Design of double layer WLCSP using DOE with factorial analysis technology

Newer, faster and smaller electronic packaging approaches with high I/O counts and more complex semiconductor devices are emerging steadily and rapidly. Wafer level chip scaling package (WLCSP) has a high potential for future electronic packaging. However, the solder joint reliability for a large chip size of about 100 mm² without underfill is remains a troubling issue that urgently requires a solution. To this end, a double-layer WLCSP (DL-WLCSP) with stress compliant layer and dummy solder joint is proposed in this research in order to enhance the solder joint fatigue life. To ensure the validity of the analysis methodology, a test vehicle of Rambus DRAM is implemented to demonstrate the applicability and reliability of the proposed DL-WLCSP. The results of the thermal cycling in the experimental test show good agreement with the simulated analysis. Furthermore, to investigate the reliability impact of the design parameters, including solder volume, the arrangement of the die-side and substrate-side pad diameter, pitch, compliant layer thickness, die thickness, and the printed circuit board (PCB) thickness, a design of experiment (DOE) with factorial analysis is adopted to obtain the sensitivity information of each parameter by the three-dimensional nonlinear finite element models (FEM). The statistics results of the analysis of variance reveal that the thickness of the stress compliant layer and the volume of the solder joint can effectively reduce the stress concentration phenomenon, which occurs around the outer-corner of the solder joint. In addition, the evident interaction between design parameters can also be obtained. The smaller thermal strains can be achieved through a better combination of design parameters of the geometry so as to provide the actual requirement of the physical information prior to manufacturing.