Thermomechanical Analysis on Stress Mitigation of FCPBGA with Low Melting Temperature Solder and Low Elastic Modulus Cu Pillar

Increasing thermomechanical stress in complex and large flip-chip packages is a critical issue in maintaining mechanical integrity and reliability of the packages. Effects of varying mechanical properties of joining materials and organic substrate on stress mitigation in low-k dielectric layer under Cu pillar bump were studied using thermomechanical analysis. Lowering coefficient of thermal expansion (CTE) of organic substrate and lowering melting temperature of solder contribute to significant stress reduction. The effect of Cu pillar's elastic modulus becomes relevant when its value is greatly reduced.