CN118064943B

CN118064943B - Stabilizer for cyanide-containing gold-silver electroplating solution, electroplating solution and electroplating method

Info

Publication number: CN118064943B
Application number: CN202410077009.6A
Authority: CN
Inventors: 任长友; 王彤; 邓川; 刘松; 邓威
Original assignee: Shenzhen United Blue Ocean Applied Materials Technology Co ltd
Current assignee: Shenzhen United Blue Ocean Applied Materials Technology Co ltd
Priority date: 2024-01-10
Filing date: 2024-01-19
Publication date: 2024-12-06
Anticipated expiration: 2044-01-19
Also published as: CN118064943A

Abstract

The invention discloses a stabilizer of cyanide-containing gold-silver electroplating solution, the electroplating solution and an electroplating method, and belongs to the technical field of electroplating. The stabilizer is hydantoin or derivatives thereof, and the molar ratio of the stabilizer to silver ions in the cyanide-containing gold-silver electroplating solution is more than 2. The gold and silver electroplating solution comprises water-soluble cyanide-containing gold salt, water-soluble cyanide-containing silver salt, conductive salt, the stabilizer and water, wherein the concentration of gold ions is 1-20g/L, the concentration of silver ions is 0.3-10g/L, and the concentration of conductive salt is 10-120g/L. The stabilizer has excellent protection capability on silver ions in the cyanide-containing gold-silver electroplating solution, can also play a role of a leveling agent, has extremely high stability, does not need to additionally add free cyanide ions and the leveling agent, is friendly to the manufacturing process of photoresist related to semiconductor chip manufacturing, does not change electroplating equipment and the electroplating solution, and can obtain gold-silver alloy plating layers with different gold contents only by changing current density.

Description

Stabilizer for cyanide-containing gold-silver electroplating solution, electroplating solution and electroplating method

Technical Field

The invention belongs to the technical field of electroplating, and relates to a stabilizer of a cyanide-containing gold-silver electroplating solution, an electroplating solution and an electroplating method.

Background

In packaging semiconductor chips, pure gold is generally used to prepare a package structure based on excellent characteristics of gold in order to secure chip performance. And compared with pure gold, the gold-silver alloy has lower resistance and higher heat conductivity coefficient, thereby being beneficial to improving the heat dissipation performance of the chip and increasing the interconnection density of electric appliances.

The gold-silver alloy packaging structure prepared by using the electroplating process replaces a pure gold packaging structure, has the same technical requirements for gold-silver electroplating liquid as other application scenes, such as high stability and the like, and has different requirements for other application scenes, such as (1) the photoresist on a chip to be packaged cannot be swelled, and (2) gold-silver coating layers with different gold contents can be conveniently manufactured to form a composite structure so as to simultaneously meet the multi-aspect requirements of the packaging structure on hardness, roughness and sulfuration oxidation resistance.

Gold and silver alloy plating solutions are classified into two major classes, cyanide-containing and cyanide-free, in which gold ions and silver ions and cyanide ions are more stable than cyanide-free plating solutions due to their higher complexing constants. However, silver ions are photosensitive and, in addition to the cyanide moiety of the potassium silver cyanide, it is often necessary to introduce free cyanide ions into the bath to ensure that the silver ions do not decompose in the presence of light. Besides the great potential safety hazard, the free cyanide ions can swell the photoresist, so that the manufacturing precision of the circuit is reduced. Therefore, developing a stabilizer for the cyanide-containing gold-silver electroplating solution and the corresponding electroplating solution, which does not contain free cyanide ions, and simultaneously ensures high stability of the electroplating solution, is a difficult point in the current industry.

CN104099653B discloses a technical scheme for preparing gold-silver alloy by using gold potassium cyanide and silver potassium cyanide electroplating solution, the pH of the electroplating solution is 6.5-7.0, and the electroplating solution contains oxalate of 2-5mL/L as leveling agent (see paragraphs 160-165 of the specification). Because the plating solution does not contain a protective agent of silver ions, such as free cyanide ions, and in addition, in the pH range, the cyanide ions provided by the main salt have poor coordination ability to the silver ions, the stability of the plating solution is poor.

GB2046794A and CH629260A5 also disclose the technical proposal of preparing gold-silver alloy by using gold potassium cyanide and silver potassium cyanide electroplating solution, wherein the electroplating solution does not contain free cyanide ions, but uses high molecular additives such as polyethyleneimine or polyethylene oxide as brightening agent. Firstly, the stability of the plating solutions is poor due to the absence of a silver ion protective agent, and secondly, polyethyleneimine or polyethylene oxide is easy to co-deposit in the plating layer, so that the internal stress of the plating layer is too high, holes are formed in the plating layer after annealing, and the corrosion resistance of the plating layer is reduced.

Disclosure of Invention

The invention aims to provide a stabilizer of cyanide-containing gold-silver electroplating solution, a corresponding electroplating solution and an electroplating method, wherein the stabilizer has excellent protection capability on silver ions in the cyanide-containing gold-silver electroplating solution, avoids photodecomposition of the silver ions, and simultaneously can play a role of a leveling agent, so that the gold-silver electroplating solution does not need to be additionally added with the leveling agent.

In one aspect of the invention, a stabilizer of the cyanide-containing gold-silver electroplating solution is provided, wherein the stabilizer is hydantoin or a derivative thereof, and the molar ratio of the stabilizer to silver ions in the cyanide-containing gold-silver electroplating solution is more than 2, preferably 2-20.

The stabilizer hydantoin or the derivative thereof provided by the invention can be used as a complexing agent of silver ions to prevent photoreduction of the silver ions and improve the stability of the electroplating solution. The molar ratio of hydantoin or its derivative to silver ion is above 2, preferably 2-20, to ensure coordination complex with silver ion, and above 20, the molar ratio of hydantoin or its derivative to silver ion may result in too high a viscosity of the plating solution and thus reduced uniformity of plating thickness. On the other hand, in addition to the complexation with silver ions, hydantoin or its derivative and silver ions achieve a synergistic effect, acting as a crystallization modifier. The crystallization regulator is added into the electroplating solution, and the technical scheme of the invention does not need to add the crystallization regulator, so that a very flat plating layer can be obtained.

Further, the derivatives of hydantoin are 5, 5-dimethylhydantoin (CAS number: 77-71-4), 1, 3-dimethylol-5, 5-dimethylhydantoin (CAS number: 6440-58-0) or 1-aminohydantoin (CAS number: 6301-02-6), preferably 5, 5-dimethylhydantoin.

In still another aspect, the invention provides a gold-silver electroplating solution, which comprises water-soluble cyanide-containing gold salt, water-soluble cyanide-containing silver salt, conductive salt, the stabilizer and water, wherein the concentration of gold ions is 1-20g/L, the concentration of silver ions is 0.3-10g/L, and the concentration of conductive salt is 10-120g/L.

Further, the pH value of the gold and silver electroplating solution is 8-10. When the pH is less than 8, the complexing ability of the stabilizer and silver ions is lowered, resulting in deterioration of the stability of the plating solution, and when the pH is more than 10, the photoresist is adversely affected.

Further, the water-soluble cyanide-containing gold salt is preferably potassium aurous cyanide (formula KAu (CN) ₂), and the gold ion concentration is 1-20g/L. When the concentration is less than 1g/L, the plating layer tends to burn during plating, and when the concentration exceeds 20g/L, the cost increases due to the effect of carry-over. The water-soluble cyanide-containing silver salt is preferably silver potassium cyanide (molecular formula KAg (CN) ₂), the concentration of silver ions is 0.3-10g/L, and the proportion of gold and silver in the plating layer can be regulated by regulating the concentration of silver ions in the electroplating solution according to different scene requirements.

The inventors found that when gold ions and silver ions are in the above concentration ranges, the gold content of the gold-silver alloy plating layer obtained can be changed by simply adjusting the current density using the same plating solution and plating apparatus. Specifically, the gold content in the gold-silver alloy plating layer obtained by the lower current density is low, and the gold content in the gold-silver alloy plating layer obtained by the higher current density is high. The inventors have also found that when the gold content in the gold-silver alloy plating is in a moderate range (20-50 wt%), the hardness and roughness can meet the requirements of semiconductor chip packaging, while the resistance to sulfidation oxidation requires a gold content of over 60 wt%. Therefore, the composite packaging structure can be designed, wherein the gold content of the packaging structure main body is in a moderate range, so that the hardness and the roughness meet the requirements, and meanwhile, a protective layer with a much smaller thickness is formed on the outer surface of the packaging structure main body, the integral hardness and the roughness are not influenced, and the requirements of sulfuration and oxidization resistance are met.

Further, the conductive salt is preferably pyrophosphate, and can be one or more of sodium pyrophosphate, potassium pyrophosphate or ammonium pyrophosphate. The pyrophosphate can improve the conductivity of the plating solution, improve the electroplating uniformity, and the concentration is too low to meet the conductivity requirement of the plating solution, and too high concentration can lead to high viscosity of the plating solution so as to influence the electrodeposition of gold or silver.

In another aspect, the present invention provides a plating method, wherein the gold and silver plating solution is used for plating, the operation temperature of plating is 20-40 ℃, and the current density of plating is 0.3-1.5A/dm ². The operation temperature is lower than 20 ℃, the temperature is not easy to control, the electroplating uniformity is poor, and the roughness of the plating layer is larger than 40 ℃, so that the subsequent welding performance or bonding effect can be influenced. The current density of the electroplating is operated within the range of 0.3-1.5A/dm ², and the current density is improved and the gold content in the plating layer is increased for the electroplating solution with the same composition. When the current density is less than 0.3A/dm ², the plating speed is slow to affect the production efficiency, and when the current density is more than 1.5A/dm ², the plating layer is at risk of scorching.

Further, when the gold content in the gold-silver plating layer is required to be 50wt% or less, the current density of the plating is 0.3-0.6A/dm ², and when the gold content in the gold-silver plating layer is required to be 60wt% or more, the current density of the plating is 1.1-1.5A/dm ².

Further, annealing treatment is carried out on the obtained gold-silver alloy electroplated layer, wherein the annealing temperature is 270-300 ℃, and the annealing time is 5-120min. By annealing treatment, the stress can be eliminated, and the performance of the gold-silver alloy electroplated layer can be improved. The annealing temperature and the annealing time can be determined through experiments according to the area and the thickness of the gold-silver alloy electroplated layer within the above parameter ranges, and the general rule is that the larger the area is, the thicker the thickness is, the higher the annealing temperature is required and the longer the annealing time is.

It is a further aspect of the present invention to provide an article obtained by the above-described electroplating method.

The stabilizer provided by the invention has the beneficial technical effects that the stabilizer has excellent protection capability on silver ions in the cyanide-containing gold-silver electroplating solution, avoids photodecomposition of the silver ions, and simultaneously can play a role of a leveling agent. The electroplating solution provided by the invention has extremely high stability, free cyanide ions are not required to be added additionally, the preparation process related to photoresist for manufacturing the semiconductor chip is friendly, and meanwhile, a leveling agent is not required to be added additionally. According to the electroplating method provided by the invention, gold-silver alloy plating layers with different gold contents can be obtained by only changing the current density on the premise of not changing electroplating equipment and electroplating liquid.

Drawings

Fig. 1 is a schematic diagram of a gold-silver alloy package obtained by using the plating solution provided by the invention.

Reference sign 1-gold-silver alloy packaging structure main body, 2-thin layer with higher gold content and 3-chip substrate.

Detailed Description

The technical scheme of the invention is clearly and completely described below with reference to the attached drawings. It will be apparent that the described embodiments are only some, but not all, of the embodiments of the invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the present invention without making any inventive effort, are intended to fall within the scope of the present invention.

The present invention uses the following analytical test methods to test the gold and silver plating solutions and gold and silver alloy plating layers obtained in each of examples and comparative examples.

(1) And (5) testing the stability of the plating solution. After the plating solution is prepared, the plating solution is placed for 24 hours at room temperature, if the plating solution is clear and no metal film is formed on the surface of the plating solution, the plating solution is judged to be stable, and if the plating solution is turbid or a layer of metal film is formed on the surface of the liquid, the plating solution is judged to be unstable.

(2) And (5) testing the gold content of the plating layer. A seed layer of 300nm TiW and 80nm Au was first sputter deposited on a silicon wafer using PVD, and then gold and silver alloy was electroplated onto the wafer, and the gold and silver content in the plating was tested using SEM-EDX (Zeiss, sigma 300).

(3) And testing the roughness of the plating layer. A seed layer of 300nm TiW and 80nm Au was first sputter deposited on a silicon wafer and then plated with gold and silver alloy using a laser confocal microscope (KEYENCE, VK-2050) to test the roughness of the plating.

(4) And testing the hardness of the plating layer. A seed layer of 300nm TiW and 80nm Au was first deposited on a silicon wafer by PVD sputtering, and then a gold-silver alloy of 10 μm thickness was electroplated onto the wafer, and the hardness of the coating was tested using a hardness tester (Mitutoyo, HM-200).

Example 1

600ML of deionized water was added to a 1L beaker, 60g of potassium pyrophosphate and 30g of hydantoin were added, respectively, and the solution was sufficiently dissolved, then 10g of gold potassium cyanide and 5g of silver potassium cyanide were added, respectively (molar ratio of hydantoin to silver ion was 6.5), the pH of the plating solution was adjusted to 9, and deionized water was added to make the volume of the plating solution 1000mL. The plating temperature was set at 30℃and a 3 ́ cm gold-plated silicon wafer was used, the current density was 0.3A/dm ², and the plating thickness was 10. Mu.m. The annealing temperature after electroplating was 290 ℃ and the annealing time was 30 minutes.

The gold content in the plating layer after electroplating is 26wt%, the roughness Ra is 57nm, the hardness of the plating layer before annealing is 102HV, the hardness of the plating layer after annealing is 65HV, and the plating solution is stable after being placed for 24 hours.

Example 2

The difference from example 1 is that the current density is changed to 0.4A/dm ², and the other conditions are the same.

The gold content in the plating layer after electroplating was 31wt%, the roughness Ra was 63nm, the hardness of the plating layer before annealing was 113HV, the hardness of the plating layer after annealing was 62HV, and the plating solution was stable after 24 hours.

Example 3

The same difference as in example 1 is that the current density is changed to 0.5A/dm ², and the other conditions are the same.

The gold content in the plating layer after electroplating was 44wt%, the roughness Ra was 83nm, the hardness of the plating layer before annealing was 133HV, the hardness of the plating layer after annealing was 95HV, and the plating solution was stable after 24 hours.

Example 4

The difference from example 1 is that the gold ion concentration was changed to 20g/L, the silver ion concentration was changed to 10g/L, the potassium pyrophosphate concentration was changed to 120g/L, the stabilizer was changed to 23.7g/L of 5, 5-dimethylhydantoin (the molar ratio of 5, 5-dimethylhydantoin to silver ion was 2), the pH was changed to 8, the temperature was changed to 20℃and the other conditions were the same.

The gold content in the plating layer after electroplating is 38wt%, the roughness Ra is 82nm, the hardness of the plating layer before annealing is 112HV, the hardness of the plating layer after annealing is 85HV, and the plating solution is stable after being placed for 24 hours.

Example 5

The difference from example 4 is that the pH was changed to 10, the plating temperature was changed to 40℃and the current density was changed to 0.6A/dm ², and the other conditions were the same.

The gold content in the plating layer after electroplating was 49wt%, the roughness Ra was 91nm, the hardness of the plating layer before annealing was 157HV, the hardness of the plating layer after annealing was 104HV, and the plating solution was stable after 24 hours.

Example 6

The difference from example 4 is that the pH was changed to 9, the plating temperature was changed to 30℃and the current density was changed to 0.9A/dm ², and the other conditions were the same.

The gold content in the plating layer after electroplating was 57wt%, the roughness Ra was 158nm, the hardness of the plating layer before annealing was 177HV, the hardness of the plating layer after annealing was 136HV, and the plating solution was stable after 24 hours.

Example 7

The difference from example 1 is that the current density was changed to 1.0A/dm ², the plating thickness was changed to 0.5. Mu.m, and the other conditions were the same.

The gold content in the plating layer after electroplating was 66wt%, the roughness Ra was 24nm, and the plating solution was stable after 24 hours of standing.

Example 8

The difference from example 1 is that the current density was changed to 1.3A/dm ², the plating thickness was changed to 0.5. Mu.m, and the other conditions were the same.

The gold content in the plating layer after electroplating was 73wt%, the roughness Ra was 27nm, and the plating solution was stable after 24 hours of standing.

Example 9

The difference from example 1 is that the current density was changed to 1.6A/dm ², the plating thickness was changed to 0.5. Mu.m, and the other conditions were the same.

The gold content in the plated layer after electroplating was 77wt%, the roughness Ra was 34nm, and the plating solution was stable after 24 hours of standing.

Example 10

The difference from example 1 is that the gold ion concentration is changed to 1g/L, the silver ion concentration is changed to 0.3g/L, the potassium pyrophosphate concentration is changed to 10g/L, the stabilizer is changed to 4.7g/L of 5, 5-dimethylhydantoin (the molar ratio of 5, 5-dimethylhydantoin to silver ion is 19.8), the electroplating temperature is changed to 40 ℃, the electroplating thickness is changed to 0.5 mu m, and the rest conditions are the same.

The gold content in the plating layer after electroplating was 35wt%, the roughness Ra was 12nm, and the plating solution was stable after 24 hours of standing.

Comparative example 1

The difference from example 3 is that the hydantoin is changed to 0g/L (i.e., no hydantoin is added), with the same conditions.

The gold content in the plating layer after electroplating was 38wt%, the roughness Ra was 83nm, the hardness of the plating layer before annealing was 135HV, the hardness of the plating layer after annealing was 78HV, and a silver film was found on the plating solution surface after 24 hours of standing the plating solution.

Comparative example 2

The difference from example 3 is that the hydantoin is changed to 5g/L (the molar ratio of hydantoin to silver ion is changed to 1.1), and the other conditions are the same.

The gold content in the plating layer after electroplating was 40wt%, the roughness Ra was 79nm, the hardness of the plating layer before annealing was 145HV, the hardness of the plating layer after annealing was 85HV, and a silver film was found on the surface of the plating solution after 24 hours of standing the plating solution.

Comparative example 3

The difference from example 3 is that the hydantoin is changed to 5g/L sodium oxalate monohydrate at 30g/L, and the other conditions are the same.

The gold content in the plating layer after electroplating was 35wt%, the roughness Ra was 155nm, the hardness of the plating layer before annealing was 127HV, the hardness of the plating layer after annealing was 74HV, and a silver film was found on the surface of the plating solution after the plating solution was left for 24 hours.

Comparative example 4

The difference from example 3 is that the hydantoin is changed to 15g/L ethylenediamine at 30g/L, and the other conditions are the same.

The gold content in the plating layer after electroplating is 37wt%, the roughness Ra is 340nm, the hardness of the plating layer before annealing is 168HV, the hardness of the plating layer after annealing is 126HV, and the plating solution is stable after being placed for 24 hours.

The parameter conditions and performance comparison results of the above examples and comparative examples are shown in table 1.

Table 1 parameter conditions and performance comparison results for each example and comparative example

Note that "NA" in the table, i.e., "Not Applicable", indicates inapplicability.

As can be seen from the data in table 1, in each example, the stabilizer of the present invention was added to the plating solution, and no silver film was generated on the surface of the plating solution after the plating solution was left for 24 hours, indicating that the stabilizer of the present invention solves the stability problem of the plating solution.

It can also be seen from the data in table 1 that the annealing hardness and roughness of the plating layer have a great relationship with the gold content in the plating layer, and when the gold content in the plating layer is controlled to be below 50wt%, the roughness Ra of the plating layer is lower than 100nm and the annealing hardness of the plating layer is lower than 105HV, which is of great significance for replacing pure gold with gold-silver alloy in the semiconductor chip package structure. It is apparent from the comparison of examples 1 to 3, examples 4 to 6 and examples 7 to 9, respectively, that increasing the current density increases the gold content in the plated layer, and that this also provides a method for producing a gold-silver alloy having a higher gold content, in view of the oxidation resistance or sulfidation resistance of a gold-silver alloy, particularly a gold-silver alloy having a lower gold content, which is inferior to that of pure gold, thereby improving the oxidation resistance or sulfidation resistance of the gold-silver package structure. However, there is a problem in that the roughness and the annealing hardness of the plating layer are large (Jin Gaoyu wt%) at a high gold content, for example, 6, and are not suitable for packaging of semiconductor chips (the required roughness Ra of a general package is less than 100nm and the annealing hardness is less than 120 HV). Based on this, the plating solution of the present invention can be used as a two-layer package structure, as shown in fig. 1, a gold-silver alloy package structure body 1 with a low gold content is prepared by using a low current density, and combined with a chip substrate 3, thereby ensuring a low roughness and a low hardness, and the plating thickness is about 8-12 μm, and then a thin layer 2 with a high gold content is prepared on the top surface by using the same plating solution through a high current density, thereby improving the oxidation resistance or the vulcanization resistance of the gold-silver package structure, and the plating thickness is about 10-500nm. In this way, the requirements of low hardness and low roughness are ensured, and the reliability of subsequent welding or bonding can be ensured. Examples 7 to 9 show that the roughness Ra of the gold-silver alloy plating layer with high gold content is only 24 to 37nm when the plating thickness is 0.5 mu m.

In comparative example 3, sodium oxalate monohydrate was used as an additive (refer to CN 104099653B), and it was found that after the plating solution was left for 24 hours, a white silver film was formed on the surface of the plating solution, which resulted from photodecomposition of silver ions, indicating that sodium oxalate did not stabilize the silver ions. In comparative example 1, since the hydantoin derivative was not used as the silver ion stabilizer, the plating solution was unstable after 24 hours of standing, and the roughness of the plating layer was found to be relatively high, again demonstrating that the synergistic effect of the hydantoin and silver ions acts as a grain conditioner. In comparative example 2, the molar ratio of hydantoin to silver ion was less than 2, and a small amount of white silver film was also found on the surface of the plating solution after 24 hours of standing, indicating that a certain amount of hydantoin was required to complex silver ion to function as a stabilizer.

In addition, silver ions and hydantoin or a derivative thereof were used in combination with a crystallization modifier, and comparative examples 3 and 4 had a roughness of 83nm when 5, 5-dimethylhydantoin was used and a roughness of 340nm when ethylenediamine was used at the same plating thickness. Although ethylenediamine is used as an additive, the plating solution has good stability, the roughness of the plated layer after electroplating is too high, and the prepared gold-silver alloy is obviously unsuitable for packaging semiconductor chips.

Although embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are illustrative and not to be construed as limiting the invention, and that variations, modifications, alternatives, and variations may be made in the above embodiments by those skilled in the art without departing from the spirit and principles of the invention. The protection scope of the present invention is defined by the claims and the equivalents thereof.

Claims

1. A gold and silver electroplating solution is characterized by comprising water-soluble cyanide-containing gold salt, water-soluble cyanide-containing silver salt, conducting salt, stabilizing agent and water, wherein a crystallization regulator is not needed, the gold ion concentration is 1-20g/L, the silver ion concentration is 0.3-10g/L, the conducting salt concentration is 10-120g/L, the stabilizing agent is hydantoin or derivatives thereof, and the molar ratio of the stabilizing agent to silver ions in the cyanide-containing gold and silver electroplating solution is more than 2.

2. The gold-silver electroplating solution according to claim 1, wherein the molar ratio of the stabilizer to silver ions in the cyanide-containing gold-silver electroplating solution is 2-20.

3. The gold and silver plating solution according to claim 1, wherein the derivative of hydantoin is 5, 5-dimethylhydantoin, 1, 3-dimethylol-5, 5-dimethylhydantoin or 1-aminohydantoin.

4. A gold and silver plating solution according to claim 3, wherein said hydantoin derivative is 5, 5-dimethylhydantoin.

5. The gold and silver plating solution according to claim 1, wherein the pH of the plating solution is 8 to 10.

6. The gold and silver plating solution according to claim 1, wherein the water-soluble cyanide-containing gold salt is potassium gold cyanide and the water-soluble cyanide-containing silver salt is potassium silver cyanide.

7. The gold and silver plating solution according to claim 1, wherein the conductive salt is pyrophosphate.

8. The gold and silver plating solution according to claim 7, wherein the pyrophosphate is one or more of sodium pyrophosphate, potassium pyrophosphate, and ammonium pyrophosphate.

9. An electroplating method characterized in that the gold and silver electroplating solution according to any one of claims 1 to 8 is used for electroplating, the operation temperature of electroplating is 20 to 40 ℃, and the current density of electroplating is 0.3 to 1.5A/dm ².

10. The plating method according to claim 9, wherein when the gold content in the gold-silver plating layer is required to be 50wt% or less, the current density of the plating is 0.3 to 0.6A/dm ², and when the gold content in the gold-silver plating layer is required to be 60wt% or more, the current density of the plating is 1.1 to 1.5A/dm ².

11. Electroplating method according to claim 9 or 10, characterized in that the obtained gold-silver alloy electroplated layer is annealed at 270-300 ℃ for 5-120min.