Effects of Warpage on Fatigue Reliability of Solder Bumps: Experimental and Analytical Studies

Out-of-plane displacement (warpage) has been a major thermomechanical reliability concern for board-level electronic packages. Printed wiring board (PWB) and component warpage results principally from coefficient of thermal expansion mismatch among the materials that make up the PWB assembly (PWBA). Warpage occurring during surface-mount assembly reflow processes and normal operations may lead to severe solder bump reliability problems. In this research, the effect of initial PWB warpage on the low cycle thermal fatigue reliability of the solder bumps in plastic ball grid array (PBGA) packages was studied using experimental and analytical methods. A real-time projection moire¿ warpage measurement system was used to measure the surface topology of PWBA samples at different temperatures. The thermal fatigue reliability of solder bumps was evaluated from experimental thermal cycling tests and finite element simulation results. Three-dimensional (3-D) models of PWBAs with varying board warpage were used to estimate the solder bump fatigue life for different types of PBGAs mounted on PWBs. In order to improve the accuracy of FE results, the projection moire¿ method was used to measure the initial warpage of PWBs, and this warpage was used as a geometric input to the finite element method. The simulation results were validated and correlated with the experimental results obtained using the projection moire¿ technique and accelerated thermal cycling tests. An advanced prediction model was generated to predict board level solder bump fatigue life based on the initial PWB warpage, package dimensions and locations, and solder bump materials.