Note: Descriptions are shown in the official language in which they were submitted.
~ J7~,~
~ase No. S-ll ,137
6~LD ALLOY PLATIN~ B~T~I AND ~ROCESS
BACKGROUND OF THE INVENTION
This invention relates to a bath for
electroplating gold alloys and more particularly to a
bath for the high speed plating of a hardened gold
alloy.
It is known to co-deposit various metals with
gold during electrodeposition to give a harder
electrodeposit than is achievable with pure gold. It
is usual for a complexing agent for the alloying metal
to be present and in a kno~n gold-cobalt plating bath,
citrate is present as a complexing agent and as a
buffering agent for the bath.
It might be expected that a bath for the
electrodeposition of a gold-nickel alloy might be
derivable from a bath for the electrodeposition of a
gold-cobalt alloy merely by replacing the source of
cobalt with a source of nickel. However, if this is
tried, it is found that the plating speed is impaired
and that unwanted precipitates are obtained in the
bath.
SUMMARY OF THE INVENTION
It has surprisingly been found that citrate
appears to be the agent responsible for these
deficiencies, especially when the bath is for
depositing gold-nickel alloys.
In accordance with the present invention,
there is, therefore~ provided a citrate-free bath for
the electrodeposition of a gold alloy, the bath
comprising a bath soluble source of gold, a bath
soluble source of alloying metal, oxalic acid and
formic acid.
It will be understood that throughout the
specification reference to a weak organic acid (such as
oxalic acid or formic acid) and its anion are used
interchangeably: the nature of the species present
will, of course, depend on the pH of the bath.
DETAILED DESCRIPTION OF TH-E INVENTION
The bath soluble source of gold is preferably
a gold (I) salt, which could, for example, be an alkali
metal gold (I) cyanide or ammonium gold (I) cyanide.
The gold may be present in an amount of from 4 to 50
g/l, preferably 4 to 20 g/l, for example 8 to 12 g/l.
It is particularly preferred that the
alloying metal be nickel. In such case, the bath
soluble source of alloying metal preferably comprises
nickel sulfate. The nickel may be present in an amount
of from 0.5 to 20 g/l, preferably 1 to 5, for example 2
to 3 9/l
The oxalic acid is believed to have two
primary functions in the bath. First, it is believed
to act as a complexing agent for the nickel ions;
secondly, it acts as a buffering agent for the bath.
The oxalic acid will, therefore, normally be present in
an amount sufficient for it to fulfill these functions.
When choosing the amount of oxalic acid to be used, the
relatively limited solubility of the acid at lower
temperatures must be taken into account. Preferred
baths in accordance with the invention are those in
which oxalic acid is present in an amount of from 20 to
loo g/l, preferably 30 to R0 g!lz for exa~plP ~n tn' ~Q
g/l. More specifically, the oxalic acid may be present
in an amount from 45 to 55 g/l, for example 50 g/l,
which was found to De the most suitable concentration
for use. Small variations of oxalic acid concentration
around this leYel have hardly any effect on the plating
speed at which the bath may be operated.
The formic acid is believed to be an
essential ingredient for obtaining high plating speeds.
It appeared to operate as an anti-burning agent or an
inhibitor for metal transport in the high current
density areas. The formic acid may be present in an
-` 15 amount of from 20 to 100 ml/l, preferably 30 to 80
ml/l, although from 30 to 40 ml/l appeared to be the
optimum concentration range. A particularly preferred
concentration of formic acid was 35 ml/l.
A pH adjusting agent, for example potassium
hydroxide or another alkali metal hydroxide, may be
present in the bath, preferably in an amount which will
provide a final bath pH of from 3.9 to 5.1, more
particularly 4.1 to 4.9.
Although it is not necessary for the bath to
contain any further ingredi~nts, other additives may be
used to modify and/or further improve brightness,
ductility, grain refinement and the like. Components
for these and other purposes, as are conventional in
the art, may be added in accordance with known
practices. In doing so, however, the components added
--3--
must be compatible with the other bath components and
not have any adverse effect on the bath or its
operation.
Additionally~ in accordance with the present
invention, there is provided a process for plating gold
alloy, particularly gold nickel, on a substrate~
especially at high speed. The process comprises
immersing the substrate to be plated as a cathode in a
bath in accordance with the present inv~ention and
passing current between the~cathode and an anode in the
bath.
The bath may be operated at a temperature of
from 20C to 80B, preferably 30 to 70C, for example
40 to 60C.
After plating, the duly plated substrate is
preferably rinsed in softened or deionized water so as
to avoid deposits of calcium oxalate.
In laboratory scale plating processes carried
out in accordance with the invention in agitation
equipment, it was found that plating speeds of 3.8
microns per minute at current densities of 13 ASD could
be achieved without loss of brightness. With the use
of suitable equipment with higher solution and handling
capabilities, for example, jet plating, much higher
plating speeds at ~uch higher current densities (up to
200 ASD) can be achieved.
Baths in accordance with the present
invention are particularly suitable for use in high
speed plating. In such commercial use of baths of the
3Q invention, current densities of from 10, 20, 50 or 100
to 200 ASD may be used to take advantage of the high
speed plating potential of the baths. When plating at
high speeds, it is generally desirable to agitate the
solution, preferably to achieve such a high level of
agitation that the solution is turbulent.
For a better understanding of the invention,
the following non-limitiny examples are given.
EXAMPLE 1
A bath having the~following composition was
made up;
INGREDIENT AMOUNT
Gold (in the form o~ potassium
gold (I) cyanide) 10 g/l (*)
Oxalic acid 2H~0 63 g/l
Formic acid 40 ml/l
Nickel (in the form of nickel sulfate) 2 9/1 (*)
Potassium hydroxide ~ to pH 4.1
Distilled Water to 1 liter
The bath formulated as above was placed in a
laboratory scale turbulent agitation plating system.
Electrolyte was pumped through two pipes into a 500
ml/l beaker and was directed through holes in the pipes
onto the substrate, which was immersed as a cathode in
the beaker. Electrolyte solution was pumped away
through a third pipe in the beaker. The cathode is
located between the two supply pipes and anodes are
placed around the supply pipes at such a position that
they do nt disturb the solution flow.
(*) Metal concentration
--5--
The solution is pumped around the system at a
flow rate of 5 liters per minute (measured with water
at room temperature).
It was found that fully briaht gold nickel
electrodeposits up to about 5 microns in thickness were
deposited from the above bath at a deposition rate of
3.8 microns per minute, using a current density up to
13 ASD.
EXAMPLE 2
An electrolyte bath having the following
composition was prepared: -.
INGREDIENT AMOUNT
Gold j(in the form of potassium
gold (I) cyanide) 10 g/l (*
Oxalic acid 2H20 50 g/l
Formic acid 35 ml/l
Nickel (in the form of nickel sulfate) 3 g/l (*)
Potassium hydroxide to pH 4.4
~O Distilled Water to 1 liter
Under the conditions described from Example
1, it was found that fully bright gold-nickel
electrodeposits were obtained at a deposition rate of
3.75 microns per minute using a current density of up
to 12 ASD.
(*) Metal concentration
--6--
EXAMPLE 3
With a bath as described above in Example 2,
except that 20 ml/l formic acid and 2 g/l nickel were
used, fully bright deposits were obtained at a plating
speed o~ 2.5 microns per minute at a current density of
8 ASD. Fully bright deposits were also obtained at a
plating speed of 2.8 microns per minutes (9 ASD) and
3.1 microns per minute (10 ASD). In all cases, the
temperature of the bath was 50C.
EXAMPLE 4
Using an electroplating bath as prepared in
Example 2, except that the concentration of formic acid
was 40 ml/l and the concentration of nickel was 2.0
g/l, fully bright deposits of gold-nickel alloy were
obtained at a plating speed of 3.0 microns per minute
with a current density of 10 ASD. Fully bright
deposits were also obtained at plating speeds of 3.3
microns per minute (11 ASD) and 3.55 microns per minute
(12 ASD). In all cases the bath temperature was 50C.
It will be further understood that the
examples set forth above are illustrative only, and
that they are subject to further changes and
. modifications without departing from the broader
aspects of this invention.
