Effect of Die Size and Die Tilt on Solder Reliability under Thermal Cycling

Solder is widely used in electronic packaging to connect die and substrate in die attach process. It is important to get a uniform solder distribution to provide good electrical conductivity, mechanical support as well as excellent thermal properties for packages, especially for power packages with high current densities. While nonuniform solder distribution would emerge when die moves and tilts in the reflow process due to the unbalanced capillary force induced by the liquidus solder. It would affect the solder reliability and the package life. In this paper, we investigated the geometry evolvement of large area lead-free solder paste used in power electronic packaging with Surface Evolver, and focused on the effect of die size and die tilt on solder reliability. Surface Evolver, an interactive program for the study of surfaces shaped by surface tension and other energies as well as subject to various constraints, was used to simulate the shape of solder. The quality of solder was then evaluated by finite element method (FEM) after evolvement. 2D meshes exported from Surface Evolver were imported to the FEA software for modeling. Thermal-structural analysis was conducted to study the thermal-mechanical performance of solder. As the large mismatch of coefficient of thermal expansion of solder, die and substrate, thermal expansion was considered. Simulations of different die sizes without die tilt were performed as control groups. Compared to the control groups, solder with die tilt suffers from higher inelastic strain and inelastic strain concentrates on the thinner side of solder. Both are adverse to the solder reliability. Larger die size would not only increase the inelastic strain on the corner of solder, but also induce larger die stress.