Tin plating and whisker

1 introduction Whisker growth (Whisker) refers to the metal surface of filament, needle single crystal, it can grow on the surface of solid material, easy happening in tin, zinc, cadmium, silver, and other low melting point metal surface, usually occurs in the 0.5-50 microns, sedimentary thickness of very thin metal surface. Typical whiskers are 1-10 microns in diameter and 1 to 500 microns in length.
The damage of tin whiskers
Whisker growth in essence belongs to a kind of spontaneous, without being limited by the conditions, such as electric field, humidity and air pressure surface growth phenomena, and to contain Tin plating layer of Tin whisker growth on the surface of the most typical. The crystals may bend under electrostatic or airflow and may fall off in the movement of electronic equipment causing short circuit or damage. In a low-pressure environment, the tin whiskers may even have an arc discharge between adjacent conductors, causing serious damage. The following table lists some accidents caused by the whiskers.
3 Tin plating whisker various forms
The shape of the tin whiskers is varied, and there are long needles, curved, and noodle kinks, and open forked and so on, as shown in the picture below.
4 methods for the resistance of tin crystals
4.1 plating matte
Traditional bright Tin platingprocess in order to increase the Tin plating layer of gloss, is generally going to add some whitening agents, but generally easy to grow bright Tin Tin whisker; Tin plating without our agent, and use matte Tin plating, against the growth of Tin whisker has certain effect.
Tin plating layer can influence on the grain size and the crystallization of the Tin whisker growth. A large number of experimental studies have found that the tin coating of small grains is more likely to form the tin whisker than the tin coating of large grains. Tin plating layer of grain to a certain size (3 to 8 microns), within the scope of the relative internal stress between the grain size and grain size is small, to achieve a state of thermodynamic equilibrium, Tin crystal tendency towards minimum an elderly male character long.
Tak tin, SPIRIT of science and technology T - 310/410 matte pure tin plating process using special grain growth control agent, the stability control of pure tin coating grain size within the scope of the 3 to 8 microns, effectively inhibit the growth of tin whisker must be the trend.
4.2 heat treatment
There are three methods for heat treatment of surface coating: Annealing, Fusing, and Reflow. The latter two actually melt the coating before solidification, the most commonly used is the method of annealing. After the Tin plating on the baking in the oven or 170 ℃ 150 ℃ / 2 h / 1 h, can achieve the effect of annealing; Tin plating after reflow, coating can be melting and solidification, Hot Dip is also an effective method to suppress the whisker whiskers without electroplating.
4.3 intermediate coating
Intermediate coating is to point to in the Tin plating before plating a layer of other metal elements as barrier layer, and then Tin plating. The commonly used intermediate materials are Ni and Cu. The Ni is most commonly used, Cu is commonly used in brass or iron base plate.
4.4 plating layer alloying
The addition of Pb, Ag, Bi, Cu, Ni, Fe, Zn and other metal elements in Sn can effectively inhibit the growth of Sn crystals. In Japan and South Korea, the lead-free solder end and pin have the sn-bi or sn-ni coating. 4.5 increase the Tin plating layer thickness
Increase the Tin plating layer thickness can inhibit Tin crystal an elderly male character long. Generally increase tin layer thickness 8-10um.
The growth mechanism of tin whiskers
The growth of tin whiskers belongs to a spontaneous appearance. Bell LABS earlier reported spontaneous growth of tin whiskers on tin electroplating. The structural properties of tin whiskers are studied and the crystal whiskers are single crystal structures. The growth of the tin whisker is from the bottom (heel) rather than the top. There are three main explanations for the growth mechanism of the tin whisker, the mechanism of dislocation motion, the mechanism of recrystallization and the rupture mechanism of the oxide layer.
References:
1. The growth of tin whiskers, the special draft of universal SMT and encapsulation, Dr. Wu yiping, researcher of wuhan optoelectronic national laboratory, professor/director of huazhong university of science and technology
2. Tin whisker problem, ChinaSMTonline (WeChat ID: ChinaSMTonline)
3. The Tin plating of whisker growth problem, Tak Tin, inside of science and technology information
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