JPH0573837B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an improvement in the electrolytic tin plating process using a plating bath such as a sulfate bath or a borofluoride bath.

[Conventional technology]

Electrolytic tin plating is applied to the exposed metal parts of electronic components for the purpose of rust prevention and solderability. Usually, a sulfate bath or a borofluoride bath is used for this tin plating, but whisker-like crystals called whiskers occur in the film plated with such a tin plating bath several days after plating. There is. Such whiskers cause problems such as short circuits in electric circuits, so they must be suppressed as much as possible, and measures such as applying a special base plating or heat treatment after plating are generally taken.

By the way, in the above-mentioned electrolytic tin plating method, needle-like or thread-like abnormal deposits may occur on the surface of the plating film during plating. Unlike whiskers, this is a polycrystalline body formed during electrodeposition. Although the mechanism of this abnormal precipitation is not yet clear, dendrite precursors are formed due to the crystal structure of the cathode surface, the anisotropic growth of crystals, chemical affinity, etc. This is thought to be because a current with a current density flows and grows at an accelerated rate. Similar to the whiskers in the case of tin plating, this abnormal precipitation causes short circuits in electrical circuits and impairs the reliability of electronic components, so it is important to prevent this from the standpoint of ensuring reliability. This was also strongly requested to improve the yield of Metsuki products. In order to prevent such abnormal precipitation during tin plating, conventional methods have been used to pre-treat the material to be plated, such as acid immersion, electrolytic degreasing, or chemical polishing, or to perform base plating. The actual situation is that the number of steps increases, making the manufacturing process complicated, and that abnormal precipitation cannot always be sufficiently suppressed. Furthermore, this abnormal precipitation can be prevented by adding a brightening agent to the plating bath to achieve bright plating, but this method cannot be applied to matte tin plating specifications.

[Problem that the invention seeks to solve]

The purpose of the present invention is to solve the trouble of abnormal precipitation,
The object of the present invention is to provide a method for stably and inexpensively performing electrolytic tin plating without requiring additional steps such as pre-treatment or base plating or adding brighteners.

[Means for solving the problem] In order to achieve this object, the method of the present invention involves intermittently stopping the current supply in an electro-tinning method using a plating bath such as a sulfate bath or a borofluoride bath. The points are distinctive.

In the method of the present invention, energization is carried out in the forward direction for plating formation, and the energization is stopped. The preferred time for energizing and stopping cannot be determined uniformly because it is affected by the material of the object to be plated, the composition of the plating bath, the current density, etc., but at a normal current density of 1.5 A/dm ² , the energizing time is approx. 80 seconds or less, more preferably 20
One unit is ~60 seconds, and the energization is stopped for 3 seconds or more, more preferably 5 to 20 seconds.

From the point of view of the effect of preventing abnormal deposition, the shorter the energization time and the longer the stop time, the better; however, if the energization time is too short or the stop time is too long, the efficiency of plating formation will decrease, which is undesirable. Furthermore, if the current application time exceeds 80 seconds or the stop time is too short, such as less than 3 seconds, it is difficult to sufficiently prevent abnormal deposition.

The current waveform caused by intermittent stopping of energization as described above is not particularly limited, and for example, a rectangular wave,
It may be a step wave, a triangular wave, a sawtooth wave, etc.

[Effect]

The reason why abnormal precipitation is effectively prevented by the method of the present invention is not necessarily clear, but it is presumed as follows. In other words, the diffusion double layer formed near the cathode plays an important role in the process in which the dendrite precursors formed due to the above-mentioned factors grow into abnormal precipitation, and the cations near the cathode are absorbed by electrodeposition. As dendrite precursors are consumed and the concentration gradient of the diffusion double layer increases, the dendrite precursors grow into protrusions, and a current with a high current density flows through these protrusions, causing the protrusions to grow at an accelerated pace and become abnormally precipitated. When the current supply is stopped intermittently according to the method of the present invention, cations move from offshore of the cathode surface to the diffused double layer during the stopped time, and the concentration gradient of the diffused double layer is relaxed, and as a result, the growth of dendrite precursors is suppressed. It is considered that the

Examples of the present invention are shown below along with comparative examples.

〔Example〕

Example...30g/SnSO ₄ as a plating bath,
120g of H ₂ SO ₄ , 40g of Starter Kurumo (trade name of tin plating additive made by Raybolt Co., Ltd.)
ml/ml, using a plating device equipped with a cathode locker, using tin as an anode,
A 42 alloy test piece attached to a cathode rack was tin-plated. The plating conditions are: distance between electrodes 200mm, cathode current density 1.5A/dm ² , temperature 20℃
The energization and de-energization times are each 60 seconds,
It was set to 5 seconds. I plucked for 9 minutes under these conditions,
No abnormal precipitation was observed.

Comparative Example: Plating was carried out for 8 minutes using the same plating bath and equipment as in the example, and under the same plating conditions as in the example, except that normal direct current plating was used. Acicular deposits approximately 100μ in length were observed on the test piece.

〔Effect of the invention〕

The present invention has effectively prevented needle-like and thread-like abnormal precipitation during electroplating, and has greatly contributed to improving the reliability of plated products and improving product yield. The method of the present invention does not require pretreatment of the object to be plated or base plating, and since no special additives are used, there is no need to manage them, making it a simple, inexpensive, stable, and highly practical method. .

Claims (1)

[Claims] 1. An electric tinning method using a plating bath such as a sulfate bath or a borofluoride bath, which is characterized by intermittent stopping of current supply.