Underfill constraint effects during thermomechanical cycling of flip-chip solder joints

Abstract

The presence of an “underfill” encapsulant between a microelectronic device and the underlying substrate is known to substantially improve the thermal fatigue life of flip-chip (FC) 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 thermomechanical cycling (TMC), has been used to investigate the impact of the constraint imposed by the underfill on a solder joint. Finite element (FE) modeling has been used to demonstrate that the SJS sample geometry captures most of the deformation characteristics of an FC joint and to provide insight into 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 allow 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.