JPS61194196A - Electroplating method of tin-lead alloy - Google Patents

Abstract

PURPOSE:To eliminate the generation of the abnormal deposition of the aciculae on a plating coated surface by stopping or reversing intermittently the electric conduction in case of performing the electroplating of tin-lead alloy by using an organic sulfonic acid bath. CONSTITUTION:In case of performing the electroplating of tin-lead alloy by using an organic sulfonic acid bath for plating tin-lead alloy, both the forward electric conduction for forming the plating and the stop of this electric conduction or the electric conduction in the reverse direction are intermittently performed. For example, in case of 2A/dm<2> current density, about <=80sec preferably about 20-60sec are made to one unit in the forward electric conduction and about >=3sec preferably about 5-20sec are made to one unit in the stop or the reverse of the electric conduction. Thereby the abnormal deposition of the aciculae and the yarns is prevented to increase the reliability and the product yield of the plated material.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to improvements in tin-lead alloy plating methods using organic sulfonic acid baths.

[Prior art] In the past, exposed metal parts of electronic components were electroplated with tin for rust prevention and soldering purposes, but this plating caused whisker-like crystals called voice ker. Due to problems such as short circuits in electric circuits, electronic parts that have become smaller and more precise in recent years, such as the outer leads of IC packages, are coated with tin-lead alloy plating, which causes fewer whiskers. A borofluoride bath has conventionally been used in large quantities as a bath for such tin-lead alloy plating, but when this bath is used, it is necessary to remove borofluoride ions from the waste water. However, it is difficult to remove borofluoride ions sufficiently, so a plating bath to replace the borofluoride bath is required, and an organic sulfonic acid bath was announced and put into practical use as such a tin-lead alloy plating bath. It has reached this point.

By the way, when this organic sulfonic acid bath is used, needle-like or thread-like abnormal precipitation may occur on the plated surface. Although the mechanism behind this abnormal precipitation is not yet clear, dendrite precursors are formed due to the crystal structure of the cathode surface, anisotropic crystal growth, 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 mentioned above, this abnormal precipitation causes short circuits in electric circuits and impairs the reliability of electronic parts, so it is important to prevent this from the viewpoint of ensuring reliability.
There was also a strong request from both sides to improve the yield of plated products.

Conventionally, in order to prevent abnormal precipitation in tin-lead alloy plating using an organic sulfonic acid bath as described below, the object to be plated is subjected to 1ij treatment such as acid immersion, electrolytic degreasing, or chemical polishing, or the base plating is Although there is a method of applying the same, it increases the number of work steps and complicates the manufacturing process, and it is not always possible to sufficiently suppress abnormal precipitation. Another known method is to add a smoothing agent to the plating bath to inactivate the active material, but in order to effectively prevent abnormal precipitation, the selection of the smoothing agent and the control of the plating bath must be strictly controlled. Because it is requested・K”
L<1 This is a difficult method to implement.

[Problems to be Solved by the Invention] The object of the present invention is to overcome the problems of the prior art described in J. The purpose is to solve the problem of the need for additional steps such as pre-treatment and base plating, or the need for difficult selection and management of smoothing agents. In other words, the purpose of the present invention is to eliminate the problem of abnormal precipitation occurring in the tin-lead alloy plating method using an organic sulfonic acid bath without using additional steps such as pretreatment or base plating or using additives. The object of the present invention is to provide a method for stably and inexpensively performing tin-lead alloy plating.

[Means for solving problems] The present invention aims to solve the above problems.

Electric tin using organic sulfonic acid bath for tin-lead alloy plating
The present invention provides an electric tin-lead alloy plating method characterized by intermittently stopping or reversing current flow in the lead alloy plating method.

In the method of the present invention, energization in the forward direction of plating formation and stopping or reversal of the energization (current energization in the opposite direction) are performed intermittently. This cannot be said uniformly because it is affected by bath composition, current density, etc., but at a normal current density of 2A/drn', each practical preferred time is about 80 seconds or less for one energization, more preferably 20
One unit is ~60 seconds, and the stopping or reversal of energization is within a range of 3 seconds or more, more preferably 5 to 20 seconds. From the viewpoint of the effect of preventing abnormal precipitation, 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 coating formation will decrease, which is undesirable. Also, +11! If the current flow exceeds 80 seconds or the stop time is too short, such as less than 3 seconds, it will be difficult to sufficiently prevent abnormal deposition. The same effect can be obtained with a shorter reversal time than in the case of , but since the deposited plating layer will dissolve during reversal, it is necessary to select an appropriate time so that it is not too long compared to the forward current application time. be.

The waveform of the current generated by intermittently stopping or reversing the current flow as described above is not particularly limited, and may be any one such as a rectangular wave, step wave, triangular wave, or harmonic wave.

The organic sulfonic acid bath for tin-lead alloy plating used in the method of the present invention includes tin and lead, alkanol sulfonic acid,
Known as plating baths whose main ingredients are salts of organic sulfonic acids such as phenolsulfonic acid, such as alkanolsulfonic acid solder plating bath (AS bath) and phenolsulfonic acid solder plating bath (PS bath) manufactured by Ishihara Pharmaceutical Co., Ltd. Baths and the like can be mentioned as slotlets made by Max Schletter.

[Effect] Although the reason why abnormal precipitation is effectively prevented by the method of the present invention is not necessarily clear, it is presumed as follows.

As mentioned above, the diffusion double layer formed near the cathode is thought to play an important role in the process in which dendrite precursors formed due to various factors grow into abnormal precipitation. In other words, the cations near the cathode are consumed by electrodeposition, and the concentration gradient of the diffused double layer increases. As a result, 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. It is estimated that abnormal precipitation will occur. If the current supply is stopped intermittently using the method of the present invention, the diffusion double layer hechaotine will migrate from offshore of the cathode during the stop time, and the concentration gradient of the spreading double layer will be alleviated. It is thought that as a result, the growth of dendrite precursors is suppressed. It is thought that when the current supply is intermittently reversed, the concentration gradient is partially alleviated quickly.

[Example] The method of the present invention will be specifically explained below using examples.

Example 1 AS-S (stannous alkanol sulfonate) 200 g/1. A.S.
-P (lead alkanol sulfonate) 20g1Q, M
Filled with a plating bath (Ishihara AS bath, trade name) containing 70 g/l of S-A (alkanesulfonic acid) as the main component,
A tin-lead alloy plate was used as the anode, and the energization and deactivation times were 60 seconds and 5 seconds, respectively, the distance between the electrodes was 200 mm, the cathode current density was 2 A/d, the temperature was 20°C, and the plate was installed on the cathode rack for 12 minutes. 42 alloy test piece was plated, but no abnormal precipitation was observed.

Example 2 Slotlet SN (tin) 180 g/vert, Slo, Tore, To PB (lead) 7 g/vert, Slotlet A (acid) 15
As a result of plating in the same manner as in Example 1, using a plating bath containing 0 g/i as the main component (slot rent bath, trade name) and setting the energization and stop times to 20 seconds and 5 seconds, respectively, no abnormal precipitation occurred. I was not able to admit.

Example 3 Plating was carried out in the same manner as in Example 1 except that instead of stopping the energization, energization was conducted in the opposite direction during the period, but no abnormal deposition was observed.

Comparative Example Using the same plating bath as in Example 2, plating was carried out for 12 minutes using a normal DC power supply. Approximately 1 length on the test piece
Acicular precipitation of about 0.00 l was observed.

[Effect of the invention] According to the present invention, electric tin-lead metals and needles in electric tin-lead metals are formed.

Abnormal string-like precipitation was effectively prevented, making a significant contribution to improving the reliability of plated products and improving product yield.

Another advantage of the present invention is that variations in plating thickness, composition, etc. can be reduced.

Furthermore, the method of the present invention can be used for pre-treatment of objects to be plated and
Also, since no special additives are used, there is no need to manage them, and it is a simple, paper storage, stable, and highly practical method.

Claims (1)

[Claims] 1. An electric tin-lead alloy plating method using an organic sulfonic acid bath for tin-lead alloy plating, which is characterized by intermittent stopping or reversal of current flow.