Underfill Constraint Effects during Thermo-Mechanical Cycling of Flip Chip Solder Joints

The presence of an 'underfill' encapsulant between a micro-electronic device and the underlying substrate is known to substantially improve the thermal fatigue life of flip-chip solder joints, primarily due to load-transfer from the solder to the encapsulant. In this study, a new single joint-shear (SJS) test, which allows the measurement of the strain response of an individual solder ball during thermo-mechanical cycling (TMC), has been utilized to investigate the impact of the constraint imposed by the underfill on a solder-joint. Finite element modeling has been utilized to demonstrate that the SJS sample geometry captures most of the deformation characteristics of a flip-chip joint, and to provide insight into the experimental observations. It has been shown that the strain response of a eutectic Pb-Sn solder joint is influenced significantly by in-situ microstructural coarsening during TMC, which in turn is dependent on the underfill properties. In general, underfill properties, which allowed the imposition of large compressive hydrostatic stresses on the solder joint, were the most effective in reducing coarsening. Phase coarsening prevented the stabilization of the stress-strain response of the solder even in the absence of crack-damage, and was found to depend strongly on the local inelastic strain state within the joint. This necessitates that future solder deformation models account for strain-history dependent microstructural evolution, and that underfill properties be optimized to minimize the extent of coarsening during TMC in order to maximize joint life.