Here's an interesting patent I bumped into by accident. "fatigue resistant" solder alloy. Hmmmm.

Title:Fatigue resistant lead-tin eutectic solder

Document Type and Number:United States Patent 5308578

Link to this page:http://www.freepatentsonline.com/5308578.html

Abstract:The fatigue resistance of lead-tin eutectic solder is increased by doping the solder with about 0.1 to 0.8 weight % of a dopant selected from cadmium, indium antimony and mixtures thereof. The doped eutectic solder exhibits increased resistance to thermally or mechanically induced cyclic stress and strain. As a result, the fatigue resistance of the solder joint is increased. Combination of dopants, such as indium and cadmium, in combined amounts of less than 0.5 weight % are especially effective in increasing the fatigue resistance of the lead-tin eutectic solder.