JPS61186494A - Method for silver plating - Google Patents

Abstract

PURPOSE:To plate silver having arbitrary luster simply and stably, by plating silver on material by repeating conduction and nonconduction of electricity thereon by using silver plating liquid. CONSTITUTION:Neutral or alkaline silver plating liquid is used, conduction and nonconduction times of electricity are provided during preset plating time without adding luster agent substantially, and this as one period is repeated by plaural periods in plating time. Furthermore, ratio of electricity conduction time to nonconduction time and periods of these are varied, to control luster degree arbitrarily from high luster to nonluster. Plated film having high luster range obtd. by the method has high tensile strength and elongation ratio and contains no impurites. It is suitable method for s1ilver plating to lead frame made of Cu, Cu alloy, Fe-Ni alloy, etc. as raw material.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a plating method for silver plating lead frames used for mounting transistors, ICs, etc., for example.

More particularly, it relates to a method for obtaining high gloss to matte silver plating deposits without the use of brighteners.

In addition, copper flushing, which has traditionally been done to obtain better adhesion to the substrate. Rurui is about a silver plating method that can omit silver flash plating.

Conventional technology Conventionally, as a method for controlling the glossiness of silver plating and brush deposits, a silver plating solution containing substantially no brightening agent was mixed with selenate, selenocyanate, selenium oxide, iodide, or bromide. The degree of gloss can be controlled from high gloss to matte by adding brighteners such as cobalt salts, thiourea, alone or in combination, or by adjusting the amount of brighteners added.

In these conventional methods for controlling the brightness of silver plating solutions, the concentration of the brightener added is extremely small, on the order of 10 to 1 ooppm! In addition, since the brightener is incorporated into the plating precipitated crystals and consumed, strict concentration control is required to stably obtain a certain level of brightness. However, as mentioned above, there is no practical way to measure the concentration of the brightener because it is in an extremely small amount of 10 to 1100 pp. Therefore, it is difficult to determine the amount of brightener added by measuring the glossiness of the resulting plating deposit. What is currently being controlled is the plating solution.

There are problems in that the amount of lubrication work required for management is large and the variation in quality is also large.

Furthermore, from another point of view, a disadvantage of the conventional method is that silver plating on lead frames requires strict control over impurities, and there is a tendency to regard the brightening agent injected into the deposited silver plating as an impurity. In silver plating methods in which high gloss is obtained by adding a brightening agent, the brightening agent is incorporated into the plating film, which poses a fundamental problem.

Furthermore, the method of obtaining high gloss using a brightening agent has the drawback that the flexibility of the silver plating film is significantly impaired, residual stress tends to remain, and cracks are likely to occur.

The present invention eliminates the drawbacks of the conventional methods described above, and aims to provide a silver plating method that allows the glossiness of silver plating to be controlled in a simple manner.

Means for Solving the Problems In order to achieve the above-mentioned object, the silver plating method of the present invention uses a neutral or alkaline silver plating solution to which no brightening agent is added, and conducts electricity during a preset plating time. This method is characterized in that a period of time and a period of non-energization are provided, and this period is regarded as one cycle, and a plurality of cycles are repeated during the plating time.

Effect The present inventors have determined that in order to arbitrarily control the gloss level from high gloss to matte, the ratio of energized time (hereinafter referred to as ON time) to non-energized time (hereinafter referred to as OFF time) (hereinafter referred to as duty ratio) is determined. We have discovered that this can be achieved by changing the period of these changes.

Also, as a feature of the silver metsu precipitate according to the present invention.

It has also been found that the tensile strength and elongation of the plated film obtained in the high gloss range are both higher than those of the plated film of the same gloss obtained using a brightener.

In addition, there is a concern that the plating time will be longer to obtain the same plating thickness when using the energization method of the present invention, but since the current density during the ON time can be higher than that of the normal continuous energization method, it is practically plating time will not be increased. This is considered to be because the limiting current density increases during the OFF time because the formed diffusion layer disappears.

Example Examples of the present invention will be described below.

(Example 1) Copper cyanide strike powder was applied to a 427a IeC lead frame pretreated by a conventional method. , 3 μm was applied, and jet sinter plating was applied to 4 μm using a silver plating solution that does not contain a brightening agent as shown below (the plating time was controlled by τ). Glossiness measured by reflection densitometer 0 KAg(ON)2 100177e KCN 3og/e Bath temperature 6o'c and 70°CFlow rate 8
．． 3 m/sea Current density SOmsn The results are shown in Table 1.

Table 1 (Example 2) 1μm electroless plating applied on sterol resin.

Furthermore, sodium cyanoselenate so ppi was added to the silver plating solution used in Example 1 on the base plated with bronze copper plating.
15860 (film thickness approx. 6
μm) to obtain a sample with a gloss level of 0.81.

Using the silver plating solution used in Example 1 on the same substrate, 40
ASD 70℃, duty ratio s (0N10FF=
0-310.1 (sac)) ON and OFF for 20 seconds
F was repeated to obtain a sample with a film thickness of 6 μm and a gloss level of 0.83. The styrene resin was dissolved and the copper was further peeled off to obtain silver foil. This was tested for tensile strength using a micro tensile tester.
and the elongation rate was measured. The measurement results are shown in Table 2.

(Margin below) Table 2 (Example 3) 42 alloy and 196 alloy that had been subjected to normal pretreatment were directly plated with silver to a thickness of 4 μm without flash plating. The Ginme Tsugi solution used in Example 1 was used. After that, the plated surface was subjected to a heat resistance test at 500"C for 6 minutes in the air.
Observation was made using a stereomicroscope.

The results are shown in Table 3.

(The following is a margin) Table 3 It can be seen from Table 3 that the smaller the duty ratio, the better the adhesion to the substrate.

Effects of the Invention As is clear from the above explanation, according to the silver plating method of the present invention, a neutral or alkaline plating solution containing no brightener is used, and electricity is applied during a preset plating time. By setting the time and non-energizing time, it is possible to obtain a glossy finish of any desired luster.

Claims (3)

[Claims] (1) A silver plating method characterized by using a silver plating solution and applying silver plating to the material by repeatedly applying and de-energizing the material. (2) The silver plating method according to claim 1, wherein the silver plating solution is a neutral or alkaline silver plating solution. (3) The silver plating method according to claim 1 or 2, wherein the material is a lead frame material made of copper, copper alloy, or iron-nickel.