Effects of Microstructural Heterogeneity on Bga Reliability*

The near eutectic 60%-40Pb alloy is the most commonly used solder for electrical interconnections in electronic packages. This alloy has a number of processing advantages (suitable melting point of 183°C and good wetting behavior). However, under conditions of cyclic strain and temperature (thermomechanical fatigue), the microstructure of this alloy undergoes a heterogeneous coarsening and failure process that makes the prediction of solder joint lifetime complex. A viscoplastic constitutive model for solder with an internal state variable that tracks microstructural evolution is currently under development. This constitutive model was implemented into several finite element codes. With this computational capability, the thermomechanical response of solder interconnects, including microstructural evolution, can be predicted. This capability was applied to predict the thermomechanical response of a ball grid array (BGA) solder interconnect. BGAs with both homogeneous and heterogeneous initial microstructures were evaluated. In this paper, the constitutive model used to describe the solder will first be briefly discussed. The results of computational studies to determine the thermomechanical response of BGA solder interconnects will then be presented.