Wafer level package with thermal-stress-absorbing interface structure and elongated pad

The increasing demand for high performance, cost-effective, and smallest packages has led to wafer level package (WLP). One of the biggest concerns about this technology is the interconnection reliability during thermal cycling. To improve solder joint reliability in WLP a thermal-stress-absorbing interface structure consisting of two stress-buffering polymer layers and elongated pad design was investigated in this study. To efficiently help absorbing thermal stresses during thermal cycling, the second polymer layer, polybenzoxazole (PBO), was constructed to extend substantially over the entire first polymer layer, epoxy contained polysiloxane. For the reliability test, the board-mounted chips without underfill were subjected to temperature cycling tests, with temperature ranging from -25 °C to 125 °C. The results of this experiment show a significant improvement in solder joint reliability when using the thermal-stress-absorbing interface structure together with the elongated pad design.