EP1285104B1 - Electrolyte and method for depositing tin-silver alloy layers - Google Patents

Description

The present invention relates to an acidic electrolyte for the deposition of tin-silver alloys, a process which uses this electrolyte by the process available coatings and the use of the Electrolytes for coating electronic components.

In the production of electronic assemblies is the soft soldering using the eutectic solder alloy SnPb (63% by weight) Sn 37 wt.% Pb) the standard method of joining technique. Voted on it is customary to maintain solderability the components to be connected by galvanic To provide processes with a lead tin layer. The Bleizinnschichten can in principle any Have alloy composition, as are the pure Metals can be used. The most common alloys with 3 to 40 wt.% Pb, in particular 5 to 20 wt.% Pb. High lead alloys with e.g. 95 wt.% Pb become for Special applications used when higher Melting temperatures are desired. Coatings with Pure tin are also widely used, although here fundamental problems because of the not excludable Risk of whiskering exist.

Although the mentioned lead tin alloys are very good Soft soldering properties are very large Strive for a substitution of lead. At a Scrapping and landfill of equipment with leaded solder joints there is a risk that by Corrosive processes lead converted into water-soluble form can be. This can make it a long term one corresponding pollution of groundwater.

A promising alternative for the eutectic Lead tin solder is the tin-silver alloy. Here too will again suitably the eutectic composition used as these are the lowering of the Processing temperatures to a minimum. Too high Processing temperatures may e.g. when soldering by PCBs and components of electronic assemblies too cause irreversible damage. The eutectic Composition of the tin-silver alloy consists of 96.5 Wt.% Sn and 3.5 wt.% Ag. The melting point of the eutectic is 221 ° C. By alloying small amounts of Cu (about 1 Wt.%) Is a further lowering of the melting point 217 ° C possible.

The tin-silver solder SnAg3.5 is therefore also suitable as Alternative for the Bleizinnlot, because the former already as Lot is used for special applications and therefore already Practical experience available. In case of using the Tin-silver-Lotes, in turn, it is desirable that the Components for maintaining the solderability galvanic with Coatings of a tin-silver alloy are coated. There the main component of the solder is tin, would be one too Component coating with pure tin with the solder alloy compatible. Pure tin layers are but because of the already mentioned risk of whisker formation less desirable.

An electrolyte for coating with silver-tin alloys with a silver content of> 80% by weight is described in J. Cl. W. Fluehmann, Plat. Surf. Finish. Jan. 1983, 46 ". The deposition takes place here from an alkaline Electrolytes with pyrophosphate as complexing agent for tin and Cyanide as complexing agent for silver. Another basic Electrolyte based on tetravalent Stannations, Silver nitrate and hydantoin as a complexing agent for silver (Ohhara, M. Yuasa, Tokyo Univ. of Science [1996]).

However, a high silver content of the coating is on the one hand not desirable for economic reasons. on the other hand should the silver content of the tin-silver alloy advantageously ≤ 10 wt.%, in order to reduce the solderability of the Coating at low temperatures, i. near the Eutectic of the tin-silver alloy, to enable. In addition, the use of cyanide due to its high Toxicity can be avoided.

Furthermore, alkaline electrolytes are the following Disadvantages connected. Plants so far for the deposition used by tin-lead alloys of acidic electrolytes are not technically wastewater treatment cyanide electrolytes designed. Further use these conventional systems for the deposition of tin-lead alloys However, for economic reasons, it is also for the deposition of tin-silver alloys is desired.

In addition, the deposition rate is alkaline Electrolytes comparatively low. Since tin in the alkaline Milieu in tetravalent form, is the Deposition rate compared to a Sn (II) acidic electrolyte containing reduced by 50%.

The problem with the development of an acidic electrolyte for the deposition of tin-silver alloys with a low silver content is the large potential difference between the metals Sn and Ag. The standard potentials are: Sn ²⁺ + 2e → Sn ⁰ : -0.12 V Ag ⁺ + e → Ag ⁰ : + 0.8V

Is in an electrolyte, the two separable metals contains, the potential difference of the two metals is large prefers the metal with the more positive standard potential deposited. That from a tin-silver electrolyte preferably silver deposited.

To make matters worse in acidic tin-silver systems added that tin in divalent form is a reducing agent. Sn ²⁺ → Sn ⁴⁺ + 2e: E ₀ = + 0, 15V

Here, there is a difference to the standard potential of silver of 0.65 V. If an acidic, Sn (II) -containing solution is therefore mixed with an acidic, silver-containing solution, the silver is reduced according to the following equation: Sn ²⁺ + 2Ag ⁺ → 2Ag + Sn ⁴⁺

The reaction is recognizable by precipitation of finely divided black silver powder or the deposition of a silver level on the container wall. Another problem is that, in general, the base materials to be coated have a more negative standard potential than silver. For electronic components, the base material is often copper or a copper alloy. The value for the standard potential Cu → Cu ²⁺ is + 0.35 V. The difference to silver thus amounts to 0.45 V. This potential difference causes silver to be deposited in the charge exchange on the copper surface. Such a reaction may impair the adhesion of the subsequently deposited layers.

Prerequisite for the successful deposition of tin-silver alloys from a strongly acidic, divalent It is therefore appropriate to use suitable electrolytes containing tin ions Find compounds that complex the silver and thereby a shift of the standard potential of the Silver to produce more negative values. In addition, the Chelating agents selectively act on silver. At the same time Complexation of tin would also be a shift of the Standard potential to more negative values. The original potential difference of uncomplexed ions would be restored.

As a complexing agent for silver is in the patent application EP 0 893 514 called potassium iodide, which is the standard potential of silver shifts by -870 mV. This gives you almost an approximation of the standard potential values for tin and Silver.

Disadvantageous for the use of potassium iodide is that it is in large excess of the amount of silver to be complexed must be used. There are concentrations of e.g. 300 g / l necessary. Since potassium iodide is an expensive compound, Such a process could not be economically viable become. In addition, a pH of 4 to 6 must be set become. In this area bivalent tin is only in Presence of complexing agents soluble. This effect in turn, a shift of the standard potential of tin and thus increase the potential difference between Tin and silver. Effective complexing agents for tin are e.g. Hydroxy carboxylic acids. These complicate the precipitation of Heavy metal compounds in wastewater treatment and are therefore undesirable.

In addition, weakly acidic electrolytes (pH 4 to 6) have only a low electrical conductivity. Such electrolytes can be used only for the deposition of metal layers at low cathodic current densities (0.1 to 5 A / dm ² ), ie in the so-called drum and frame technology with deposition rates of usually 0.05 to 2.5 microns / min. They are not suitable for high cathodic current densities (5 to 100 A / dm ² ), which are present in the high-speed deposition (continuous plating), with which deposition rates of 2.5 to 50 microns / min are achieved. In summary, therefore, the method according to EP 0 893 514 is associated with a number of disadvantages.

EP 0 854 206 describes aromatic thiol compounds as Complexing agent. With such connections is a Displacement of the resting potential of silver by approx. -400 mV reachable. The values are thus sufficient to one to achieve common deposition of tin and silver and to get a stable electrolyte.

Although in the said patent application both the Thiol compounds RSH as well as those derived therefrom Disulfide RSSR (R = aromatic radical) as complexing agent can be described by simple measurements, that complexing only by the thiol compounds is reached. Thus, for example, for that in Scripture called 2-mercaptopyridine a shift of Resting potential of silver around -564 mV. The corresponding Disulfide, 2,2'-dithiopyridine, causes only one Displacement of -5 mV, thus virtually does not complex more.

Since aromatic thiol compounds are easily oxidized to disulfide In the long run, there is no sufficient stability of the Electrolytes containing these aromatic thiol compounds as Complexing agents include, i. one permanent operation of the electrolyte is not feasible.

Aromatic compounds also often have one poor biodegradability and thus can too Lead to problems in biological wastewater treatment.

In the Japanese application Toku-Gan H7-330437 be Thiourea or compounds derived therefrom Called complexing agent for silver. With these connections one also reaches a sufficient displacement of the Resting potential. The disadvantage of thiourea and its Derivatives a not inconsistent health risk. Some of these compounds are particularly toxic to Aquatic organisms.

The object of the invention is therefore one against oxidation stable, acidic electrolytes for the deposition of tin-silver alloys to provide for the Use both at low cathodic current densities (Drum or frame technology) as well as at high cathodic current densities (continuous plating process) in Existing plants previously used for the separation of standard lead-tin layers are used is suitable and is not toxic, in particular the Wastewater treatment is not difficult, i. none Environmental degradation.

The object is achieved by an acidic aqueous electrolyte for the deposition of tin-silver alloys containing one or more alkyl or alkanolsulfonic acids, one or more soluble stannous salts, one or more soluble silver (I) salts and one or more organic sulfur compounds, wherein the organic sulfur compounds have the general formula: XR ¹ - [ZR ² ] _n -ZR ³ -Y wherein each Z represents a sulfur atom or an oxygen atom and the radicals Z are the same or different, R ¹ , R ² and R ³ independently represent each an alkylene group having 2 to 10 carbon atoms, n = 1 to 20, X and Each Y is independently -OH, -SH or -H, and in the case where only one Z represents a sulfur atom, X and / or Y is -SH, and in the case where Z is exclusively an oxygen atom, X and Y are each -SH are.

Alkylene groups having 2 to 5 carbon atoms are preferred, e.g. Ethylene, n-propylene, iso-propylene, n-butylene, iso-butylene and tert-butylene groups.

Bis-(hydroxyethyl)-sulfid: HO-CH₂-CH₂-S-CH₂-CH₂-OH

3,6-Dithiaoctandiol-1,8: HO-CH₂-CH₂-S-CH₂-CH₂-S-CH₂-CH₂-OH

3,6-Dioxaoctandithiol-1,8: HS-CH₂-CH₂-O-CH₂-CH₂-O-CH₂-CH₂-SH

3,6-Dithia-1,8-dimethyloctandiol-1,8: HO-CH(CH₃)-CH₂-S-CH₂-CH₂-S-CH₂-CH(CH₃)-OH

4,7-Dithiadecan: H₃C-CH₂-CH₂-S-CH₂-CH₂-S-CH₂-CH₂-CH₃

3,6-Dithiaoctan: H₃C-CH₂-S-CH₂-CH₂-S-CH₂-CH₃

3,6-Dithiaoctandithiol-1,8: HS-CH₂-CH₂-S-CH₂-CH₂-S-CH₂-CH₂-SH

Furthermore, the following organic sulfur compounds are preferred:

Bis (hydroxyethyl) sulfide: HO-CH ₂ -CH ₂ -S-CH ₂ -CH ₂ -OH

3.6 dithiaoctanediol-1.8: HO-CH ₂ -CH ₂ -S-CH ₂ -CH ₂ -S-CH ₂ -CH ₂ -OH

3.6 Dioxaoctandithiol-1.8: HS-CH ₂ -CH ₂ -O-CH ₂ -CH ₂ -O-CH ₂ -CH ₂ -SH

3,6-dithia-1,8-dimethyloctandiol-1.8: HO-CH (CH ₃ ) -CH ₂ -S-CH ₂ -CH ₂ -S-CH ₂ -CH (CH ₃ ) -OH

4,7-dithiadecane: H ₃ C-CH ₂ -CH ₂ -S-CH ₂ -CH ₂ -S-CH ₂ -CH ₂ -CH ₃

3,6-dithiaoctane: H ₃ C-CH ₂ -S-CH ₂ -CH ₂ -S-CH ₂ -CH ₃

3.6 Dithiaoctandithiol-1.8: HS-CH ₂ -CH ₂ -S-CH ₂ -CH ₂ -S-CH ₂ -CH ₂ -SH

The molar ratio of the organic sulfur compound to soluble silver (I) salt (molar amount of organic Sulfur compound: molar amount of soluble silver (I) salt) is preferably at least 1, more preferably 5: 1 to 1: 1, particularly preferably 3: 1.

The tin (II) may be present in the electrolyte as a salt of mineral, Alkyl sulfonic or alkanol sulfonic acids present. Examples for salts of mineral acids are sulfates and Tetrafluoroborates. Preferred salts of alkylsulfonic acids are e.g. Methanesulfonates, ethanesulfonates, n- and iso-propanesulfonates, Methanedisulfonates, ethanedisulfonates, 2,3-propanedisulfonates and 1,3-propanedisulfonates. usable Alkanol sulfonates are 2-hydroxyethanesulfonates, 2-hydroxypropanesulfonates and 3-hydroxypropanesulfonates. Particularly preferred is tin (II) methanesulfonate.

The tin (II) salts are preferred in the electrolyte in one Amount of 5 to 200 g / l electrolyte, more preferably 10 to 100 g / l of electrolyte, calculated as tin (II), present.

The silver (I) is in the electrolyte preferably in the form of Salts of mineral, alkylsulfonic or alkanolsulfonic acids in front. Examples of mineral, alkylsulfone or Alkanol sulfonic acid salts are as described above for Tin (II) salts mentioned compounds. Particularly preferred Silver (I) methanesulfonate.

In the electrolyte are preferably 0.05 to 50 g / l, especially preferably 0.1 to 20 g / l, silver (I) salts, calculated as Silver (I), present.

The soluble silver salts can be generated upon preparation of the electrolyte by adding silver compounds which dissolve in the acidic range to form salts. Examples of silver compounds which dissolve in the acidic range to form salts are silver oxide (Ag ₂ O) or silver carbonate (Ag ₂ CO ₃ ).

The electrolyte may also contain various additives, the usually in acidic electrolytes for the separation of Tin alloys are used, e.g. grain refining Additives, wetting agents and / or brighteners included.

As grain refining additives, for example, nonionic surfactants of the general formula RO- (CH ₂ -CH ₂ -O) _n -H, wherein R is an alkyl, aryl, alkaryl or aralkyl group having 1 to 20, preferably 1 to 15, carbon atoms and n = 1 to 20.

The grain refining additive is preferably in an amount from 0.1 to 50 g / l of electrolyte, preferably 1 to 10 g / l Electrolyte, before.

The wetting agent may be used in an amount of 0.1 to 50 g / l Electrolyte, preferably 0.5 to 10 g / l electrolyte present.

The alkyl sulfonic acid and the alkanol sulfonic acid have preferably 1 to 10, more preferably 1 to 5, Carbon atoms on. As the alkylsulfonic acids, e.g. Methanesulfonic acid, ethanesulfonic acid, n-propanesulfonic acid, iso-propane sulfonic acid, methanedisulfonic acid, Ethanedisulfonic acid, 2,3-propanedisulfonic acid or 1,3-propanedisulfonic acid available. usable Alkanol sulfonic acids are e.g. 2-hydroxyethanesulfonic acid, 2-hydroxypropanesulfonic acid and 3-hydroxypropanesulfonic acid.

The alkyl and / or alkanol sulfonic acid is in the electrolyte preferably in a concentration of 50 to 300 g / l Electrolyte, more preferably 100-200 g / l electrolyte.

The pH of the acidic electrolyte is preferably 0 to < 1.

The present invention further provides a method for the electrolytic coating of substrates with tin-silver alloys, in which using the Electrolyte according to the invention, an anode of metallic Tin and a cathode of the substrate to be coated the Coating applied by passing direct current becomes made available.

The applied with the inventive method tin-silver alloys can silver in a proportion of 0.1 to 99.9 wt.% Contained. To the solderability of the alloys Allow low temperatures, they prefer a silver content of 0.5 to 10 wt.%, more preferably 2 to 5% by weight, on. The silver content may e.g. by Variation of the concentration ratios of tin and Silver salts in the electrolyte, the electrolyte temperature and the Flow rate of the electrolyte, based on the coating material, to be adjusted.

The current density can be 0.1 A / dm ² (drum or rack technology) up to 100 A / dm ² (high-speed systems).

The temperature of the electrolyte is preferably in the range from 0 to 70 ° C, more preferably in the range of 20 to 50 ° C.

As a substrate to be coated, e.g. copper surfaces or surfaces of copper-containing alloys.

The electrolyte according to the invention can be used for the coating of electronic components are used.

The present invention will become apparent from the following Embodiments explained.

example 1

A tin-silver electrolyte was prepared as follows: 150 g / l 70% aqueous methanesulfonic acid 20 g / l Tin (II), as Zinnmethansulfonat 1 g / l Silver (I) as silver methanesulfonate 2 g / l 3.6 dithiaoctanediol-1.8 4 g / l Nonylphenol ethoxylate with 14 EO groups (Lutensol AP-14 from BASF)

With this electrolyte copper sheets were coated in the frame technology at a current density of 0.5 to 2 A / dm ² . The electrolyte temperature was 20 ± 2 ° C. The electrolyte had a pH of 0. Finely crystalline bright, silky-gloss layers were obtained without signs of dendrite formation. The measurement of the alloy composition by means of X-ray fluorescence methods gave the following values: Deposition at 0.5 A / dm ² 5.8% by weight Ag Deposition at 2 A / dm ² 3.9% by weight Ag

Example 2

The following tin-silver electrolyte was prepared: 150 g / l 70% aqueous methanesulfonic acid 40 g / l Tin (II), as Zinnmethansulfonat 1.5 g / l Silver (I) as silver methanesulfonate 4 g / l 3.6 dithiaoctanediol-1.8 4 g / l ethoxylated bisphenol A (Lutron HF-3 from BASF)

The deposition of the tin-silver coating from this electrolyte on a copper sheet was carried out at 40 ± 2 ° C in a high-speed system in the current density range of 5 to 20 A / dm ² . The electrolyte was stirred vigorously (magnetic stirrer, 40 mm stirring bar, stirring speed 700 rpm). Light gray, semi-matt deposits were achieved. The determination of the alloy composition by means of X-ray fluorescence measurement gave the following values: Deposition at 5 A / dm ² 5.7% by weight Ag Deposition at 10 A / dm ² 4.6% by weight Ag Deposition at 15 A / dm ² 4.1% by weight Ag Deposition at 20 A / dm ² 4.4% by weight Ag

Example 3

The following tin-silver electrolyte was prepared: 150 g / l 70% aqueous methanesulfonic acid 20 g / l Tin (II), as Zinnmethansulfonat 0.5 g / l Silver (I) as silver methanesulfonate 2 g / l 3.6 dithiaoctanediol-1.8 4 g / l Nonylphenol ethoxylate with 14 EO groups (Lutensol AP-14 from BASF)

The deposition was carried out as indicated in Example 1. A uniform light gray-smooth deposit was obtained. The silver content at 2 A / dm ² was 2 wt.%.

Example 4

In order to determine the oxidation resistance of the organic sulfur compounds according to the invention, which is essential for the stability of the electrolyte, the following solution was prepared: 150 g / l 70% aqueous methanesulfonic acid 0.5 g / l Silver (I) as silver methanesulfonate 3.64 g / l 3.6 dithiaoctanediol-1.8

The solution was stirred in an open beaker at a temperature between 20 and 25 ° C with a stirring frequency of 300 rpm. The shift of the resting potential of the silver through the organic sulfur compound was measured using a silver / silver chloride electrode immediately after the preparation of the solution, after one day, after three days, and after six days. The results are shown in Table 1. Time [d] Resting potential shift [mV] 0 -360 1 -360 3 -360 6 -360

The constant value of the shift of the rest potential of the Silver through the complexing organic Sulfur compound over the entire duration of the experiment clarifies that the organic sulfur compound unchanged as a complexing compound. A Oxidation to a compound that does not have the silver complexed, i. the potential of the silver not too smaller values, does not take place. One Electrolyte in which the organic Sulfur compounds used to complex the silver Thus, it has excellent stability.

Comparative Example 1

The experiment carried out in Example 4 was repeated except that the 3,6-dithiaoctanediol-1,8 was replaced by the aromatic sulfur compound 2-mercaptoaniline (2.5 g / l). Table 2 shows the test results. Time [d] Resting potential shift [mV] 0 -380 1 -350 3 -80 6 -20

The test results make it clear that the proportion of the Complexed silver in the solution with progressive Duration of the test decreases sharply. After 6 days is the Resting potential value of uncomplexed silver is almost reached. The decrease of the rest potential shift is on the Oxidation of 2-mercaptoaniline to 2,2'-dithioaniline attributed, which has no complexing effect. A stable electrolyte is thus when using the opposite Oxidation of unstable aromatic sulfur compounds not to obtain.

Claims (8)

1. Acid aqueous electrolyte for the deposition of tin-silver alloys comprising
      one or more alkyl sulphonic acids and/or alkanol sulphonic acids,
      one or more soluble tin (II) salts,
      one or more soluble silver (I) salts and
      one or more organic sulphur compounds,
   characterised in that the organic sulphur compounds have the following general formula: X-R¹-[Z-R²]_n-Z-R³-Y where Z denotes in each case a sulphur atom or an oxygen atom and the other Z are the same or different, R¹, R² and R³ each denote independently of each other an alkylene group which has 2 to 10 carbon atoms, n is 1 to 20, X and Y are each independently of each other -OH, -SH or -H, and in the event that only one Z denotes a sulphur atom, X and/or Y is -SH, and in the event that Z is exclusively an oxygen atom, X and Y are each -SH.
2. Electrolyte according to claim 1, characterised in that the molar ratio of the organic sulphur compound to the soluble silver (I) salt (molar quantity organic sulphur compound : molar quantity soluble silver (I) salt) is at least 1.
3. Electrolyte according to claim 1 or 2, characterised in that the tin (II) salts are the salts of mineral, alkyl sulphonic or alkanol sulphonic acids.
4. Electrolyte according to one or more of claims 1 to 3, characterised in that the silver (I) salts are the salts of mineral, alkyl sulphonic or alkanol sulphonic acids.
5. Electrolyte according to one or more of claims 1 to 4, characterised in that a grain refining additive is contained.
6. Electrolyte according to claim 5, characterised in that non-ionic surface-active agents having the general formula RO-(CH₂CH₂-O)_n-H, where R denotes an alkyl, aryl, alkaryl or aralkyl group and n is 1 to 20, are present as the grain refining additive.
7. Method for the electrolytic coating of substrates with tin-silver alloys, characterised in that coating is carried out while passing direct current through, using an electrolyte according to any of claims 1 to 6, an anode made of metallic tin and a cathode made of the substrate to be, coated.
8. Use of the electrolyte according to any of claims 1 to 6 for the coating of electronic components.