Effect of Voids on Thermo-Mechanical Durability of Pb-Free BGA Solder Joints: Modeling and Simulation

Abstract

The effect of process-induced voids on the durability of Sn-Pb and Pb-free solder interconnects in electronic products is not clearly understood. Experimental studies have provided conflicting ambiguous conclusions, showing that voids may sometimes be detrimental to reliability, but they may sometimes even increase the reliability of joints, depending on the size and location. Because of the higher level of process-induced voids in Pb-free solders, this debate is more intensified in Pb-free joints. This study presents Finite Element Analysis of the influence of voids size, location, and spacing on the durability of Pb-free solders. A three dimensional, global-local, visco-plastic FEA is conducted for a CTBAG132 assembly under thermal cycling. The displacement result of the global FEA at the top and bottom of the critical ball is used as the boundary condition in a local model which focuses on the details of a single ball of the CTBGA package under temperature cycling. Parametric study is conducted to model a solder ball with voids of different sizes, and locations. The maximum void size modeled is up to 6% of the ball volume. An energy-partitioning model for cyclic creep-fatigue damage is used to estimate the damage and monitor the trends as the size and location of voids are varied. Potential sites for maximum damage and crack initiation are identified. Strain based and energy based damage models are compared, for later verification with experimental studies in a future paper.