USE OF A TIN-LEAD-BISMUTH ALLOY AS A SUBSTITUTE FOR EUTECTIC TIN-LEAD IN WAVE SOLDERING.

This paper compares the properties of 60/40 tin-lead with those of alloys containing 3% bismuth, about 50% tin and the rest lead, from the viewpoint of wave soldering. Theoretical and laboratory investigations, and industrial wave soldering trials have been carried out. The principal motivation for using the tin-lead-bismuth alloy is that, due to its lower tin content, it is cheaper than 60/40; in addition, the matt solder joints produced make inspection easier. Laboratory investigations using the wetting balance have shown that wetting properties are comparable to those for 60/40, while available results indicate no deleterious effects on mechanical properties for additions of up to 5% Bi to 60/40. The 3% Bi alloy has a higher liquidus temperature and a longer freezing range than 60/40. The matt joint appearance results from solidification contraction at the joint surface. Industrial wave soldering trials gave results for the 3% Bi alloy at least as good as those for 60/40 with the same soldering time and temperature.