Hygrothermal delarninations in lead-free solder reflow of electronic packages

The use of environmentally friendly lead-free solder in electronic assembly requires a higher process temperatures that double the saturated steam pressure inside the electronic package. We studied the interfacial delamination fractures created by the trapped steam inside the defect voids in packages by computationally modeling the steam diffusion and evaporation processes and the hygrothermal fracture behaviors. These results were compared with results from the conventional saturated steam pressure approach. The comparisons revealed that the saturated steam approach is appropriate for small defects, while the evaporative-diffusion approach is suited for delamination analyses of small and large defects. Our results showed that the strain energy release rate increased with initial defect growth, but reached a defect-size independent plateau when the defect had grown larger than the temperature-dependent critical defect size. To control delamination in packages undergoing lead-free solder reflow, the interfacial fracture energy release rate should be engineered to be above this plateau by controlling the interfacial adhesion, and the applied strain energy release rate should be reduced by reducing the diffusion and evaporation rate of water in the materials.