JP2024526925A - Silver electrolyte for deposition of silver dispersion layers - Google Patents

Abstract

The present invention is directed to a silver electrolyte and corresponding method for the galvanic deposition of silver onto a conductive substrate. The silver electrolyte features certain additives that help prevent electrolyte foaming while not adversely affecting electrodeposition.

Description

The present invention is directed to a silver electrolyte and corresponding method for the galvanic deposition of silver onto a conductive substrate. The silver electrolyte features certain additives that help prevent electrolyte foaming while not adversely affecting the electrodeposition.

Today, electrical contacts are used in virtually all electrical appliances. These applications range from simple plug connectors in the communications sector, for the automotive industry or aerospace technology, to sophisticated safety-related switching contacts. Here, the contact surfaces must have good electrical conductivity, low contact resistance with long-term stability, as well as good corrosion and wear resistance with as low an insertion force as possible. In electrical engineering, plug contacts are often coated with hard gold alloy layers made of gold-cobalt, gold-nickel or gold-iron. These layers have good resistance to wear, good solderability, low contact resistance with long-term stability, and good corrosion resistance. The rising price of gold calls for cheaper alternatives.

As an alternative to hard gold plating, coatings with silver have proven to be advantageous. Silver and silver alloys are among the most important contact materials in electrical engineering, not only because of their high electrical conductivity and good oxidation resistance. These silver layers have similar layer properties to the hard gold layers and layer combinations used so far, such as palladium-nickel with gold flash. Furthermore, the price of silver is relatively low compared to other precious metals, especially hard gold alloys.

For the silver coating of substrates and for preparing contact surfaces, special silver electrolytes are used. These are usually cyanide-containing or cyanide-free silver-containing solutions, which serve to electrochemically, in particular galvanically, silver the surfaces. In this case, the silver electrolyte solutions can contain a wide variety of further additives, for example grain refiners, dispersants, brightening agents or solid components. For applications in the electrical and electronic sector, in particular in plugs and plug and switch contacts, the electrical conductivity, contact resistance and friction coefficient are of particular importance.

If two silver-coated surfaces are moved over one another, a relatively high force is required for this purpose, which depends on the applied normal force and the material-specific friction coefficient of the surfaces, respectively. The disadvantages of contacts with silver surfaces are evident here, since silver surfaces have a relatively high friction coefficient, which leads in particular to high insertion and pulling forces in the case of plug contacts. Due to the high friction coefficient, wear occurs in the case of silver surfaces, as a result of which the number of possible plug cycles is severely limited. Furthermore, the problem arises that the silver surfaces are prone to wear. However, a contact resistance as low as possible of a system of two silver-coated surfaces is particularly advantageous.

Various cyanide-containing silver electrolytes for producing contact surfaces have already been found in the prior art (DE 2543082 A1, WO 9114808 A1, DE 10346206 A1, DE 102008030988 A1, DE 102015102453 A1, CN 105297095 A). DE 102018005352 A1 also describes a cyanide-containing silver electrolyte for producing contact surfaces in which solid components are dispersed as dry lubricants. These solids are intended to be incorporated into the silver layer as highly dispersed and as easily distributed as possible. The aim is to ensure the highest possible self-lubricating properties of the silver layer by homogeneously dispersing the solid components in the silver layer (Arnet, R. et al., "Silver dispersion layers with self-lubricating properties", Galvanotechnik 2021, Vol. 1, p. 21 et seq.). The problem with known silver electrolytes is that they tend to foam (the total volume of the electrolyte increases by about 40%), especially when significant electrolyte migration is used, for example when using dispersion devices. However, significant electrolyte migration is necessary for usable high current densities and therefore rapid deposition. This is due to the following problems in the coating process, among others:
a) uniform incorporation of solids into the silver layer;
b) by electrolyte loss due to bubbles exiting the process vessel; and c) by a marbling effect on the part surface due to the adhesion of bubbles during extension and lifting of the part.
This can have adverse effects and therefore result in adverse product or process problems.

The object of the present invention is therefore to identify possibilities for maximally suppressing bubble formation in such electrolytes during silver deposition, while at the same time ensuring that the homogeneous deposition of the silver dispersion layer is as unaffected as possible.

These and other objects apparent from the prior art are achieved by the use of the electrolyte according to claim 1 of the present invention. Advantageous embodiments of the electrolyte according to the present invention are described in dependent claims 2 to 7 which are dependent on claim 1. Claims 8 to 11 relate to a method comprising preferred embodiments thereof.

1. An aqueous silver electrolyte for the galvanic deposition of a silver layer on a conductive substrate, comprising the following components:
a) at least one soluble silver compound;
b) free cyanide in an amount of 20 to 200 g/l;
c) at least one luster additive in an amount of 0.2 to 10 g/l;
d) at least one wetting agent in an amount of 0.1 to 15 ml/l;
e) at least one solid component in an amount of 2 to 200 g/l;
Including,
f) at least one antifoaming agent in an amount of 0.2 to 20 g/l,
Surprisingly, achieving the above objective is easy.

During deposition of the silver layer from the electrolyte according to the invention, the electrolyte should be kept constantly moving in order to prevent settling of solid components and ensure the highest possible current density. Foam formation is reliably prevented/greatly reduced by the antifoaming agent. It is also surprising that the addition of a corresponding antifoaming agent does not have a significant adverse effect on the desired properties of the silver layer, such as the layer composition (solid incorporation) and the contact resistance.

In principle, all compounds available to the skilled artisan that have a defoaming effect and are inactive in a given system can be used as defoamers (http://de.wikipedia.org/w/index.php?title=Entsch%C3%A4umer&oldid=187879609). Examples include defoamers from the group consisting of polyethers (BASF Pluriol® E Series), fatty alcohol alkoxylates (BASF Degressal® SD20), phosphoric acid esters (BASF Degressal® SD40) or alkyl polyglycol ether carbonates (CLARIANT COL® 100). The use of defoamers based on polyethers is particularly advantageous. In this context, polyalkylene glycols, in particular those selected from the group of polyethylene glycols, are advantageous (http://de.wikipedia.org/w/index.php?title=Polyethyleneglycol&oldid=210432692). These are commercially available under different trade names, for example Pluriol® Series E200 to E9000, the numerical designation corresponding to the average molecular weight of the substance. It has also proven advantageous if the polyalkylene glycols, in particular the polyethylene glycols, used have a higher average molar mass of more than 200 g/mol, preferably at least 400 g/mol. The average molar mass of the polyethylene glycols used is very preferably between 1000 and 9000 g/mol, very particularly preferably between 4000 and 8000 g/mol. The average molar mass of the polymer (http://de.wikipedia.org/w/index.php?title=Mittler_molare_Masse&oldid=188125754) is determined by methods known to those skilled in the art (http://www.chemgapedia.de/vsengine/vlu/vsc/de/ch/9/mac/charakterisierung/d3/gpc/gpc.vlu/Page/vsc/de/ch/9/mac/charakterisierung/d3/gpc/auswertung.vscml.html). The amount of antifoam in the electrolyte can be defined by those skilled in the art. This ranges from 0.2 to 20 g/l, preferably from 1 to 10 g/l, and particularly preferably from 1 to 5 g/l.

In the context of the present invention, a brightener additive is a substance which shifts the particle size of the silver deposit to a smaller particle size. Brightener additives selected from the group consisting of substituted and unsubstituted mononuclear arylsulfonic acids, as well as thioalkylcarboxylic acids, thiocarbamides have proven to be effective as such additives. In this context, arylsulfonic acids, such as phenolsulfonic acid and benzenesulfonic acid, toluenesulfonic acid, are particularly preferred. However, thiolactic acid, thiobarbituric acid and 1-phenyl-1H-tetrazole-5-thione in particular have also proven to be advantageous. Compounds which do not adversely affect the solid deposit are advantageous. Brightener B is used in the electrolyte according to the invention in a concentration of 0.2 to 10 g/l, preferably 0.5 to 10 g/l, particularly preferably 1.0 to 5 g/l. It is particularly advantageous if naphthalenesulfonic acid, naphthalenesulfonic acid derivatives (for example naphthalenesulfonic acid condensation products with aldehydes) or mixtures thereof are not present in the electrolyte according to the invention. In contrast, arylsulfonic acids in which the aryl group is a benzene ring, such as the Na or K salts of benzenesulfonic acid, are highly preferred. The aryl group may be optionally substituted. In particular, in relation to the polyether-based antifoaming agents identified above, especially the polyethylene glycol ethers, the latter, despite the high current density and the resulting significant electrolyte movement, produce only small amounts of foam and therefore allow good dispersion of solid components in the silver deposit (see examples).

The electrolyte contains solid components, the function of which is well recognized in the prior art. In the sense of the present invention, "solid components" means components that are not in solution but are present as solids in the electrolyte. In particular, solid components such as those mentioned in DE 102018005352 A1 in this respect can be used. These are preferably selected from the group consisting of graphite, graphite fluoride, graphite oxide, coated graphite, graphene, carbon black, fullerenes, diamond, Al ₂ O ₃ , cubic boron nitride, or mixtures thereof, preferably graphite, graphite fluoride, graphite oxide, Al ₂ O ₃ coated graphite, or mixtures thereof, more preferably graphite, graphite oxide, or mixtures thereof, even more preferably graphite. Suitable methods of conditioning of materials are well described in the literature and have little, if any, influence on the invention described herein. The solid components are present in the electrolyte dispersed, in particular physically dispersed. This is achieved in particular by corresponding devices such as stirrers, dispersing devices (e.g. Ultraturrax®) or dispersing disks. The amount of solid components used can be specified by those skilled in the art according to their discretion. In general, the concentrations here are from 2 to 200 g/l, preferably from 20 to 150 g/l, particularly preferably from 80 to 130 g/l. The diameter of the solid components must be selected by those skilled in the art according to the application profile. As a rule, the average particle diameter (http://de.wikipedia.org/w/index.php?title=Partikelgr%C3%B6%C3%9Fenverteilung&oldid=186369602) is from 1 to 50 μm, preferably from 2 to 20 μm, particularly preferably from 2 to 10 μm. For this purpose, the mean particle diameter (d50 of the Q3 distribution) is measured using a Beckmann Tornado dry dispersion module according to ISO 13320-1 (latest edition as of the filing date). In the sense of the present invention, the mean particle size (d50) indicates that 50% of the particles of the solid component have a diameter smaller than a particular value.

Wetting agents are also present in the electrolyte. The manner in which they are selected is known to those skilled in the art. Typically, ionic and non-ionic surfactants, such as polyethylene glycol adducts, fatty alcohol sulfates (e.g. sodium lauryl sulfate), alkyl sulfates, alkyl sulfonates, aryl sulfonates, alkylaryl sulfonates, sulfonated castor oil are used as wetting agents (see also Kanani, N: Galvanotechnik; Hanser Verlag, Munich Vienna, 2000; page 84 et seqq.). The silver coatings deposited using the baths thus provided are generally white and glossy to high gloss. The wetting agents result in a layer without pores. Compounds based on alkyl sulfates are preferably used as wetting agents. These may be linear or branched alkyl sulfates with C ₁ -C ₂₅ alkyl groups, preferably linear or branched alkyl sulfates with C ₂ -C ₂₀ alkyl groups, which may be unsubstituted or optionally substituted. The wetting agents preferably contain linear or branched alkyl sulfates with C ₃ -C ₁₅ alkyl groups, which may be unsubstituted or optionally substituted, preferably linear or branched alkyl sulfates with C ₃ -C ₁₂ alkyl groups, which may be unsubstituted or optionally substituted. The wetting agents can also be present in the form of their salts, for example sodium salts, potassium salts. Very particular preference is given in this connection to those selected from the group consisting of 2-ethylhexyl sulfate-Na salt, lauryl ether sulfate-Na salt, sodium monoalkyl sulfates, such as sodium tetradecyl sulfate, sodium dodecyl sulfate, sodium ethylhexyl sulfate, sodium decyl sulfate, sodium octyl sulfate, and mixtures thereof. The content of the at least one wetting agent is between 0.1 and 15 ml/l, preferably between 0.2 and 10 ml/l, more preferably between 0.5 and 7 ml/l, even more preferably between 1 and 6 ml/l.

Optionally, salts containing Se or Te can also be present in the electrolyte. The selenium or tellurium compounds used in the electrolyte can be selected as appropriate by the skilled artisan. Suitable selenium and tellurium compounds are those in which selenium or tellurium is present in the form of anions in the oxidation state of +4 or +6. Selenium and tellurium compounds are advantageously used in electrolytes in which selenium or tellurium is present in the oxidation state of +4. Selenium and tellurium compounds are particularly preferably selected from tellurite, selenite, tellurous acid, selenious acid, telluric acid, selenic acid, selenocyanates, tellurocyanates, and selenates and tellurates. In this case, the use of selenium compounds is generally preferred over tellurium compounds. It is very particularly preferred to add selenium to the electrolyte in the form of a salt of selenic acid, for example potassium selenite. It is highly preferred to add it as potassium selenocyanate. The amount of these compounds in the electrolyte can be selected as required by the skilled artisan. It ranges from 0.1 mg to 500 mg/l, preferably from 0.5 mg to 100 mg, particularly preferably from 0.5 mg to 10 mg/l, based on selenium or tellurium.

In addition to the abovementioned raw materials, the electrolyte also contains a certain amount of free cyanide. This is preferably added to the electrolyte in the form of a water-soluble salt of hydrocyanic acid. Alkaline salts are advantageous, potassium cyanide being very particularly preferably used. The amount of free cyanide in the electrolyte can be set in accordance with values known to those skilled in the art. As a rule, free cyanide is present in the electrolyte in a concentration of 20 to 200 g/l, preferably 80 to 180 g/l, particularly preferably 100 to 150 g/l (in each case based on CN ions). The cyanide salts used can also serve as conductive salts.

Another essential component of the electrolyte is silver, which is precipitated in dissolved form. It can be introduced into the electrolyte as required by the skilled artisan in the form of water-soluble salts. These silvers can be from the group consisting of silver methanesulfonate, silver carbonate, silver phosphate, silver pyrophosphate, silver nitrate, silver oxide, silver lactate, silver fluoride, silver bromide, silver chloride, silver iodide, silver thiocyanate, silver thiosulfate, silver hydantoin, silver sulfate, silver cyanide and alkali silver cyanides. In this context, potassium silver cyanide is very particularly advantageous. The silver salts are used in starting concentrations in the electrolyte of 2 to 200 g/l, preferably 10 to 100 g/l, particularly preferably 20 to 50 g/l (in each case based on Ag metal).

It should be noted that in a preferred embodiment, further ions may be present in low concentrations in dissolved form in the electrolyte. These are in particular those which result in a harder layer compared to the deposition of a pure silver layer (known as hard silver). These are ions selected from the group consisting of Sb, Bi, In, Sn, W, Mo, Pb, As, Cu, Ni. These are generally added in concentrations of 0.001 to 30 g/l, preferably 0.01 to 20 g/l, particularly preferably 0.1 to 10 g/l (in each case based on the corresponding metal).

The present invention also relates to a method for the galvanic deposition of a silver layer on an electrically conductive substrate, in which the electrically conductive substrate is immersed in an electrolyte according to the present invention and a current flow is established between an anode in contact with the electrolyte and the substrate as a cathode.

The temperature prevailing during the deposition of the dispersion layer can be selected as desired by the skilled person or the temperature is established by itself by external influences (frictional heat from the dispersion module). This will depend on the one hand on a sufficient deposition rate and on the usable current density range, and on the other hand on economical aspects or the stability of the electrolyte. It is advantageous to set the temperature of the electrolyte to 10°C to 70°C, preferably 15°C to 40°C, particularly preferably 20°C to 30°C.

In principle, the pH of the electrolyte can be adjusted as required by the skilled person. However, in this case, the skilled person will follow the aspect of introducing as little as possible into the electrolyte additional substances that may adversely affect the deposition of the corresponding layer. In a particularly preferred embodiment, the pH is therefore set only by adding a base. All compounds that are suitable for the corresponding application from the point of view of the skilled person can therefore be used. This is preferably done by using alkali hydroxides, oxides or carbonates. During electrolysis, the electrolyte according to the invention is set in a basic pH range, preferably above 8. Optimal results can be achieved with a pH of the electrolyte of 8 to 13, more preferably 9 to 11. In principle, the pH can be adjusted as required by the skilled person. However, in this case, the skilled person will follow the aspect of introducing as little as possible into the electrolyte additional substances that may adversely affect the deposition of the corresponding layer. Therefore, in a particularly preferred embodiment, the pH is set only by adding a base. All compounds that are suitable for the corresponding application from the point of view of the skilled person can therefore be used. Fluctuations may occur with respect to the pH of the electrolyte during electrolysis. Therefore, in a preferred embodiment of the method, the skilled person will monitor the pH during electrolysis and, if necessary, proceed to adjust the pH to the set value. The methods described here are known to those skilled in the art.

The current density established between the cathode and the anode during the deposition process in the electrolyte according to the invention can be selected by the skilled person on the basis of the efficiency and quality of the deposition. Depending on the application and the type of coating system, the current density in the electrolyte is advantageously set at 0.2 to 100 A/dm ^2. If necessary, the current density can be increased or decreased by adjusting system parameters such as, for example, the design of the coating cell, the flow rate, the relationship of the anode or cathode. Current densities of 0.2 to 50 A/dm ² are advantageous, 0.2 to 10 A/dm ² are preferred, 1 to 5 A/dm ² are very particularly preferred.

Instead of direct current, pulsed direct current or reverse pulse plating can also be applied, whereby the current flow is interrupted for a certain time (pulse plating) or the current flow is reversed. The use of pulse plating and reverse pulse plating in the form of current interruption for silver-graphite dispersion deposition is described in the literature (Arnet, R. et al, "Silver dispersionsschichten mit selbstschmierenden Eigenschaften", [Silver dispersion layers with self-lubricating properties], Galvanotechnik 2021, Vol. 1, p. 21 et seq.).

The electrolyte according to the invention and the method according to the invention can preferably be used for technical applications, for example for the galvanic deposition of silver layers for electrical plug connections and printed circuit boards. For technical applications, coating in continuous flow systems can also be used.

For technical applications, especially with current densities in the range of 0.5 to 50 A/ ^dm2 , layer thicknesses in the range of 0.1 to 100 μm are typically deposited in the rack process. For technical applications, layer thicknesses up to 200 μm or even 500 μm thick may also be deposited in continuous plants. For this purpose, the current densities are within the ranges mentioned above.

When using an electrolyte, various anodes can be used. Soluble or insoluble anodes are suitable, as well as combinations of soluble and insoluble anodes. When using soluble anodes, it is particularly preferred to use silver anodes (DE 1228887, Praktische Galvanotechnik, 5th edition, Eugen G. Leuze Verlag, p. 342 et seq., 1997).

Preferred insoluble anodes are anodes made from materials selected from the group consisting of platinized titanium, graphite, stainless steel, mixed metal oxides, glassy carbon anodes, and special carbon materials ("diamond-like carbon", DLC), or combinations of these anodes. Insoluble anodes of platinized titanium or titanium coated with mixed metal oxides are advantageous, the mixed metal oxides being preferably selected from iridium oxide, ruthenium oxide, tantalum oxide, and mixtures thereof. Iridium-transition metal mixed oxide anodes composed of iridium-ruthenium mixed oxide, iridium-ruthenium-titanium mixed oxide, or iridium-tantalum mixed oxide are also advantageously used for the practice of the invention. Further information can be found in Cobley, A. J et al. (The use of insoluble anodes in acid sulphate copper electrodeposition solutions, Trans IMF, 2001, 79(3), pp.113 and 114).

Possible electrically conductive substrates are those that can be coated with the electrolyte according to the invention in the acidic pH range. These are preferably precious metal-containing substrates or even non-precious metal substrates such as nickel or copper surfaces. Preferably, the silver layer according to the invention is deposited on a nickel or nickel alloy layer or on a copper or copper alloy layer. Suitable substrate materials advantageously used herein are copper-based materials such as pure copper, brass, bronze (e.g. CuSn, CuSnZn), or special copper alloys for plug connectors such as alloys containing silicon, beryllium, tellurium, phosphorus, or iron-based materials such as iron or stainless steel, or nickel, or nickel alloys such as NiP, NiW, NiB, gold or silver. The substrate material may also be a multilayer system that is galvanically coated or coated using other coating techniques. This concerns, for example, iron materials that are nickel- or copper-plated and then optionally gold-plated, palladium-pretreated, platinum-pretreated or silver-pretreated. In this case, the nickel- or copper-plated intermediate layer may also be made from the corresponding alloy electrolyte (e.g. NiP, NiW, NiMo, NiCo, NiB, Cu, CuSn, CuSnZn, CuZn, etc.). A further substrate material may be a wax core pre-coated (electroformed) with a conductive silver lacquer.

The term "electrolyte bath" is understood in the present invention to mean an aqueous solution that is placed in a corresponding container and used with an anode and a cathode under current flow for electrolysis. In the present case, the only solid components used represent an exception here.

The electrolyte according to the invention is aqueous. With the exception of added solid components and possibly insoluble heterogeneous defoamers, the compounds used in the electrolyte are soluble in the electrolyte. The term "soluble" refers to a compound that dissolves in the electrolyte at the operating temperature, which in this case is the temperature at which the electrolytic deposition is carried out. In the context of the present invention, a substance is considered soluble if at least 1 mg/l of the substance dissolves in the electrolyte at the operating temperature.

The electrolytes according to the invention have long-term stability. By combining the described brightening additives for silver deposition with the use of antifoaming agents, it has been possible to obtain coatings suitable for the described applications. These have a sufficiently low contact resistance and, moreover, maintain a surprisingly high surface integrity and therefore a low contact resistance in the contact circuit even after many plugging and rubbing processes. This could not be expected from the available state of the art.

[Example]
Electrolyte composition aqueous solution A:
30g/l as potassium silver cyanide
Potassium cyanide 130g/l
Potassium carbonate 5g / l
Se as potassium selenocyanate 2 mg/l
Ethylhexyl sulfate 4g/l
Benzene sulfonic acid 1g / l
100 g/l of graphite powder with an average particle size (d50) of 2 to 4 μm
Current density 1.5A/ ^dm2
Temperature: 25°C
Anode: Soluble pure silver anode

Graphite incorporation is not adversely affected by the addition of antifoaming ingredients.

Electrolyte composition aqueous solution B:
30g/l as potassium silver cyanide
Potassium cyanide 130g/l
Potassium carbonate 5g / l
Se as potassium selenocyanate 2 mg/l
Ethylhexyl sulfate 5g/l
Benzene sulfonic acid 1g / l
100 g/l of graphite powder with an average particle size (d50) of 2 to 4 μm
Current density 1.5A/ ^dm2
Temperature: 25°C
Anode: Soluble pure silver anode

Graphite incorporation is not adversely affected by the addition of antifoaming ingredients.

Claims (11)

1. An aqueous silver electrolyte for the galvanic deposition of a silver layer on a conductive substrate, comprising:
a) at least one soluble silver compound;
b) free cyanide in an amount of 20 to 200 g/l;
c) at least one gloss additive in an amount of 0.2 to 10 g/l;
d) at least one wetting agent in an amount of 0.1 to 15 ml/l;
e) at least one solid component in an amount of 2 to 200 g/l;
Including,
f) at least one antifoaming agent in an amount of 0.2 to 20 g/l. The electrolyte of claim 1, characterized in that the antifoaming agent is selected from the group of polyalkylene glycols. The electrolyte of claim 2, characterized in that the antifoaming agent has an average molar mass of at least 200 g/mol. The electrolyte according to any one of claims 1 to 3, characterized in that the gloss additive is selected from the group consisting of arylsulfonic acids. Electrolyte according to any one of claims 1 _to 4, characterized in that the solid component is selected from the group consisting of graphite, graphite fluoride, graphite oxide, diamond, _Al2O3 coated graphite, or mixtures thereof. An electrolyte according to any one of claims 1 to 5, characterized in that the wetting agent is selected from the group of alkyl sulfates. An electrolyte according to any one of claims 1 to 6, further comprising at least 0.1 to 500 mg/l of a salt of Se or Te anion (based on selenium or tellurium). 1. A method for the galvanic deposition of a silver layer on a conductive substrate, comprising:
A method comprising immersing an electrically conductive substrate in the electrolyte according to any one of claims 1 to 7 and establishing a current flow between an anode in contact with said electrolyte and said substrate as a cathode. The method according to claim 8, characterized in that the temperature of the electrolyte is between 20°C and 90°C. 10. The method according to claim 8 and/or 9, characterized in that the current density during electrolysis is between 0.2 and 100 A/ ^dm2 . The method according to any one of claims 8 to 10, characterized in that the pH during electrolysis is continuously set to a value greater than 8.