Characterisation of the cyclic softening properties of solder

Abstract

In drop impacts, solder joints connecting a die and board can fail from fatigue loading resulting from differential flexing induced by the impact pulse. Some solders exhibit strain softening under cyclic loading and this property must be incorporated into finite element modelling to capture the response accurately. This investigation characterises the cyclic softening behaviour of three solder alloys ¿ SAC305, Sn100C and eutectic SnPb through a series of fatigue tests. Cyclic stress-strain curves are analysed to extract isotropic softening and kinematic hardening constitutive parameters. Application of the proposed material model, together with the parameters determined, is demonstrated via simulation of solder joints in a vibrating board. A comparison of elastic-plastic and elastic-cyclic-softening material properties is also undertaken. A significant difference in stress values, ranging from 15-30%, for results based on the two material models is observed, indicating that cyclic softening is significant and must be considered in such computational modelling.