Thermal fatigue modelling for SnAgCu and SnPb solder joints

Thermal fatigue of solder joints is investigated by means of the finite element method (FEM). During the usual thermal cycle regime with relatively slow temperature ramping rates, solder constitutive response is dominated by secondary creep. In the paper secondary creep laws are given for SnPb and SnAgCu solders, based on our own measurements and literature data. These secondary creep behaviours are compared to other data recently published. Additional primary creep terms are introduced in the creep description. To relate calculation results to fatigue life, criteria of Manson-Coffin type are proposed. These empirical laws are based on either the cyclic inelastic strain or dissipated energy. Effects of the choice of different creep laws on the calculated creep strains and energy densities are studied for two standard components on FR-4 board, a ceramic chip resistor (CC) of size 0805 and a plastic ball grid array with 225 I/O (PBGA 225). It is shown that the choice of the creep law, e.g. the inclusion of primary creep, does significantly affect only the results for the PBGA solder balls.