Reliability enhancement of wafer level packages with nano-column-like hollow solder ball structures

In this paper, hollowed solder ball structures in wafer level packages are investigated. Detailed 3-D finite element modelling is conducted for stress and accumulated inelastic strain energy density or creep strain analysis. Three cases are studied in this paper: a regular solder ball, a solder ball with a polymer core of sphere shape, and a hollowed solder ball. Two different creep models are applied: 1) anand model; and 2) hyperbolic sine creep model. Testing data of polymer-cored solder ball WLP are used for verification. Hollowed ball WLP appears to have further improved thermal-cycling performance. When anand model is used, some unexpected results are obtained: 1) maximum inelastic strain energy density and von Mises stress occur in the balls of the middle of the diagonal row; and 2) von Mises stress is almost equally distributed among all balls. However, when hyperbolic sine creep model is used, the obtained results are consistent with the observations in conventional solder ball WLP. Nevertheless, both creep models predict the significant reduction in solder ball stress and accumulated creep strain or inelastic strain energy density for hollow ball WLP. Existing experimental results of polymer-cored WLP from literature can indirectly validate the benefit from hollowed balls.