"I’ve heard the terms tin pest and tin whisker. Are they the same?"
I’ve heard the terms tin pest and tin whisker. Are they the same?
Tin pest, sometimes referred to as tin disease, is an allotrope (a new molecular configuration of an element, with new physical properties) that occurs when tin is exposed to very cold temperatures. Tin is stable in the metallic (beta) form at room temperature, but it will change to the powdery (alpha) form at temperatures below 8.24ºF (-13.2ºC). This change is said to have made the buttons on Napoleon’s troops to turn to dust, causing him to lose the war in Russia - after all, his soldiers couldn’t fight and hold up their pants. Though it is unlikely that this situation really happened, it does make a good story. (For more information about changes in tin, you might enjoy Napoleon’s Buttons: How 17 Molecules Changed History, by P. Couteur and J. Burreson.)
Another story about this transformation says that at the end of the 19th century, a Russian Czar shipped tin across Russia. When it arrived at its destination, it was a gray powder. The Czar’s officials thought the transporter had robbed the Czar, so he was put to death. When the material was analyzed, it was in fact pure tin, just not in the beta state expected.
In plating shops, tin pest is not a concern. Tin whiskers, however, are important.
Tin whiskers are single crystals of tin that grow spontaneously from a tin-plated surface. As they grow, they can cause electrical shorts. Efforts to remove lead from circuit boards and solder have led to an increased occurrence of tin whiskers. It is estimated that this phenomenon has caused billions of dollars in damage to satellites and other electronics.
Some feel that increased stress might promote diffusion within the deposit and lead to greater whisker propensity. Other experiments show the reverse. Also, whiskers may have a long incubation period. Their growth can be quick or very slow. This means that it is difficult to evaluate whether a tin-plated part will produce whiskers. Another theory is that pure tin electroplated surfaces, especially those that employ brighteners in the plating process, are more susceptible to whisker formation. If you are planning to use pure tin, discuss the potential problems with your bath vendor.
Using tin-silver or other alloys seems to keep the whisker problem in check, but plating might be more difficult. According to the NASA Tin Whisker website (http://nepp.nasa.gov/whisker), plating chemistry can influence whisker growth. Pure tin is the most prone, while alloys such as tin-copper (SnCu), tin-bismuth (SnBi) and tin-lead (SnPb) are less susceptible.
Other metals also can grow whiskers, including zinc, cadmium gold aluminum, lead and indium. While gold and silver can grow whiskers, a bigger problem with precious metals is that they often grow “legs” and walk away from plating facilities if they are not properly secured, particularly as these materials increase in market value. Copper has this problem as well.
It is interesting to note that whiskers rarely are found in tin-lead or solder plating. Theories suggest that any or all of these might be involved: brighteners, incorporated hydrogen, co-deposited carbon, pH, current density, bath temperature and agitation. Other possible suspects include the substrate material; stresses induced by stamping, etching and, annealing; the formation of intermetallic compounds; and substrate element diffusivity into tin.
With the potential for tin whiskers should you plate tin? Sure, just be aware of the possible problems and try to find an appropriate alloy for your project. Some research has been done using foam-type coatings over the tin, but these coatings have not completely resolved the problems. Generally, working with your bath vendor and your customers should produce satisfactory products with limited tin whisker growth.
Pat Plater is a regular feature of Finishing Today magazine. Send your questions to firstname.lastname@example.org.