Since the passing of the RoHS in 2006 it is now against the law for lead to be mixed into solder and other coatings using in electronics, the problem of tin (and zinc) whiskers damaging critical systems is likely to rise. Tin whiskers are filaments that grow from tin covered surfaces, and can be mm long. They cause short circuits and in some circuits can act like antenna, which can cause all kinds of problems. It takes years for whiskers to grow, which is why you have not heard of massive recalls of cell phones caused by tin whiskers. These items are disposable anyways so it is likely it will never be a serious problem, but by as early as ’09 electronics with long lives like the avionics of the new Airbus planes might have problems. The mechanism behind whisker growth is unknown, but is likely do to stresses in the materials.
From studying what has been published by NASA and others on tin whisker formation it seems that gamma or electron bombardment of the component followed by annealing to reduce the stress should significantly reduce the formation of tin whiskers, far more than annealing alone. It also seems possible that the addition of small amounts of bismuth, aluminum or titanium to the tin (and possibly zinc) should also help prevent or slow the formation of whiskers. The reason I believe any of this would help is all of these things will at the atomic level alter the crystalline structure of the tin (or zinc) and slow the formation of the whiskers. Now there is a chance that doing this would actually make the problem worse, since the mechanism of whisker formation is not known. However? If it turns out the mechanism of whisker formation has anything to do with the tin (or zinc) changing phase to as a result of the broken crystals lattices or needing an asymmetric site to begin the growth then these suggestions will make the problem way worse. However the lead seems to prevent the formation of whiskers by preventing the tin from forming crystals large enough to create a whisker, since lead is not soluble in tin.
I think the biggest preventives will be in the form of a wax or polymer that is non-conductive that can be sprayed or bath plated on to electronics to prevent the formation of whiskers. For example components could be submerged in a bath of the monomers then a focused magnetic field used to heat the only the areas near metal up enough to cause the polymerization. The heat could also be produced by energizing the components, but this would result in uneven coating. This would insulate all the bare metal and perhaps compress it enough to prevent the growth of whiskers. Another excellent method would be to make the solder out of nano-crystalline materials and not really melt it on in the traditional sense but use ultrasonic waves in a vacuum to cause the material to pack together and fuse the crystals to form bonds. (It would likely be helpful if the leads of the component being “soldered” or plated were made of, or coated with a metal (or more like metal oxide) that was soluble in the material being used to allow “wetting” of the surface.) This would be for the moment a very expensive way to solder or plate components but for certain industries well worth it.
Some experiments I would like to do to study metallic whisker formation are:
Isotopic studies of zinc or tin whiskers, to determine if there is over or under representation of one or more isotopes. (I assume chemical studies have already been done, and no smoking gun was found.) Electron beam or other microetching of the plating to see if “expansion” gaps in could prevent or at least control whisker formation. Finally I would like to see what affects the microstructure of the metal beneath the tin/zinc has on whisker formation. It seems entirely possible the whiskers are build like mountains or volcanoes, where forces from underneath displace the tin “crust” to form whiskers.
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