On the absence of the β to α Sn allotropic transformation in solder joints made from paste and metal powder

Until July 2006, most solder joints in the electronics industry were made of the alloy 63Sn37Pb or 62Sn36Pb2Ag. After this date, the European environmental Restriction of Hazardous Substances directive (RoHS) forced many manufacturers to use Pb-free alloys. These substitutes for SnPb are Sn-rich alloys (over 90% Sn) of various compositions. Below 13.2 °C, Sn potentially transforms into a different phase. This occurs with catastrophic effects, as the transforming material becomes extremely brittle and falls apart. The purpose of this paper is to investigate if this allotropic transformation also occurs in samples prepared from solder paste or metal powder. This work compares the transformation propensity of samples prepared with bulk solder, solder paste, and tin powder. Different conditions and geometries are used in the investigation and experiments with both commercial and specifically prepared solder pastes are carried out. Samples prepared from bulk solder transform into the α phase as expected, whilst samples prepared from solder paste and tin powder do not transform. The residual organic compounds from the flux are believed to be responsible for this behaviour. The tin oxide (SnO2) retained in the bulk after melting could also play a role. This paper shows, for the first time, a relationship between the ability of tin to transform and the nature of the starting material and in particular that the tin β/α allotropic transformation does not occur when samples are prepared from paste or powders. The new lead-free alloys can therefore be used with more confidence in mission-critical applications.