JPH0622252B2 - Gold plating method for electronic components for semiconductor integrated circuits - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for applying gold plating to required parts of electronic components for semiconductor integrated circuits such as plastic packages and ceramic packages.

(Problems to be Solved by Conventional Techniques and Inventions) Electronic components for semiconductor integrated circuits, such as plastic packages, have been conventionally plated with gold because high reliability is required. Due to the sharp rise, it is desired to improve the productivity by performing gold plating on only the necessary portions at a high speed.

Conventionally, gold plating on electronic parts for semiconductor integrated circuits is performed by a direct current method using a plating bath containing an additive such as thallium at 10 mg / or more. In this method, in order to increase the plating rate and improve the productivity, a high current density (for example, 1
When trying to perform plating work with A / dm ² or more), a large amount of thallium (100 ppm) as an additive is present in the plating film.
Above). However, when this large amount of thallium is co-deposited, an oxide of thallium is formed on the gold-plated surface, and the wire and the gold-plated film are not well bonded in the wire bonding step which is a post-process. Defects such as bonding force due to diffusion movement occur.

Therefore, in order to reduce the co-deposition amount of thallium, plating work is currently performed by a direct current method at a low current density of about 0.25 A / dm ² . As a result, it takes a long time for plating, and the productivity of plating is low.

On the other hand, when gold plating is performed by the direct current method using a plating bath that does not contain additives such as thallium, the gold plating film is only matte and rough, and the gold for electronic components for semiconductor integrated circuits is obtained. It does not have satisfactory characteristics as plating.

(Means for Solving the Problems) The present invention provides a gold plating method for electronic components for semiconductor integrated circuits, which solves the above problems, and reduces the amount of thallium eutectoid in the gold plating film to 100 ppm or less. It is possible to obtain a gold plating film of an electronic component for a semiconductor integrated circuit having excellent characteristics such as wire bonding property at a high speed, that is, containing citric acid or citrate and containing 1 mg of thallium. / A gold plating method for an electronic component for a semiconductor integrated circuit, characterized in that an electronic component is gold-plated by a constant current pulse method using a gold cyanide plating bath contained below.

In the present invention, citric acid or a citrate salt is contained in the plating bath, which allows the proper pH of the gold plating bath to be stably maintained, and is usually 30 to 8
0 g / content is included. As the gold cyanide, potassium gold (I) cyanide, ammonium (I) gold cyanide, etc. are used, and the content thereof in the plating is 5 to 50.
g / degree.

By the way, conventionally, thallium is contained in the gold plating bath. The reason for this is that thallium has the function of adjusting the gold-plated particles electrodeposited while repeating adsorption and desorption on the gold-plating surface into crystals of an appropriate size. Because there is. Therefore,
When attempting to perform highly productive plating work at a high current density (1 A / dm ² or more), desorption of thallium on the gold plating surface is hindered, resulting in an increase in the amount of eutectoid and a defective gold plating film layer. Is formed.

Therefore, in the present invention, the constant current pulse method is adopted.

This method is a plating method in which an electric current is intermittently applied, and ON / OFF of the electric current is periodically repeated. In the present invention, by adopting the constant current pulse method,
It is considered that the amount of thallium co-deposited in the gold plating film decreases because the desorption of thallium is promoted during the time when the current is turned off, that is, when the current does not flow. According to the present invention, the amount of thallium added to the plating bath is set to 1 of the conventional method.
/ 10 (1 ppm) and increase the pulse current density (current density of the current that flows when the current is on) (2 A /
dm ² or more) When gold plating is applied, it is possible to obtain a gold plating film having the same characteristics as a product manufactured by a DC method with a low current density using a conventional plating bath containing 10 mg / thickness of thallium. Became.

On the other hand, when a plating bath containing no thallium is used, the conventional DC method results in a rough plating film. However, when the constant current pulse plating method according to the present invention is adopted and the pulse current density is set to be extremely high (10 A / dm ² or more) and plating work is performed, a large number of gold crystal nuclei are momentarily in a high range. It is thought that the gold plating film is formed, and as a result, a dense and glossy gold plating film is obtained.

The conditions for using the constant current pulse in the present invention include pulse current density of 10 to 1 in the case of a thallium-free plating bath.
00A / dm ² , pulse on time 0.1 to 10 mse
c. When the average current density is 0.1 to 0.5 A / dm ² and the thallium content is 1 mg /, the pulse current density is 2 to 5 A /
dm ² , pulse on time 0.1 to 10 msec. The average current density is preferably 0.1 to 1 A / dm ² .

The gold plating bath used in the present invention is a citric acid- or citrate-containing bath, for example (A) citric acid 50 g /
, Sodium citrate (or potassium citrate,
Alternatively, ammonium citrate) 50 g /, and further, potassium primary cyanide 10 to 50 g / is used as a basic bath, and (B) citric acid 30 g /
, Sodium citrate (or potassium citrate,
Alternatively, ammonium citrate) 50 g /, potassium dihydrogen phosphate 140 g /, and potassium primary gold cyanide 10-50 g / are used as the basic bath, and thallium 0-1 mg / is added. The pH of these plating baths is 3.7 to 4.5, and the plating operation is performed at a bath temperature of 50 to 70 ° C. In addition, potassium dihydrogen phosphate (dihydrogen phosphate salt) plays a role of a buffering agent for suppressing the PH fluctuation of the plating bath according to the present invention in combination with the above citric acid. By the way, when the thickness of the gold plating film produced by the conventional direct current method is thin (2 μm or less), a large number of small holes are present in the gold plating film, and the underlying nickel diffuses on the gold plating surface to form an oxide. As a result, there is a problem that the fusion of the IC chip and the gold plating is obstructed in the die bonding process. Therefore, at present, the film thickness is increased to 3 to 5 μm to prevent nickel from diffusing and the above problem is addressed.

The constant-current pulse method of the present invention is an effective plating method for reducing the number of holes, and even if the thickness is reduced to about half that of the conventional method, a gold plating film having the same function can be obtained. This greatly contributes to the cost reduction of manufacturing electronic components for integrated circuits.

(Example) Example 1 Plating bath having a thallium content of 0 mg / citric acid 50 g / sodium citrate 50 g / potassium primary potassium cyanide 50 g / bath temperature of about 60 ° C while stirring using the above gold plating bath
Then, gold plating was performed on the ceramic package substrate under the following pulse conditions.

Pulse conditions Pulse current density 10 A / dm ² Pulse on time 0.1 ms Pulse off time 1.9 ms When gold plating work was performed under the above plating bath and conditions, there was no co-deposition of thallium in the gold plating layer and for semiconductor integrated circuits. A gold plating film having excellent wire bonding properties, die bonding properties and corrosion resistance required for gold plating for electronic parts was obtained.

The average current density in the constant current pulse method corresponds to the current density in the direct current method, and in the case of Example 1, 0.5 A / dm ²
Therefore, the general current density of the conventional DC method is 0.25 A / d.
Compared to m ² , the plating operation is performed at twice the current density, that is, at twice the speed. The average current density in the present invention is preferably 0.1 to 0.5 A / dm ² from the viewpoint of plating productivity and film characteristics.

Example 2: Plating bath with thallium content of 1 mg / citric acid 30 g / sodium citrate 50 g / potassium dihydrogen phosphate 140 g / potassium dihydrogen cyanide 50 g / bath temperature of about 60 ° C. using the above gold plating bath Then, gold plating was performed on the ceramic package substrate under the following pulse conditions.

Pulse condition Pulse current density 2A / dm ² Pulse on time 0.1ms Pulse off time 0.1ms Under this pulse condition, the average current density is 1A / dm ² , which is compared with the normal current density of 0.25A / dm ^{2 in} the DC method. It became possible to perform plating work at a current density four times higher, that is, at a speed four times faster. The amount of thallium eutectoid in the gold plating film produced under the conditions of Example 2 was about 70 ppm, which is lower than the amount of thallium eutectoid, which is said to start causing wire bonding defects. ,
Has good wire bonding properties.

(Effects of the Invention) As described above, a gold cyanide plating bath containing citric acid or a citrate and containing 1 mg / l or less of thallium is used to perform gold plating on electronic components for semiconductor integrated circuits by a constant current pulse method. According to the gold plating method of the invention, a gold plating film of an electronic component having excellent characteristics such as wire bondability can be obtained at high speed (high productivity) using a gold plating bath having an extremely low thallium content.

Claims (3)

[Claims] 1. A semiconductor integrated circuit characterized in that an electronic component is gold-plated by a constant current pulse method using a gold cyanide plating bath containing citric acid or a citrate and containing 1 mg / l of thallium. Plating of electronic components for automobiles. 2. The gold plating method for electronic components for semiconductor integrated circuits according to claim 1, wherein the gold cyanide plating bath is substantially free of thallium. 3. The gold plating method for an electronic component for a semiconductor integrated circuit according to claim 1, wherein the gold cyanide plating bath contains a dihydrogen phosphate salt.