Effect of initial microstructure, frequency and temperature on the low cycle fatigue behaviour of the soldering alloys 96.5Sn–3.5Ag and 63Sn–37Pb

Abstract

Due to the differences in the coefficients of thermal expansion between the components of electronic assemblies (substrate-solder-circuit) the variations of temperature (either by heating during the current passage or by environmental conditions) promote cyclical deformation in the component and in the solder alloys. Bench checks of the assemblies indicate that the failures happen after relative few cycles, being deduced that the alloys work in the low cycle fatigue (LCF) regime. The study of the behaviour of solder alloys to LCF is therefore necessary to understand better the reliability of the electronic assemblies. For this reason LCF tests of the two eutectic Sn–Pb and Sn–Ag solder alloys have been carried out at temperatures that varied between −20 °C and 80 °C, and at different frequencies. Also the initial microstructure of each alloy was introduced like variable (by means of variation of the solidification rate). The behaviour to fatigue was analyzed by means of the Coffin–Manson model. The exponent of ductility to fatigue of the solder alloy Sn–Pb is in the interval from 0.27 to 0.59, whereas in the Sn–Ag alloy ranges between 0.34 and 0.70. Both exponents depended on the temperature, the frequency and the initial microstructure. The resistance to fatigue of both alloys diminished when increasing the frequency, In addition, the resistance to fatigue of the Sn–Pb alloy was minor than in the eutectic Sn–Ag.