanide, (b) 1-H-tetrazole, (c) redox mediators and (d) mixtures of any of these.
12. A method according to claim 11, characterised in that the aqueous liquor is according
to any one of claims 2 to 10.