The whole purpose of alloying tin together with lead was because in the 1800s german pipe organs made from pure tin would deteriorate after about 2 years. This was called 'tin pest' which is more literally translated as 'tin plague'. Modern organ pipes use a 50/50 alloy of tin to lead which cures the phenomenon completely. The problem was traced to pure tin not liking any tempreture below 53 degreees F.
Raw tin is found in the alpha state and when melted and processed into a useful form becomes beta. Because of the crystaline structure of tin is different in both states the molecules of the structure break them selfs appart when going below 53 F Reverting Beta tin back into Alpha tin, occasionally this can be heard as a "tin cry". The process is just like when Aluminum wire deteriorates back into a ceramic powder. As for tin whiskers, no one, not even NASA understands why they grow. It is not magnetic fields or electirc fields, it just seems to happen. I feel it is most likely due to disimilar metal tempreture coeffieceints causeing tiny dimples of the plated metal to push molecuels of tin out. Kinda like puting a Mentos into Diet Coke. Current lead free solder is about 99% tin 0.6% copper and 0.4% other which is usually antimony and bismuth. Silver bearing solder is seen in 96% tin and 3.8% silver and 0.2% other. Lead free solder also tends to eat soldering iron tips faster for what ever reason. Thanks to RoHS and people forgeting what they already knew and the Wal-Mart throw it away instead of fixing it mentality in about 4 or 5 years we should see a rash of consumer electronics filling land fills when they fail due to whiskering or beta-alpha reversion. I'm sticking with good old fashioned non-RoHS complient 63/37 solder, just like your Dad used.
