Simultaneous Characterization of Both Coefficients of Thermal Expansion and Thermal Warpages of Flip-Chip Packages With a Cap Using Strain Gauges

A relatively low-cost and easy-to-use strain gauge measurement is proposed here as an alternative method for simultaneously determining the thermally induced warpages and the coefficients of thermal expansion (CTEs) of the flip-chip packages with a cap (or lid). The feasibility of the strain gauge method is evaluated by the shadow moire experiment and the finite element analysis. The stiffness effect of the thermal interface materials (TIMs) in the flip-chip packages with a cap is also considered. The result suggests that the general back-to-back gauge measurement method on the top and bottom of the specimen would cause significant errors as the TIM is relatively compliant, but not for stiff TIMs. This study also proposes a modified gauge method with a few related strain equations for those cases with compliant TIMs to effectively determine their thermal warpages and CTEs by the gauge measurement. The finite element method (FEM) simulation, consistent with shadow moire, has further validated the effectiveness of the modified gauge method. Therefore, the newly developed and modified method of strain gauges is found to be feasible and workable for simultaneously determining both thermal warpages and CTEs of the flip-chip packages with a cap, especially with a compliant TIM.