Dynamic mechanical behavior of SnAgCu BGA solder joints determined by fast shear tests and FEM simulations

To allow realistic stress assessments of BGA modules during drop and shock events, the dynamic mechanical behavior of SAC BGA solder joints was determined by a combination of fast shear tests and FEM simulations. The goal has been the development of a validated material model for SnAg1.0Cu0.5 BGA solder joints that accounts for dynamic material hardening and the dependency of the fracture mode on the shear speed. Therefore, fast shear experiments of single joints have been performed applying a DAGE® tester to BGA solder balls being attached to Cu+Ni/Au pads with recording the peak shear forces and fracture surface plots. Dynamic 3-D finite element simulations than have been performed applying both codes, ABAQUS explicitTM and ANSYS/LS-DYNATM, to replicate the shear tests virtually. The simulations involved a rate dependent model for the solder material as well as a cohesive zone approach for the fracture in the IMC layer and included a shear criterion for the damage within the bulk solder. The coefficients of all these models have been calibrated iteratively based on the shear test results. Combining experiments and FEM simulations this way, realistic parameters have been determined for the dynamic hardening of the solder as well as for the solder bulk and the IMC strengths. Applying these parameters, the simulated peak forces match the experimental values at all shear speeds and also the range of speed, in which the transition between bulk damage and IMC fracture takes place, is precisely met. These validated models now allow highly dynamic events of BGA modules to be assessed realistically by means of FEM.