Pure tin and its main problems. Tin plague
Modern society pays great attention to issues related to preserving and protecting the environment. In particular, a large spread of soldering technology lead-free. Lead is a material causing serious harm to humans, but at the same time, the refusal of his application in electronics would still be premature. Solders that contain no lead, have a high melting point, which complicates and worsens the controllability of the soldering process. For example, to cover the contact pads of circuit boards after the transition to lead-free soldering, and began to use pure tin. But it uses a number of new problems related to the properties of this material that violates the performance and reliability of the equipment in challenging conditions. It is, first and foremost, filament growths of tin and the ability to «sick» in the cold («tin plague»).
The coating of pure tin.
Contact coating, treated with pure tin, are used for reliable connection and corrosion protection of metal. The transition to lead-free soldering technologies has led to the fact that most manufacturers began using pure tin to cover the contact surfaces. However, the beginning of active use of pure tin, as well as steadily growing demand for the miniaturization of parts has led to new manifestations have long been known for the negative qualities of this metal, the so-called «tin plague» and «whiskers» of tin.
Education «mustache» is — a characteristic phenomenon not only for this metal, it slopes also cadmium and zinc. Mustache tin is a thin thread, growing vertically, spiral, and also in the form of a lug or hook-shaped crystals. The length of such whiskers may be equal to 150 µm and cause serious danger due to closures of adjacent elements of the conductive pattern of the printed Board. Mustache, looking up or bending in the production process of the goods or their operation, may form the connecting conductors between the conductive elements. At high amperage mustache may melt and cause equipment failures.
In the electronics industry, this phenomenon is quite rare. It is possible in the case of pure tin, which at low temperatures is transferred from the α-phase white tin in the β-phase — grey amorphous substance. Soldered tin ware just crumble. The reason is the increase in the volume of metal in the cold. Alloys of lead and other metals delay this transformation, which once begun, tends the contact of a creeping expansion. The greatest velocity of propagation of the «plague» at t° -33°C. When added to the alloy of bismuth, it is possible to prevent the occurrence of plague, because the atoms of that element in the crystal lattice of pure tin prevent the restructuring and the alloy does not turn into powder even at very low temperature. A good way was the alloying of tin, cobalt, antimony and other metals. In parallel, it was found that zinc and aluminium, on the contrary, contribute to the spread of the plague.
To buy, price
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