Combined experimental and numerical investigation on flip chip solder fatigue with cure-dependent underfill properties

A cure-dependent viscoelastic constitutive relation is applied to describe the curing process of epoxy underfill in Flip Chip on Board (FCOB). The chemical shrinkage of the epoxy underfill during the curing process is applied via incremental initial strains. Thus, the stress and strain build-up, caused by the simultaneous increase in stiffness and shrinkage during the curing process, are simulated. Accelerated fatigue experiments with thermal cycles from 55°C to 80°C are carried out for a specially designed Flip Chip configuration. Based on the obtained curing induced initial stress and strain fields, thermo-mechanical predictions are presented for the test carriers. The solder bumps are modeled as temperature dependent visco-plastic properties. A combination of a Coffin-Manson based fatigue relation and a creep fatigue model is used as fatigue failure criterion. The results show that the FEM-based fatigue life predictions match better with the experimental results, if the curing induced initial stress state is taken into account