Stress Buildup of Sn3.5Ag Soldered Stacked CSPs to Board-Level Drop Impact

Solder balls, albeit mechanically soft, are the only structural support to the stacking of chip-scale packages (CSPs). This paper investigates the structural response of a Sn3.5Ag soldered bi-level stacked CSP module to board-level drop impact per the JEDEC criteria, and quantifies the stress buildup in the critical solder balls. The structural response is understood by tracking the force transmission through this 3-D packaging assembly by an elastic structural analysis, in which the solder plasticity is deliberately excluded. The elastic analysis shows that the solder balls at the ball-out level, which is the bottom level of the stacked package interconnecting to the mounting board, suffer the severest stress buildup. The results suggest that understanding of relative vibration amplitude among each level of the stacked package and the mounting board enables a good estimation of the location of critical solders without detailed modeling. For this stacked CSP module, the critical solders locate at the corners of the center package at the ball-out level. Solder balls are more severely loaded in the 0° orientation drop, about one order of magnitude larger than in the 90° orientation drop. The critical solder balls will be promptly loaded to a state in the highly plastic regime after the impact is initiated, as suggested by detailed finite element analysis which accounts for the strain-hardening of Sn3.5Ag. Larger plastic strain accumulation is observed near the interfaces to the solder pad. The solder balls at the higher level of the stacked packages are likely always free of plastic deformation in the drop testing.