GB1603632A - Electroplating of gold alloy - Google Patents

Description

(54) ELECTROPLATING OF GOLD ALLOY (71) We, JOHNSON, MAT'THEY & CO., LIMITED, a British Company, of 43 Hatton Garden, London, EC1N 8EE, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the electrode position of a gold alloy and in particular to an improved electrolyte for electrodeposition of a gold alloy.

Here and throughout the remainder of this specification and claims, the term "electrodeposition" is to be understood as referring either to electroplating, in which a relatively thin film of metal is electrodeposited onto a supporting substrate, or to electroforming, in which a somewhat thicker film of metal is electrodeposited onto a substrate which is subsequently removed leaving a self-supporting, hollow metal article, or to both, as the context requires.

In order to achieve certain effects by means of gold electroplating, for example on substrates such as an item of jewellery or on metallic spectacle frames, it has hitherto been the usual practice to electroplate a layer of substantially pure gold onto the substrate material followed by a very thin so-called "flash coating" or "gilding deposit" of, for example, an alloy of 50% gold and 50% copper or nickel. The flash coating is translucent, thus creating, in combination with the substantially pure gold layer, the required decorative effect.

The particular alloying metals of the flash coating, and their respective proportions, are selected according to the particular decorative effect desired. The flash coating also serves physically to protect the relatively soft gold layer.

One disadvantage of such a practice is that the flash coating, in order to be translucent, is required to be very thin and it therefore tends to be extremely vulnerable to abrasion, even mild abrasion, with consequent loss or modification, at least in certain areas, of the decorative effect. A further disadvantage of the practice is that the flash coating has hitherto been applied from cyanide-containing electrolytes. These are poisonous and consequently need to be handled with care. They are also rather difficult to dispose of with safety. Yet a further disadvantage of the prior art practice is that the electroplating of the substantially pure gold layer tends to be slow.

We have now found that these and other disadvantages may be overcome by electrodepositing onto a substrate a layer of an alloy of gold. We have found that a fairly thick deposit may thus be applied quickly and the need for a flash coating is therefore eliminated.

According to one aspect of the present invention, a cyanide-free gold alloy electrodeposition electrolyte comprises a nitrosulphito gold complex in a concentration of from 0.1 to 50 g.l.-l and an alloying metal comprising copper, cadmium, nickel, cobalt, zinc and/or palladium in the form of a compound or complex in a concentration of from 0.02 to 5 g.l-l.

The nitrosulphito gold complex may be prepared according to one of the methods described in our co-pending British patent application 14808/75 (Serial No. 1548887) As explained more fully in that specification, the nitrosulphito gold complex is usually associated with the nitrite used in the preparation of the complex. For example, the gold complex formed with sodium nitrite is usually associated with an equimolar amount of sodium nitrite. For use in an electrodeposition bath, it is not necessary to separate the complex from the nitrite used in its preparation and the mixture of complex and nitrite is therefore conveniently referred to as a "gold salt".

The alloying metal compound or complex which is mixed with the gold complex which is mixed with the gold complex in the electrolyte according to the invention may be added to the electrodeposition bath itself or to the gold complex or gold salt before the bath is made up ready for use. By "plating bath" we mean the entire electrodeposition system, that is to say, including tank, electrolyte, agitating means if present, electrical connections and so on.

A bath using a gold alloy electrodeposition electrolyte according to the invention is characterised by being entirely free from cyanide. We have found, in fact, that the alloying metal with gold may be added either as a compound, for example a non-cyanide salt, as a complex anion or as a suitable complex. An example of a suitable salt anion is sulphite and an example of a suitable complexing agent is EDTA. Other possible salt anions are nitrite and acetate.

The relative amounts of gold and alloying metal present are selected according to the properties required of the electrodeposit, for example colour, porosity, corrosion resistance and hardness. We have found that for many purposes the ratio by weight of alloying metal to gold need not exceed about 1:10. For example, if the concentration of gold in the electrolyte is 10 g.l-l, the concentration of alloying metal would probably be less than 1.0 gl Preferably, the concentration of the gold complex is in the range 2 to 30 g.l-1, more preferably 10 to 15 g.l-l, and the alloying metal is in the range 0.02 to 1.0 g.l , more preferably 0.02 to 0.01 g.r1.

The invention also includes a method of electrodepositing on a substrate an alloy of gold using an electrolyte according to the first aspect of the invention.

The variables of electrodeposition are discussed below.

a) Agitation. This may be effected for example by solution stirring, movement of the cathode or a combination of the two. Whatever method is used, however, we have found that, in order to obtain a consistent deposit colour from a given electrolyte, the mode and speed of agitation should be held essentially constant.

Other possible modes of agitation are gas bubbling and ultrasonic agitation.

b) pH. Copper and cadmium form deposits with gold having stable colour at high pH (that is, greater than 11.0). We have found that, in order to operate satisfactorily at lower pH values, additional additives such as polydentate amines may be required.

c) Current density. We have found that, in general, the effect, if any, of increased current density on the appearance of the deposit depends on the mode of agitation and may increase streaking, edge burning and blooming but may, on the other hand, enable finer control of colour and deposit composition to be achieved.

d) Temperature. Generally, electrodeposition baths using electrolytes according to the invention can be operated satisfactorily at room temperature.

Preparation of electrolytes. Electrolytes according to the invention may be prepared, for example, by dissolving sufficient of a nitrosulphito gold salt in water to give a solution containing the required concentration of metal, generally 0.1-50 g/l Au, preferably 2-30 g/l Au. Further sodium nitrite or other compatible alkali metal salt may also be added. Sodium hydroxide or another alkali is added to raise the pH to the value required, depending on the nature of the alloying metal. EDTA may be added to reduce the pH, if necessary. Since the pH falls during deposition, a buffer should normally be used, for example Na2B407 (borax), Na2HPO4 or NaHCO3 (but preferably Na2B4O,) for operation in the range of pH 9-11, or, for example, sodium acetate (CH7COONa) for operation under approximately neutral conditions.The buffer concentration is normally of the order of 10 gull. A salt, complex or compound containing a complex anion of the alloying metal is added in an amount sufficient to give the required concentration. The salt, complex or compound may be added to the electrolyte directly or in the form of a solution. If necessary, pH re-adjustment may be carried out.

An alternative and preferred method of preparing an electrolyte according to the first aspect of the invention is to dissolve a gold alloy salt in water and then to proceed according to the method above. The advantage of this method is that pH adjustment is carried out after all the electrodepositing species have been added and further adjustment is rendered unnecessary.

Gold alloy electrodeposits from these solutions are improved by the addition of chelating agents such as EDTA (ethylenediamine tetraacetic acid). This may be used as such, but is normally added as the sodium or other salt of EDTA.

To obtain bright deposits of gold alloy the addition of a brightener is normally required.

Preferred brighteners are those containing arsenic, antimony or selenium. Proprietary brighteners of this type are available, but the most satisfactory brightener can be prepared by reaction of As203 with EDTA.

For certain purposes, particularly for electroforming where the alloying metal is present in a relative amount to gold of about 0.03:1 or greater, it is desirable to include a stress relieving agent in the electrolyte. Examples of suitable stress relieving agents are various high boiling organic compounds such as Turkey red oil. Alternatively or additionally, to alleviate stress in the electrodeposit, the technique of pulse electrodeposition may be employed.

Electrolytes according to the invention are suitable for vat and barrel electroplating, and for electroforming, operations, and can be operated through many turnovers of the metal content by appropriate replenishment of the gold and alloying metal contents with further plating salt and of the brightener when required, and adjustment of the pH as necessary.

In vat electroplating, the substrate is positioned in the electrolyte in a suitable container and connected in the electric circuit to become the cathode. In barrel electroplating, according to one embodiment, the electrolyte container is in the form of a barrel or beaker, the bottom of which is provided with studs to act as cathodes. The anode generally comprises a central rod of platinised titanium, for example, and the axis of the barrel or beaker is inclined to the vertical. Articles to be electroplated are placed in the container which is then slowly revolved while electroplating takes place. According to another embodiment, a perforated barrel containing the articles to be electroplated is submerged in a reservoir containing the electrolyte.The barrel is caused to rotate and, suitable electrical connections being provided, the articles are tumbled inside the barrel and a deposit of metal or alloy is applied. In electroforming, a fairly thick- that is, from 25,u to 1000 > , or greater, preferably 100,u to 500,uS, - layer of metal or alloy is deposited on a cathodic substrate which is subsequently removed from the electrodeposit, leaving a hollow article of metal or alloy having high strength and, in shape, faithfully following the relief of the substrate.As an example of a gold or gold alloy electroforming operation, a wax substrate is first coated with a layer of silver by spraying silver nitrate solution and reducing the deposit so formed to metallic silver, the silvered substrate then has a relatively thick layer of gold- or gold alloy applied by electrodeposition, the wax is then melted out and the silver dissolved.

Alternatively, a copper mandrel may have a relatively thick layer of gold or a gold alloy applied by electrodeposition, the copper subsequently being dissolved.

The following experimental examples and tables are illustrative of the invention.

Example 1 This series of experiments illustrates the effect on deposit composition and appearance of adding alloving metals copper, nickel, cobalt, zinc and palladium at various concentrations to a 10 g.l~l gold nitrosulphito electrodeposition electrolyte. The palladium was added as (Pd(NO2)2(NH3)2)2 and the remaining metals as their EDTA complexes.

The copper-EDTA complex was prepared by dissolving copper (11) nitrate (59g) in water (250ml) at 50"C and adding EDTA (90g) with stirring for 1 hour. The solution was then evaporated without boiling to a volume of 150 ml and, on cooling to 20"C, a blue precipitate formed which was filtered off, washed with acetone and air dried. The yield was 98g.

For each alloying metal at each concentration, two current densities were investigated.

Electroplating was continued for l/2 hour for each experiment. In each experiment 10 ml.l1 of brightener was added. The electrolyte pH was 11.4.

Cathode efficiency, deposit composition and appearance are detailed in Table 1.

Example 2 Table 2 shows the effect on cathode efficiency and deposit colour of varying the pH of a 10 g.l-l gold nitrosulphito electrolyte to which has been added 2.5 g.l-l and 5.0 g.ll each of copper (as the EDTA complex) and palladium (as (Pd(NO2)2(NH3)2)). Each experimental deposition was continued for hour during which time the pH and current density remained constant, the latter at a value of .144 Adm. -2.

Example 3 Table 3 illustrates the deposit hardness obtained using electrolytes comprisin5 g.l-í and 10 g.l-l of gold nitrosulphito complex with additions of from 0.05 g.l-l to 5.0 g.l of copper at various pH values. Hardness was determined using a Leitz Miniload hardness tester with a loading of 25g.

Example 4 5 g.ll and 10 g.l-l gold nitrosulphito electrolytes were prepared and operated at a current density of 0.144 Adm. -2 and a pH of 11.4 with a range of copper additions. Results are quoted in Table 4a. The electrolytes were then used to prepare deposits having a-nominal thickness of 5,uw and the physical properties of these deposits are shown in Table 4b. The reflectivity figures quoted are for white light and are relative to a 211 sputtered deposit of pure gold on an optically flat glass slide.

Percentage reflectivity is generally expressesd as intensity of reflected light x 100 intensity o of reflected + scattered light Example 5 In order to determine the composition of deposits, electrolytes comprising 5 g.l-l and 10 g.l-l gold as the nitrosulphito complex and with a range of copper concentrations were operated using the cathode efficiencies from earlier experimental examples to calculate the time required to form a deposit of 0.5g. Results are quoted in Table 5.

Example 6 Vat plating trials.

In vat plating trials, electrolytes according to the invention and containing 10 g.l-1 gold, 0.02 g.l-1 copper, 15 g.l-1 gold, 0.02 g.l-1 copper and 30 g.l-1 gold 0.02 g.l-1 copper respectively were compared to the corresponding electrolytes without copper additions. It was found that all deposits for a current density of up to 0.36 Adm. -2 for 10 g.l-l gold/0.02 g.t1 copper, up to 0.4 Adm. -2 for 15 g.l-l gold/0.02 g.t1 copper and up to 0.8 Adm. -2 for 30 g.l-l gold/0.02 g.l-l copper were bright, coherent and adherent. Above these levels the reflectivity decreased slightly and the deposits exhibited mild blooming and edge burning.

The samples from the best deposits were analysed and found to contain 99.85 wt.% gold, 0.08 - 1.0 wt. % copper and < 0.08 wt. % arsenic, the latter figure corresponding to the limit of detection of this element.

Example 7 Barrel plating trials. Barrel plating trials were carried out on silver headed copper "Optecon" contact rivets as substrate in a slowly rotating, inclined 500 ml.

polythene barrel containing 300 ml. of electrolyte. Electrical contact to the charge was made via studs set into the bottom of the beaker and a centrally immersed strip of platinised titanium was employed as anode. The normal loading was 1000 contacts having an area of 203 cm2 (weight 95.42g).

Electrolytes containing 10 g.l~l and 15 g.r1 gold, each with adition of 0.02 g.l-í copper and each at a pH of 11.4, were investigated.

Example 8 Electroforming. Electroforming trials were carried out on electrolytes containing 15 g.l-1 and 30 g.l-1 gold each with addition of 0.02 g.l-1 copper. The 15 g.l-1 gold-based electrolyte was used to form a nominal 250 deposit on a copper panel at pH 11.4 using a current density of 0.4 Adm. -2 and mild agitation for 17 hours. The gold, copper and arsenic was "replenished" in advance. A similar deposition was carried out on a panel of smaller area in order to decrease the amount by which the gold, copper and arsenic required to be "replenished" in advance. The panels were dissolved in nitric acid and the deposits assayed for copper (0.1%) and arsenic ( < 0.08%). Due to the slightly noduled appearance of the deposits, further trials were conducted using continuous electroyte filtration.

Both electrolytes were used to electroform deposits for 24 hours, in each case for 16 hours at low current density, followed by a further 8 hours on a fresh panel. A mould was then electroformed to a nominal 250 m thickness to investigate the relief of the deposit. In each case the deposits were excellent in appearance and mould relief was followed to perfection.

Deposits formed from electrolytes according to the invention were also tested for porosity, corrosion resistance, solderability and contact resistance. Assessments were also made of deposit stress and electrolyte throwing power. Results are discussed below.

Porosity Deposits having a range of thickness ( < 1 to 12.5,u) were prepared on copper panels and tested electrographically using CdS paper and a current density of 0.8 Adm. -2. It was found that porosity occurred only in deposits less than 3 thick.

Corrosion resistance Samples were exposed in a dessicator to a moist 1% v/v SO2 atmosphere for 24 hours followed by a moist 1% v/v H2S atmosphere for a similar period.

An accelerated test consisted of 30 minutes' exposure to a cncentrated atmosphere of ammonium polysulphide. In general, deposits of greater than 2.5 - 3 , thickness suffered little corrosion.

Solderability 1880 thick copper wire was plated with varying thicknesses of gold alloy and lowered horizontally into a 200 mg block of melted solder, both wire and solder being fluxed. The solderability was determined by measuring the time taken for the solder to flow completely around the wire. It was found that, of the electrolytes tested (5 g.l-l gold with 0.02 g.l-l, 0.05 g.l-l, 0.15 g.l-l and 0.35 g.t1 copper respectively), soldering times were lowest for the 0.05 g.l-l copper electrolyte but higher copper levels still resulted in lower soldering times than did the 0.02 g.l-1 copper electrolyte. Results are given in Table 6.

Contact resistance Pairs of contacts plated with 5, 7.5 and 10 deposits respectively from 10 g.l-l gold/0.02 g.l-l copper and 15 g.l-l gold/0.02 g.T' copper electrolytes were measured for contact resistance by holding each pair together with a force of 0.1 and 0.25 Newtons and measuring the voltage drop, for each force, when a current of 0.5A was passed. It was found that deposits from these electrolytes had substantially better contact resistance (i.e. higher) than corresponding deposits from electrolytes comprising nitrosulphito gold salts without any copper.

Stress A strip of fully annealed beryllium copper (Be/Cu 10 x 0.9 x 0.01 cm) was masked on one side with "Donodep" stopping off paint and suspended vertically from a rigid clamp. The deflection of the strip when plated with a 12.5 deposit of gold or gold alloy is indicative of the stress of the deposit. It was found that addition of copper to a 10 g. 1 gold electrolyte caused an increase in stress but the effect lessened as the concentration of gold was increased to 15 g.t1 and 30 g.l-l.

Throwing power Throwing power may be defined as the percentage ratio of the smallest and largest coating thicknesses at particular points on an object. We have found that the throwing power of electrolytes according to the invention and containing 0.02 g.1-l copper is superior to nitrosulphito gold electrolytes without any copper. Depolarising agents may be used if required to increase still further the throwing power.

Table 1 METAL gl-1 CURRENT C.E. DEPOSIT COMPOSITION APPEARANCE DENSITY (%) A.dm -2 Cu 0.1 0.144 95.0 99.8% Au, 0.15% Cu, < 0.05% As Bright, coherent, adherent, 0.288 95.0 0.5 0.144 69.3 98% Au, 1.2% Cu, < 0.08% As Bright, rose colour, coherent, adherent 0.288 69.1 but stressed.

1.0 0.144 43.2 90% Au, 9% Cu, 0.1% As Dull pink colour at 0.288A. dm-2, rose at 0.288 45.4 0.144#.dm-2, coherent, adherent, stressed 5.0 0.144 30.7 81.3% Au, 18% Cu, < 0.08% As Dull pink at both, coherent, adherent but 0.288 36.0 Stressed.

Ni 0.1 0.144 98.0 99.8% Au, 0.15% As, < 0.05% Ni Bright, coherent, adherent.

0.288 99.0 Bright with slight streaking 0.5 0.144 94.0 " " " Very bright, coherent, adherent.

0.288 95.0 1.0 0.144 95.0 " " " Very bright, coherent, adherent, gold colour 0.288 95.0 considerably lighter.

5.0 0.144 97.0 99.8% Au. < 0.08% As, 0.12% Ni Very bright, light coloured deposit, highly 0.288 89.0 stressed.

Co 0.1 0.144 96.0 approx 99.8% Au, 0.1% As, < 0.1% Co At 0.144#.dm -2 the deposits were very bright 0.288 96.0 " " " whilst at 0.288#.dm -2 slight edge burning 0.5 0.144 96.0 took place.

0.288 96.0 " " " 1.0 0.144 94.0 0.288 95.0 " " " 5.0 0.144 90.0 0.288 91.0 " " " Table 1 (continued) METAL gl-1 CURRENT C.E. DEPOSIT COMPOSITION APPEARANCE DENSITY (%) A.dm-2 Zn 0.1 0.144 89.0 99.8% Au, 0.15% As, Zn not detected At 0.144#.dm-2 all deposits bright whilst at 0.288 94.0 0.288#.dm-2 all deposits badly bloomed.

0.5 0.144 96.0 " " " 0.288 95.0 1.0 0.144 98.0 " " " 0.288 96.0 5.0 0.144 94.0 " " " 0.288 99.0 Pd 0.1 0.144 94.0 99.78% Au, 0.1% Pd, 0.1% As Bright, coherent, adherent.

0.288 98.0 " " " 0.5 0.144 90.0 99.8% An, 0.1% Pd, < 0.08% As " " " 0.288 91.0 Slight bloom, coherent, adherent.

1.0 0.144 86.0 99% Au, 0.85% Pd, < 0.08% As Gold was light in colour at both current 0.288 84.0 densities.

5.0 0.144 54.0 92% Au, 7.8% Pd, 1.0% As Very light, highly stressed deposit.

Table 2 Concentration Solution Cathode efficiency Deposit colour Gl-l pH 2.5 Cu 10 52 bright rose - copper 11 51 bright rose - red 12 42 copper red, yellow streak 13 41 copper red, yellow streak 5.0 Cu 10 35 bright copper 11 32 12 30 13 28 2.5 Pd 10 74 light palladium 11 76 very light palladium 12 85 grey - yellow 13 86 5.0 Pd 10 50 bright palladium 11 47 12 53 13 55 dull grey yellow Table 3 (Au) 5gl-1 l Ogl-l Sgl-l 10gl-1 5gl-1 l Ogl-l pH 10.4 10.4 11.4 11.4 12.4 12.4

Copper concentration # Hardness VPN # gl-1 0.05 170-180 195-210 180-185 175-185 140-150 150-160 0.1 175-185 195-205 175-185 185-190 130-150 145-160 0.2 170-185 200-210 175-185 190-200 130-150 130-150 0.25 180-190 210-215 180-200 185-210 140-160 150-160 0.3 180-190 230-245 180-200 205-210 150-160 140-160 0.4 200-220 240-245 200-210 195-205 150-153 130-170 0.5 200-220 230-250 200-210 200-205 130-160 150-160 0.75 210-240 250-265 210-230 200-210 150-170 145-150 1.0 220-230 280-290 220-240 215-130 160-170 155-160 1.5 220-240 280-290 230-240 225-250 150-170 150-160 2.0 250-270 280-290 240-260 235-250 170-180 180-185 2.5 260-270 290-320 240-260 295-310 170-180 160-180 5.0 280-290 320-330 260-270 290-310 170-180 170-190 Table 4a Gold Copper Cathode Deposit Deposit Concentration Concentration Efficiency Thickness Appearance, Visual gl-l gl-l % 5 0 87 6.4 very bright yellow 10 ' 90 6.5 5 0.05 88 6.4 very bright off yellow 10 " 89 6.5 5 0.1 80 6.0 very bright brass 10 " 80 6.0 yellow 5 0.2 72 5.4 bright very light pink 10 " 76 5.5 5 0.25 73 5.4 bright very light pink 10 " 76 5.5 5 0.3 70 5.3 very bright light pink 10 " 76 5.5 5 0.4 64 5.0 very bright light pink 10 " 68 5. 5 0.5 62 4.9 bright deep pink 10 " 68 5.1 5 0.75 50 3.9 bright red gold 10 " 52 3.9 5 1.0 34 2.6 very bright red gold 10 " 36 2.6 5 1.5 31 2.4 very bright red gold 10 " 36 2.6 5 2.0 30 2.4 bright deep red 10 " 35 2.6 5 2.5 30 2.4 bright deep red 10 " 35 2.6 5 5.0 28 2.1 copper colour 10 " 30 2.2 TABLE 4b Gold Copper Deposit Reflectivity Concentration Concentration Thickness Hardness % Porosity and Cracking 8gl-1 gl-1 m VPN white light 5 0 5.0 125-150 98 nil 10 " 5.1 130-150 98 nil 5 0.05 160-170 95 nil 10 " 4.9 175-185 94 nil 5 0.1 170-175 95 nil 10 " 4.9 185-190 96 nid 5 0.2 185-195 95 nil 10 " 5.2 190-200 94 nil 5 0.25 185-195 95 very mild porosity 10 " 4.8 185-210 96 " 5 " 0.3 175-195 94 very light edge cracking and porosity 10 " 5.2 195-205 97 5 0.4 190-200 92 edge cracking and porosity 10 " 5.1 205-210 90 5 0.5 185-200 91 edge cracking and porosity 10 " 5.0 200-205 88 5 0.75 190-200 82 badly edge cracked and very porous 10 " 5.1 200-210 86 5 1.0 200-205 81 very cracked deposit 10 " 5.0 215-230 84 5 1.5 215-250 8 very cracked deposit 10 " 5.0 225-250 81 5 2.0 220-230 83 very badly cracked deposit 10 " 4.9 235-250 80 5 2.5 260-275 80 crazed deposit 10 " 4.9 295-310 80 5 5.0 280-300 72 crazed deposit 10 " 5.0 290-310 70 TABLE 5 Electrolyte Deposit Gold Copper Cathode Gold Copper Arsenic carat Concentration Addition Efficiency % % gl' gl4 5 0.02 88 99.85 0.1 > 0.08 24 10 " 90 98.85 0.1 > 0.08 5 1.0 35 87.5 11.68 0.82 21 10 " 36 87.8 11.4 0.80 21 5 1.5 31 82.8 15.9 1.50 19.8 10 " 36 83.4 15.1 1.50 20.0 5 2.5 30 74.2 23.6 2.2 17.8 10 ,, 35 74.1 23.7 2.2 17.8 5 5.0 28 40.0 54.4 5.8 9.6 10 " 30 39.2 54.3 6.5 9.4 TABLE 6 Deposit thickness Solder Time (Seconds), 5gl-1 Au m Cu, 0.02gl-1 Cu, 0.05gl-1 Cu, 0.15gl-1 Cu, 0.35gl-1 0.0 4.76 4.78 4.76 4.76 0.5 5.62 3.6 3.8 4.6 1.0 3.7 2.4 2.8 3.1 1.5 3.6 1.38 2.1 2.4 2.0 3.85 0.85 1.2 1.24 2.5 3.8 0.31 1.3 1.45 3.0 4.72 0.58 1.4 2.1 5.0 4.0 0.35 0.98 2.05 WHAT WE CLAIM IS:1.A cyanide-free gold alloy electrodeposition electrolyte comprising a nitrosulphito gold complex in a concentration of from 0.1 to 50 g.l-l and an alloying metal comprising copper, cadmium, nickel, cobalt, zinc and/or palladium in the form of a compound or complex in a concentration of from 0.02 to 5 g.l'.

2. An electrolyte according to claim 1 in which the gold complex has a concentration of from 2 to 30 g.l 3. An electrolyte according to claim 2 in which the gold complex has a concentration of from 10 to 15 g.l-1.

4. An electrolyte according to claim 1, 2 or 3 in which the alloying metal has a concentration of from 0.02 to 1.0 g.l-1.

5. An electrolyte according to claim 1, 2 or 3 in which the alloying metal has a concentration of from 0.02 to 0.1 g.l'.

6. An electrolyte according to any preceding claim in which the alloying metal is added as the sulphite, nitrite or acetate salt.

7. An electrolyte according to any of claims 1 to 4 in which the alloying metal is added as the EDTA complex.

8. An electrolyte according to claim 1 having a pH greater than 11.0 9. An electrolyte according to claim 1 having a pH less than 11.0 and including a polydentate amine.

10. An electrolyte according to claim 1 including a chelating agent.

11. An electrolyte according to claim 10 wherein the chelating agent is ethylenediamine tetra acetic acid (EDTA).

12. An electrolyte according to claim 11 wherein the chelating agent is added as the sodium or other salt of EDTA.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (14)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   TABLE 5 Electrolyte Deposit Gold Copper Cathode Gold Copper Arsenic carat Concentration Addition Efficiency % % gl' gl4

   5 0.02 88 99.85 0.1 > 0.08 24

   10 " 90 98.85 0.1 > 0.08

   5 1.0 35 87.5 11.68 0.82 21

   10 " 36 87.8 11.4 0.80 21

   5 1.5 31 82.8 15.9 1.50 19.8

   10 " 36 83.4 15.1 1.50 20.0

   5 2.5 30 74.2 23.6 2.2 17.8

   10 ,, 35 74.1 23.7 2.2 17.8

   5 5.0 28 40.0 54.4 5.8 9.6

   10 " 30 39.2 54.3 6.5 9.4 TABLE 6 Deposit thickness Solder Time (Seconds), 5gl-1 Au m Cu, 0.02gl-1 Cu, 0.05gl-1 Cu, 0.15gl-1 Cu, 0.35gl-1 0.0 4.76 4.78 4.76 4.76 0.5 5.62 3.6 3.8 4.6 1.0 3.7 2.4 2.8 3.1 1.5 3.6 1.38 2.1 2.4 2.0 3.85 0.85 1.2 1.24 2.5 3.8 0.31 1.3 1.45 3.0 4.72 0.58 1.4 2.1 5.0 4.0 0.35 0.98 2.05 WHAT WE CLAIM IS:1.A cyanide-free gold alloy electrodeposition electrolyte comprising a nitrosulphito gold complex in a concentration of from 0.1 to 50 g.l-l and an alloying metal comprising copper, cadmium, nickel, cobalt, zinc and/or palladium in the form of a compound or complex in a concentration of from 0.02 to 5 g.l'.
2. 2. An electrolyte according to claim 1 in which the gold complex has a concentration of from 2 to 30 g.l
3. 3. An electrolyte according to claim 2 in which the gold complex has a concentration of from 10 to 15 g.l-1.
4. 4. An electrolyte according to claim 1, 2 or 3 in which the alloying metal has a concentration of from 0.02 to 1.0 g.l-1.
5. 5. An electrolyte according to claim 1, 2 or 3 in which the alloying metal has a concentration of from 0.02 to 0.1 g.l'.
6. 6. An electrolyte according to any preceding claim in which the alloying metal is added as the sulphite, nitrite or acetate salt.
7. 7. An electrolyte according to any of claims 1 to 4 in which the alloying metal is added as the EDTA complex.
8. 8. An electrolyte according to claim 1 having a pH greater than 11.0
9. 9. An electrolyte according to claim 1 having a pH less than 11.0 and including a polydentate amine.
10. 10. An electrolyte according to claim 1 including a chelating agent.
11. 11. An electrolyte according to claim 10 wherein the chelating agent is ethylenediamine tetra acetic acid (EDTA).
12. 12. An electrolyte according to claim 11 wherein the chelating agent is added as the sodium or other salt of EDTA.
13. 13. A process of electroldepositing on a substrate an alloy of gold which comprises

    using an electrolyte according to any one of the preceding claims.
14. 14. A process according to claim 13 when carried out at a current density of up to 0.8 Adm-2.