Still worrying about the yellowing of the lead-free pure tin coating, it is because you did not read this article seriously

Tin-lead alloy has been used as a solderable coating for many years. Electronic circuit board industry.

But as people pay more and more attention to environmental issues in recent years, the harmfulness of lead has gradually become known to the public. Therefore, countries around the world continue to promulgate increasingly strict laws and regulations to limit the application of lead in electronic products. In this context, both domestic and foreign are actively developing lead-free Plating process, among which the pure tin plating process is generally recognized by electronics manufacturers.

Following this, many new problems have also appeared, the most typical of which are the problem of yellowing of the coating, the problem of tin whiskers and the problem of turbidity of the plating solution.

The emergence of these problems once plagued the field users of pure tin electroplating and affected the further promotion of pure tin electroplating. Next, we will focus on the yellowing of the coating.

Second, the yellowing problem of pure tin coating

1. The yellowing of pure tin coating

Although tin has high chemical stability and is resistant to oxidation in the atmosphere, it is not easy to react with sulfides, but pure tin coating does have a yellowing problem, and it is quite serious. It has its own rules, unlike the discoloration of silver and other metals. , Features.

The yellowing of pure tin plating is a common phenomenon, which occurs on the tin plating of lead frames, connectors, and even other decorative tin plating.

The yellowing of the pure tin coating shows that the outer surface appears yellow after being placed under certain temperature and humidity conditions or stored for a certain period of time after plating. Yellowing affects the appearance of the coating. Severe yellowing may cause poor solderability, which is not acceptable to general users.

2. The reason why the pure tin coating turns yellow

⑴Oxidation of tin

The tin-plated layer is exposed to the air for a long time, and it will oxidize with the oxygen in the air, and the oxide will turn the color of the pure tin-plated layer into yellow, which seriously affects the solderability of the product. See Table 1 for the relationship between oxide film thickness and color.

⑵There are many defects in the coating, such as pores, cracks, etc.

These defects, such as pores, cracks, etc., will cause the plating solution to penetrate into it and cannot be cleaned and removed, resulting in excessive organic inclusions. Compared with tin-lead coatings, pure tin coatings generally have coarser crystals, large irregularities in crystal grains, and more crystal defects. Therefore, the discoloration of pure tin plating is much more serious than tin-lead.

⑶Organic inclusion or adsorption

When there are more organics in the coating, and these organics are easy to oxidize and change color, it will usually make the coating yellow and change color. Under the guidance of water vapor, the organic molecules contained in the pores of the coating through capillary action will migrate to the surface of the coating and gather on the surface. Therefore, the yellowing sometimes takes a certain period of time to be placed and humid, high-heat environment will promote the yellowing phenomenon to accelerate or become serious.

3. Methods to control yellowing

⑴ Electroplating tin additive

We use two methanesulfonic acid tin plating additives, X and Y, for comparison tests. The brass terminal is the test product. Without pre-plating, the methanesulfonic acid tin plating is directly carried out, and then exposed to the atmosphere test.

It was found that the product plated with the tin plating additive of supplier X was found to be discolored after 1 week, while the product plated with the tin plating additive of supplier Y only began to change color after 1 month. Selection is also important to prevent discoloration of the tin-plated layer.

Using different kinds of additives, the crystalline condition and carbon content of the tin-plated layer can affect the anti-tarnishing ability of the tin-plated layer. Therefore, starting from additives is an important method to solve the yellowing problem.

The specific method is: use additive components with less inclusion and absorption in the coating. Improve the ability of additives to control crystallization, form as perfect crystallization as possible, and minimize crystalline defects. Make the crystalline particles arranged tightly and reduce the gaps at the boundaries between the crystalline particles. Avoid using components that are both adsorptive and colored or will develop color after oxidation. A company in Shanghai used additives SYT848A and SYT848B to solve this problem very well.

⑵Plating process control

Pay attention to the following points in the control of the electroplating process: the pre-treatment degreasing and deoxidation time of electroplating should not be too long,

Avoid pre-treatment over-corrosion; maintenance of the plating solution is very important to avoid turbidity of the plating solution; less additives are added frequently to avoid excessive additives in the plating solution; control the tin ion concentration, temperature, current density and other conditions;

Avoid introducing other impurities into the plating solution; regularly treat the bath solution. After a certain period of use, the plating solution must be replaced. Some companies do not replace the old plating solution in order to reduce costs, resulting in poor electroplating effects.

⑶ Post-plating treatment

It is very important to neutralize after electroplating, to remove the plating solution remaining on the plating layer or the substrate, to prevent the residual solution from leaking out during the placement process, and accelerate the oxidation of the plating layer to cause the plating layer to change color. Strengthen the cleaning to ensure that the deionized water used for cleaning is clean and clean, and at least one of the deionized water used for cleaning is heated deionized water, and the water temperature should be controlled at 60 ℃ ± 5 ℃. Improper post-plating treatment and insufficient cleaning are the main causes of yellowing. The use of pure water and spray has a great effect on controlling discoloration, and the neutralizer is regularly updated.

Three. Concluding remarks

Currently, lead-free pure tin coating has been widely used in electronic electroplating, but its process is still in the early stage of widespread adoption, and some technical links are not yet very mature.

There are also problems with tin whiskers, discoloration of the coating, and turbidity of the plating solution. These problems need to be studied in depth, to be resolved, and to promote the development of lead-free pure tin electroplating technology.