Finite element modeling on electromigration of solder joints in wafer level packages

This work studies the electromigration of solder joints in an encapsulated copper post wafer level package (WLP) by finite element modeling. Experimental data showed that the electromigration failure occurs in solder joints on the printed circuit board (PCB) side due to the current crowding. In order to improve the electromigration performance on the PCB side with a copper post WLP, two new line-to-bump geometry designs are proposed. Coupled electro-thermal finite element modeling is performed to obtain the electrical and thermal fields simultaneously. The ionic flux from electron wind and thermal response is calculated based on finite element solutions. The divergence of the total flux, which is the sum of the divergence of electromigration and thermomigration, is extracted at the critical locations in solder joints. Results show that the new proposed design structures can reduce the maximum current density by 19%, and the divergence of the total ionic flux by 42%. Thermal gradient is very small in solder joints, therefore, the main driving force for electromigration failures comes from the electron wind. The finite element results on mesh dependency are discussed in this paper.