KR100576584B1 - Silver electroplating bath - Google Patents

Abstract

(Problem) When performing partial plating of silver on the lead frame, it is possible to make high-speed plating with the cathode current density of 200 A / dm 2 or more, less variation in plating thickness, and ignition due to the plating bath scattered around the plating bath. It provides a silver electroplating bath that can use a cheap stainless steel anode.

(Solution) The electroplating bath contains silver potassium cyanide 40-250 g / L, nitrate 50-300 g / L, phosphate 20-200 g / L and free cyan salt 0.01-25 g / L, The content ratio of superphosphate is 1: 1 by 6: 1 by weight, and the pH is comprised by 8-12.

Description

Silver electroplating bath {SILVER ELECTROPLATING BATH}

The present invention relates to a silver electroplating bath that can partially plate a lead frame at high speed.

In the electronic industry, a method is formed in which a circuit is formed by connecting an IC chip mounted on a printed wiring board and a lead frame subjected to gold plating or silver plating by wire bonding. The plating of the lead frame is first performed by copper strike plating, followed by masking portions not to be plated, followed by partial plating for plating only portions to be wire bonded. As the plating method for partial plating, the jet plating method is the mainstream.

The jet plating method is capable of performing partial plating in a short time by changing the flow rate or bath temperature.

In recent years, electronic devices such as personal computers and mobile telephones equipped with IC chips have been required to have a low price. For this reason, the plating of lead frame is mainly silver plating which is cheaper than gold plating. In addition, in order to achieve a low price, production efficiency is required by reducing the amount of silver used or by increasing the plating speed.

As a classification method of the silver plating bath, there is a method of classifying according to the conductive salt contained in the plating bath. According to this classification method, the conventional silver plating bath is classified into a high cyan bath, a cyan phosphate bath, a cyan organic acid salt bath, and a low cyan nitrate bath.

The high cyanide bath is a plating bath containing 50 to 200 g / L of free cyanide salt of potassium cyanide or sodium cyanide as the conducting salt. This plating bath contains high concentrations of harmful cyanide, so the drainage cost is high. In addition, when continuously used at a high cathode current density, cyanide oxidized by cyan ions is generated. This cyanide product is mixed in the plating film. Therefore, there is a problem that the purity of the plating film is lowered and adversely affects the wire bonding property.

For example, in the case where the surface of the plated material subjected to copper strike plating is to be partially plated with silver, an organic substitution prevention film is formed for the substitution prevention treatment as a pretreatment. However, the gosian bath destroys the formed organic substitution prevention film. Therefore, the silver ion in the plating bath and the strike plating copper on the surface of the plated material are replaced before the silver partial plating is energized. In addition, silver substitution occurs in the masked portion. As such, there is a problem that the masked portion other than the partial plating portion is replaced with silver and contaminated with silver. This problem also causes another problem that adversely affects solder plating in the post-process of partial plating.

On the other hand, in the so-called low cyan bath where the free cyan salt concentration is several g / L or less, the phosphate bath and the organic acid bath are not sufficiently conductive. Due to this inadequate conductivity, the plating rate is slow even with the jet plating method. Therefore, a method of increasing the silver concentration of the plating bath may be adopted as a method for performing high speed plating. In this case, the plating bath adheres to the plated material after plating, and the amount of silver flowing out of the plating bath increases. That is, since the amount of wasted silver increases, manufacturing cost becomes high. From the viewpoint of such a manufacturing cost, the content of silver in the plating bath needs to be 100 g / L or less.

As another method for performing high-speed plating, a method of increasing the flow rate or bath temperature may be adopted as described above. However, when increasing the flow rate, there is a problem that the plating bath penetrates to the masked portion.

On the other hand, when the bath temperature is increased, there is a problem that the plating bath is degraded or the evaporation speed of the plating bath is increased. Therefore, the upper limit of bath temperature is practically limited to 75 degreeC.

As described above, the phosphate bath and the organic acid bath are limited in terms of any method for performing high-speed plating. Therefore, there is a problem that high-speed plating with a cathode current density of 200 A / dm 2 or more cannot be performed.

In addition, the phosphate bath and the organic acid salt bath have variations in the plating thickness due to the insufficient conductivity described above. In order to perform good wire bonding, a plating thickness of 2.5 µm or more is required. In consideration of the variation in thickness, in order to obtain a plating thickness of 2.5 μm or more at all positions of the plating part involved in wire bonding, it is necessary to set the plating thickness of 4 to 5 μm as an average value. Plating at this plating thickness increases the amount of wasted silver. Therefore, a problem arises in that the manufacturing cost increases.

As a method of reducing the variation in the plating thickness, there is a method of lowering the flow rate, temperature and / or silver concentration of the plating bath and performing plating at low current density. However, the plating method performed under such conditions cannot be subjected to high speed plating as described above. That is, there is a problem that the plating time becomes long and the productivity decreases.

On the other hand, in the low cyan bath, since the nitrate bath has high conductivity, high-speed plating with a cathode current density of 200 A / dm 2 or more is possible, and plating is finished in 2 to 3 seconds, so productivity is high.

The jet plating method ejects the plating bath at a high flow rate. For this reason, a plating bath scatters around plating baths, such as a plating apparatus. In the case of using a nitrate bath, the scattering plating bath evaporates moisture to produce dry nitrate. This dry nitrate is likely to ignite by sparks such as a motor. Therefore, there is a high risk of a fire accident.

In addition, when plating is performed at a high current density using a stainless steel anode in a nitrate bath, the stainless steel anode is corroded by nitrate. As corrosion of the anode by the nitrate bath proceeds, the nitrate bath becomes contaminated and the anode shape changes. If the shape of the anode changes, the distance between the anode and the cathode changes, which causes the plating thickness variation. To prevent this plating thickness variation, a platinum electrode more expensive than stainless or an electrode coated with platinum must be used. In particular, in the case of narrow rectangular plating apparatus of small quantity production of various kinds, it is necessary to prepare various kinds of anodes, and thus there is a problem in that the installation cost is high.

The present inventors have repeatedly studied to solve the above problems, and when using the plating bath of a predetermined composition when applying a partial plating of silver on the lead frame, high-speed plating with a cathode current density of 200 A / dm 2 or more is possible. The present invention has been completed by discovering that there is little variation in plating thickness, there is no ignition due to the plating bath scattered around the plating bath, and a stainless steel anode can be used which is cheaper than platinum.

Accordingly, an object of the present invention is to provide a silver electroplating bath which can reduce equipment cost, material cost, etc. by solving the above problems, and is capable of silver plating at high speed safely and efficiently.

The present invention to achieve the above object

[1] silver potassium cyanide 40 to 250 g / L, nitrate 50 to 300 g / L, phosphate 20 to 200 g / L and free cyan salt 0.01 to 25 g / L, the content ratio of nitrate and phosphate being weight ratio Furnace 1: 1 to 6: 1, the pH is 8 to 12 is to propose an electroplating bath.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The silver electroplating bath of the present invention is used in conventional plating baths and jet plating baths. A normal plating bath can be shared with a jet plating bath.

In the silver electroplating bath of the present invention, silver potassium cyanide is used as the silver source. Silver potassium cyanide is water-soluble and the electrodeposition potential is so high that a dense plating film is obtained.

In the case of jet plating, the content of silver potassium cyanide in the plating bath is preferably 40 to 250 g / L, more preferably 70 to 240 g / L, particularly preferably 100 to 150 g / L. When content of silver potassium cyanide is less than 40 g / L, the plating film obtained will become a rough crystal easily and wire bonding property will fall. In addition, high-speed plating cannot be performed because the cathode current density cannot be increased. Therefore, the plating bath whose content of silver potassium cyanide is less than 40 g / L is not preferable.

When the content of silver potassium cyanide exceeds 250 g / L, high-speed plating is possible. However, in this case, since the silver concentration of the plating bath is high, the amount of silver flowing out of the plating bath increases with the plating bath attached to the plated material after plating. That is, it is uneconomical because a large amount of silver is wasted.

The nitrate has the action of maintaining high conductivity of the plating bath as described in the section of low cyanide nitrate bath described above. Therefore, the high concentration of nitrate plating bath enables plating at high cathode current density, that is, high speed plating.

As nitrate used for the silver electroplating bath of this invention, potassium nitrate, sodium nitrate, ammonium nitrate etc. are mentioned, for example. These nitrates may be used individually by 1 type, and may be used in mixture of 2 or more type.

In the case of jet plating, the content of nitrate in the plating bath is preferably 50 to 300 g / L, more preferably 90 to 260 g / L, particularly preferably 100 to 200 g / L.

When the content of nitrate in the plating bath is less than 50 g / L, the conductivity of the plating bath is low. Therefore, the plated film obtained tends to become a rough crystal, and wire bonding property falls. In addition, the variation in plating thickness becomes large. In addition, high-speed plating cannot be performed because the cathode current density cannot be increased. Therefore, the plating bath whose content of nitrate is less than 50 g / L is not preferable.

When the content of nitrate in the plating bath exceeds 300 g / L, the viscosity of the plating bath is high. Therefore, the plated film obtained tends to become a rough crystal, and wire bonding property falls. In addition, problems such as a large variation in plating thickness occur. Therefore, the plating bath whose content of nitrate exceeds 300 g / L is not preferable.

Phosphate is smaller than nitrate, but has a function of maintaining the conductivity of the plating bath, so that the phosphate can be plated with a high cathode current density in cooperation with coexisting nitrates. In addition, the phosphate salt has a function of inhibiting ignition by sparking by plating as the plating bath is scattered around the bath, and when the scattering plating bath is dried, surrounding the nitrate and silver potassium cyanide. Phosphate also inhibits corrosion of the stainless steel anode by nitrates.

Examples of the phosphate used in the silver electroplating bath of the present invention include potassium dihydrogen phosphate, potassium dihydrogen phosphate, tripotassium phosphate, sodium dihydrogen phosphate, sodium dihydrogen phosphate, trisodium phosphate, potassium pyrophosphate and sodium pyrophosphate. Can be mentioned. These phosphates may be used individually by 1 type, and may be used in mixture of 2 or more type.

In the case of jet plating, the content of phosphate in the plating bath is preferably 20 to 200 g / L, more preferably 30 to 160 g / L, and particularly preferably 40 to 120 g / L.

When the content of the phosphate in the plating bath is less than 20 g / L, the contribution to the conductivity of the phosphate itself becomes small, so that the conductivity of the plating bath tends to be lowered. In addition, since the content of phosphate that suppresses ignition of nitrate is small, dry salts of the plating bath scattered around the plating bath tend to ignite by sparks. Further, corrosion of the stainless steel anode by nitrate proceeds, making it impossible to use an inexpensive stainless steel anode and increasing the plating thickness. Therefore, the plating bath whose content of phosphate is less than 20 g / L is not preferable.

When the content of the phosphate in the plating bath exceeds 200 g / L, the excess of the phosphate tends to lower the action of the nitrate which maintains the conductivity of the plating bath high, so the conductivity of the plating bath is low. Therefore, the cathode current density cannot be increased and high-speed plating cannot be performed. Moreover, this plating bath is high in viscosity. Therefore, the plated film obtained tends to become a rough crystal, and wire bonding property falls. In addition, problems such as a large variation in plating thickness occur. Therefore, the plating bath whose content of nitrate exceeds 200 g / L is not preferable.

In the silver electroplating bath of the present invention, the preferred content ratio of nitrate and phosphate is from 1: 1 to 6: 1 by weight, and more preferably from 1: 1 to 5: 1, particularly preferably 2: 1. To 4: 1.

By setting the content ratio of nitrate and phosphate in the silver electroplating bath to the above range, high-speed plating with a cathode current density of 200 A / dm 2 or more becomes possible. In addition, the variation in plating thickness can be reduced, the ignition caused by the plating bath scattered around the plating bath can be suppressed, and the corrosion of the stainless steel anode can be suppressed.

If the content of nitrate and phosphate in the plating bath is lower than 1: 1, the conductivity of the plating bath is lowered. Therefore, the plated film obtained tends to become a rough crystal, and wire bonding property falls. In addition, the variation in plating thickness becomes large. In addition, high-speed plating cannot be performed because the cathode current density cannot be increased. Therefore, a plating bath having a content ratio of nitrate and phosphate lower than 1: 1 is undesirable.

When the content ratio of nitrate and phosphate in the plating bath is higher than 6: 1, the dry salt of the plating bath scattered around the plating bath tends to ignite by sparks. In addition, corrosion of the stainless steel anode by nitrate proceeds, making it impossible to use an inexpensive stainless steel anode and increasing the plating thickness. Therefore, a plating bath having a content ratio of nitrate and phosphate higher than 6: 1 is not preferable.

The free cyanide salt has a function of stabilizing the deposition of silver plating and preventing abnormal deposition such as plating unevenness or bumps (protrusions of protrusions). Even in the case where the free cyan salt is not added during the drying of the plating bath, by performing the plating operation, for example, silver cations in potassium cyanide precipitate on the surface of the plated material to generate and accumulate the free cyan salt. Therefore, the free cyan salt may not be added at the time of the dry bath of a plating bath. On the other hand, when free cyan salt is added during the drying of the plating bath, stable plating can be performed immediately after the plating bath is dried.

The degree of generation and accumulation of the free cyan salt changes depending on the pH of the plating bath, the plating throughput, the exhaust state of the upper plating bath, and the like. Therefore, it is preferable to adjust the content of the free cyanide salt in the plating bath by analyzing the plating bath periodically and adding cyan salt, not only at the time of drying the plating bath but also during the plating operation.

As a free cyan salt used for the silver electroplating bath of this invention, potassium cyanide, sodium cyanide, ammonium cyanide, hydrogen cyanide, etc. are mentioned, for example. These free cyan salts may be used alone or in combination of two or more thereof.

In the case of jet plating, the content of the free cyan salt in the plating bath is preferably 0.01 to 25 g / L, more preferably 0.3 to 12 g / L, particularly preferably 0.3 to 5 g / L.

When content of the free cyanide salt in a plating bath is less than 0.01 g / L, since abnormal precipitation, such as a plating nonuniformity and a bump, cannot be prevented, it is unpreferable.

When the content of the free cyanide salt in the plating bath exceeds 25 g / L, a cyanide product in which cyan ion is oxidatively polymerized is generated. This cyanide product is mixed in the plating film. Therefore, the purity of the plating film is lowered, which adversely affects wire bonding properties.

This plating bath also destroys the organic substitution prevention film formed in the previous step. Therefore, good partial plating cannot be performed. Therefore, the plating bath whose content of free cyanide exceeds 25 g / L is not preferable.

The pH of the silver electroplating of the present invention is preferably from 8 to 12, more preferably from 8.5 to 11.5, particularly preferably from 8.5 to 9.5 in order to easily control the content of the free cyanide salt in the plating bath.

If the pH of the plating bath is lower than 8, the free cyan salt cannot be present stably. Therefore, abnormal precipitation, such as a plating nonuniformity or a bump, cannot be prevented similarly to the case where content of the free cyanide salt in a plating bath is less than 0.01 g / L. Therefore, plating baths having a pH lower than 8 are undesirable.

When pH of a plating bath is higher than 12, content of the free cyanide salt in a plating bath will also become high. Therefore, a cyanide product in which cyan ions are oxidatively polymerized is produced as in the case where the content of the free cyanide salt in the plating bath exceeds 25 g / L. This cyanide product is mixed in the plating film. Therefore, the purity of the plating film is lowered, which adversely affects wire bonding properties.

In addition, this plating bath destroys the organic substitution prevention film performed in the previous step. Therefore, good partial plating cannot be performed. Therefore, a plating bath having a pH higher than 12 is not preferable.

To the silver electroplating bath of the present invention, hydroxycarboxylic acids such as citric acid, tartaric acid and malic acid or salts thereof are added in order to further improve the variation in plating thickness, the uniformity of plating appearance and the denseness of the crystals of the coating film. can do.

In addition, a pH buffer may be added to the silver electroplating bath of the present invention to stabilize the pH. Examples of the pH buffer include boric acid, potassium borate, sodium borate, and the like, and one or more thereof can be added.

In addition, a polishing agent may be added to the silver electroplating bath of the present invention in order to give gloss to the plating film. Examples of the brightening agent include sulfur series organic compounds such as carbon disulfide, thiourea, and thio lactic acid, and selenium compounds such as selenium cyanide, selenic acid, and selenium oxide. What is necessary is just to add suitably these 1 or more types according to the desired glossiness of a plating film.

In the silver electroplating bath of the present invention, a platinum electrode or a platinum coated electrode as well as a cheap stainless electrode can be used as the anode material. Using the plating bath of the present invention, high-speed plating of a cathode current density of 200 A / dm 2 or more can be achieved by jet plating. In this case, the preferred flow rate of the plating bath is 2 to 10 m / sec, and the preferred bath temperature is 45 to 75 ° C.

Since the cathode current density is appropriately selected depending on the shape of the plating apparatus or the coating material, the cathode current density of less than 200 A / dm 2 may be selected in some cases. The cathode current density which is generally selected including such a case is preferably 30 to 300 A / dm 2, more preferably 35 to 270 A / dm 2, and particularly preferably 50 to 250 A / dm 2.

Example

The present invention will be described in detail with reference to the following Examples, but the present invention is not limited to these Examples.

A leadframe of 42 alloy (Fe-42 wt% Ni) QFP (abbreviated Quad Flat Package) 144 pin (0.25 mm thick) was electrolytically degreased. Subsequently, the electrolytic degreased lead frame was subjected to 0.1 μm thick copper strike plating. The lead frame subjected to the copper strike plating was rubber masked after the substitution prevention treatment. This rubber masked lead frame was used as a sample for partial plating. The partial plating test was carried out by the jet plating method and plated by the following operation using the plating baths shown in the following Examples and Comparative Examples at a bath flow rate of 5 m / sec.

It is known in advance that the desired plating thickness can be obtained by setting the cathode current density and the plating time. The target plating thickness was set to 5 µm, the cathode current density was changed every 20 A / dm 2 in the range of 20 to 260 A / dm 2, and the partial plating test was performed on these cathode current densities. The plating time in each partial plating test was set according to each said cathode current density.

A partial plating test was performed on each cathode current density, and the cathode current density range where a dense and uniform coating was obtained in the partial plating test was defined as a good cathode current density range. The results are shown in Table 1.

Example 1

Silver Potassium Cyanide 130 g / L

Potassium Nitrate 150 g / L

Dipotassium phosphate 50 g / L

Potassium cyanide 2 g / L

Boric acid 30 g / L

Potassium cyanide 1 mg / L

pH 8.9

Bath temperature 65 ℃

Example 2

Silver Potassium Cyanide 130 g / L

Potassium Nitrate 250 g / L

Dipotassium Phosphate 70 g / L

Sodium cyanide 3 g / L

Boric acid 15 g / L

Potassium cyanide 1 mg / L

pH 9.5

Bath temperature 65 ℃

Example 3

Silver Potassium Cyanide 200 g / L

Potassium Nitrate 180 g / L

Dipotassium phosphate 40 g / L

Sodium cyanide 3 g / L

Boric acid 10 g / L

Potassium cyanide 1 mg / L

pH 9.5

Bath temperature 55 ℃

Example 4

Silver Potassium Cyanide 80 g / L

Sodium Nitrate 100 g / L

Disodium Phosphate 40 g / L

Potassium cyanide 3 g / L

Ammonium citrate 25 g / L

Boric acid 10 g / L

Potassium cyanide 1 mg / L

pH 11.0

Bath temperature 75 ℃

Example 5

Silver Potassium Cyanide 150 g / L

Ammonium Nitrate 170 g / L

Potassium Pyrophosphate 150 g / L

Potassium cyanide 10 g / L

Boric acid 20 g / L

Sodium selenite 0.5 mg / L

pH 9.6

Bath temperature 60 ℃

Example 6

Silver Potassium Cyanide 180 g / L

Potassium Nitrate 180 g / L

Potassium Pyrophosphate 40 g / L

Potassium cyanide 3 g / L

Boric acid 15 g / L

pH 8.7

Bath temperature 65 ℃

Example 7

Silver Potassium Cyanide 90 g / L

Potassium Nitrate 250 g / L

Dipotassium Phosphate 120 g / L

Potassium cyanide 7 g / L

Sodium selenite 2 mg / L

pH 9.7

Bath temperature 55 ℃

Example 8

Silver Potassium Cyanide 150 g / L

Potassium Nitrate 130 g / L

Dipotassium phosphate 40 g / L

Potassium cyanide 0.5 g / L

Trisodium citrate 25 g / L

Sodium selenite 0.8 mg / L

pH 8.4

Bath temperature 65 ℃

Hereinafter, a comparative example is shown.

Comparative Example 1

Silver Potassium Cyanide 130 g / L

Dipotassium Phosphate 120 g / L

Potassium cyanide 1 g / L

Boric acid 30 g / L

Potassium cyanide 1 mg / L

pH 8.8

Bath temperature 65 ℃

Comparative Example 2

Silver Potassium Cyanide 130 g / L

Sodium cyanide 2 g / L

Trisodium citrate 150 g / L

Boric acid 30 g / L

Potassium cyanide 1 mg / L

pH 9.0

Bath temperature 65 ℃

Comparative Example 3

Silver Potassium Cyanide 130 g / L

Potassium Nitrate 150 g / L

Potassium cyanide 0.5 g / L

Boric acid 30 g / L

Potassium cyanide 1 mg / L

pH 8.5

Bath temperature 65 ℃

Example or Comparative Example Number Good cathode current density range (A / dm²) Film thickness deviation (㎛) Ignition of Dry Salt ^{* 1} Dissolution amount of positive electrode component made of stainless steel (mg / L) Fe Ni Cr Example 1 60 to 260 0.57

155 30 45 Example 2 60 to 260 0.53

150 30 35 Example 3 40 to 240 0.58

160 35 40 Example 4 40-200 0.51

145 30 35 Example 5 40 to 240 0.53

155 35 45 Example 6 60 to 220 0.54

150 35 40 Example 7 40-200 0.52

160 35 45 Example 8 60 to 260 0.51

155 30 45 Comparative Example 1 60 to 140 1.41

150 30 40 Comparative Example 2 60-100 1.24

120 25 35 Comparative Example 3 60 to 260 0.56 × 480 95 125 * 1 ignition of dry salt

: No ignition ×: Fires while igniting

Performance test 1

The variation in thickness was evaluated for the coating obtained by plating the intermediate value of the good cathode current density range. The film thickness of 144 places of the plating film obtained using the fluorescent X-ray film thickness meter was measured. These film thicknesses were aggregated for each partial plating test, and the difference between the maximum and minimum film thicknesses is shown in Table 1 as the film thickness deviation for the partial plating test.

In Table 1, the film thickness variation of the film obtained in the plating baths of Examples 1 to 8 of the present invention was 0.51 to 0.58 µm. On the other hand, the film thickness deviations of the film obtained in the plating bath of Comparative Example 1, which did not contain nitrate, and the plating bath of Comparative Example 2, which did not contain all of nitrate, were 1.41 µm and 1.24 µm, respectively.

And the film thickness deviation of the film obtained by the plating bath of the comparative example 3 which does not contain a phosphate was 0.56 micrometer, and the deviation was small to the same extent as the case of Examples 1-8. However, as described later in the performance evaluation tests 2 and 3, the performance other than the film thickness variation was inferior.

Performance Evaluation Test 2

The performance evaluation test was done about the flammability of the mixture of salt after drying a plating bath. 10 ml of each plating bath of Examples 1-8 and Comparative Examples 1-3 was put into the glass chalet, and it hold | maintained at 105 degreeC for 6 hours in the dryer, and dried the plating bath. Flint was used to spark this mixture of dried salts to check for ignition. The results are shown in Table 1.

In Table 1, the mixture of the salts which dried the plating bath of Examples 1-8 did not ignite. On the other hand, the mixture of the salt which dried the plating bath of the comparative example 3 ignited and combusted while giving off smoke. And the thing which dried the plating bath of Comparative Examples 1 and 2 did not ignite. However, as described in the performance evaluation test 1, the plating baths of Comparative Examples 1 and 2 were inferior in performance of film thickness variation.

Performance test 3

The performance evaluation test was done about the corrosion property of each plating bath in the case of using the positive electrode made from stainless steel. Using stainless steel (SUS 316) as an anode and continuously plating 30 A · hr / L at a cathode current density of 100 A / dm 2 (continue plating until the amount of electricity is 30 A · hr per 1 L of plating bath) ) Was performed. After the completion of the plating operation, the stainless steel anode was corroded and the amounts of iron (Fe), nickel (Ni) and chromium (Cr) dissolved in each plating bath were measured. The results are shown in Table 1.

In Table 1, the dissolution amount of the metal in the plating baths of Examples 1 to 8 was about the same as the dissolution amount of the metal in the plating baths of Comparative Examples 1 and 2 containing no nitrate. And the plating baths of the comparative examples 1 and 2 which are conventional plating baths generally used the anode made from stainless for practical use. Therefore, the plating bath of Examples 1-8 can use the positive electrode made from stainless steel.

On the other hand, the dissolution amount of the metal in the plating bath of Comparative Example 3 was very large for any metal as shown in Table 1. Therefore, it is not preferable to use the stainless steel positive electrode in the plating bath of Comparative Example 3.

When partial plating of silver is applied to the lead frame using the silver electroplating bath of the present invention, the cathode current density can be increased, and high-speed plating with a cathode current density of 200 A / dm 2 or more is also possible depending on the conditions. In addition, the variation in plating thickness is small and the amount of silver wasted is small. In addition, there is no ignition caused by the plating bath scattered around the plating bath, and safety is high. In addition, since it contains a lot of nitrates, the current density can be increased, and since stainless steel anodes are less corroded, stainless steel anodes can be used and plating can be performed more efficiently and at a lower cost than before.

Claims (1)

Silver potassium cyanide 40-250 g / L, nitrate 50-300 g / L, phosphate 20-200 g / L, and free cyan salt 0.01-25 g / L, and the content ratio of nitrate and phosphate is 1: 1 by weight ratio. Silver electroplating bath which is 1-6: 1 and pH is 8-12.