Drop impact analysis of Sn-Ag-Cu solder joints using dynamic high-strain rate plastic strain as the impact damage driving force

Board-level drop reliability test and analysis requires dynamic characterization of high strain-rate properties of bulk solder and solder joint failure tests. Drop impact analysis of board-level dynamic response (ie: G-levels and board bending strains) and over-simplification of deformation response of solder joints (ie: assuming elastic stress criteria) can lead to wrong conclusions in the physics-of-failure understanding in drop impact tests. Solder joint failures during drop testing is a complex failure interaction process between low cycle impact fatigue crack growth versus brittle fracture of the intermetallic interfaces. During a drop test event, dynamic hardening causes the yield stress in the solder to rise several times above the nominal monotonic tensile test yield stress. The increase in dynamic strength in the solder joint can cause dynamic strain cycling in the solder material and lead to progressive low cycle impact drop fatigue failures. On the other hand, when the drop loading is excessive, impact failure strength of the intermetallic interface will result in brittle fracture of the solder joint. Impact test were conducted with a Split Hopkinson pressure bar (SHPB) test system to study the dynamic response of bulk solder materials and impact failure of solder joint interfaces. Solder joint reliability characterization by drop impact test with clamped-clamped boundary condition were investigated for PBGA assembly with Sn-Ag-Cu solder. FEA modeling and simulation of drop impact were conducted considering different solder constitutive models such as elastic and strain rate dependent plastic model to investigate the effect of solder constitutive model on dynamic response in the solder joint. The important finding of this study is that the constitutive model used has a major impact on dynamic response of solder joint stress and strain results. It was expected that strain rate dependent plastic model gave better correlation results than the simple elastic model. This study also investigates the IMC effect on solder strain response subject to drop impact test simulation.