When power devices are operated, the assembly is thermally cycled by changes in the power input as well as variations in the ambient temperature. This produces severe stresses where there are large differences in TCE for the materials. A gold based ‘hard solder’ joint is relatively strong, so that thermal stresses result in die fracture, whereas joints made with relatively weaker indium, lead and tin containing ‘soft solders’ commonly fail because of thermal fatigue within the joint. Work by Olsen emphasised the comparative ability of hard solder devices to resist thermal fatigue as against the steady decline in performance of soft solder devices.
A number of intermediate alloys have been developed, with the aim of producing a material with a mechanical strength lying between that of hard and soft solders, avoiding damage to the die whilst resisting thermal fatigue within the joint.
‘J Alloy’, with a composition of 25%Ag/10%Sb/65%Sn, aimed at meeting this requirement, but the additives have limited solid solubility in tin, and both the normally cooled alloy and foil made from it contain coarse particles of Ag3Sn, some of which are larger than 10µm. During reflow, the tin matrix melts first, and then dissolves the Ag3Sn particles, which have a melting temperature of 480ºC. The longer time and higher temperature needed to dissolve the particles results in incomplete melting of conventional J Alloy during transient heating and cooling.
Tan found that improvements could be made to the material by using a very high cooling rate. His ‘Rapid Solidification’ process used cooling rates of around 10ºC/s, producing a fine and homogenous microstructure. The same Ag3Sn phase was present, but with the particles finely dispersed. The RS alloy had a well defined reflow solidification temperature and allows lower soldering temperatures to be used. Joints prepared with RS alloy foil showed that the finely dispersed microstructure is retained when appropriate soldering procedures are used.