The effect of cyclic thermal loading rate on the mechanical behavior of micro-bumps in CoWoS package

Abstract

Micro-bumps are one kind of the CoWoS package structures. Finite element analysis (FEA) simulation was used to more accurately investigate micro-bumps mechanical behaviour (e.g. stress-strain state, lifetime prediction) at different cyclic thermal loading rates (mimic the thermal cycling and thermal shock in JEDEC JESD22 A106B/A104E conditions). Considering the difference in thermal expansion coefficients of each material in the CoWoS structure, the material properties of each structure were fed into the simulation. The results showed that the cyclic thermal loading rate influences the maximum equivalent strain, which is larger at slower thermal loading rates. However, the maximum equivalent stress and the maximum strain energy density are larger at faster thermal loads. In addition, the regions with the maximum equivalent stress and the maximum strain energy density normally are located closed to the most distal part of the micro-bumps. Finally, this paper also presents a lifetime prediction for micro-bumps by Engelmaier model, which could be a reference only. The lifetime prediction indicated that faster thermal loading rates lead to micro-bumps greater stress-strain concentration, which in turn reduce lifetime.