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topic 33462

Lead-free solder and tin pests


(2004)

I am involved in electronics distribution and with the industry going to lead-free solder the terms tin whiskers and tin pests are starting to surface. Not having an in-depth knowledge of metals, and having read the postings on your page, it seems like tin pests could be a real life problem for boards subjected to low temperatures, like automotive. Is anyone aware of what is being done for lead-free solder, like the addition of silver? Also, with lead-free solder, at what temperatures are tin pests likely to form?

John Monteith
Electronics Distributor - Dayton, Ohio


Napoleon's Buttons

(2004)

Tin pest is a well known property of tin when it changes from its tetragonal form to a cubic form. Technically this happens at 13.2C, but only when the tin is extremely pure. In reality, it rarely happens, although it did happen to Napoleon's army when they invaded Russia. Their coat buttons were made of tin and when the temperature fell to many tens of °C below zero, the buttons fell to pieces. Once tin pest has started it is very difficult to stop. However, additions of small amounts of other metals, such as bismuth, antimony and lead result in its inhibition. Moving to a pure tin solder will not be practical because of the problems associated with tin whiskers, so other metals will be added to the solder that will not only inhibit whiskering, but will also minimise the possibility of tin pest. There is a lot of information on this in the electronics trade press.

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK



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