Impact of the combination of stress buffer layer and wafer level underfill on 3D IC assembly using thermal compression bonding

This study focuses on the investigation of a buffer layer (BL) spin-on dielectric polymer material in combination with a dry-film underfill (WLUF) on the electrical and mechanical performances of 50μm, 40μm and 20μm pitch Sn-based solder μbumps 3D stacked test structures. Thermo-mechanical stresses are generated by the coefficient of thermal expansion (CTE) mismatch between the silicon die and the other materials, including metals, underfill. The objective is to verify whether the use of a medium-stiff polymer, partially replacing the WLUF, can help to reduce the induced stresses in the stack during thermo-compression bonding (TCB) and provide more reliable interconnects. First, stacks with 3μm BL and 15μm WLUF were built by TCB so that Daisy Chains (DC's) at 50μm pitch could be electrically measured exhibiting a high yield of > ~90%. Next, package reliability testing including Thermal Cycling (TCT) and unbiased Humidity Accelerated Stress (uHAST) were conducted showing no significant changes up to 1000 cycles and 168 hours for TCT and uHAST respectively. Second, the same materials were used in a more aggressive test vehicle (TV) with μbump pitch of 40 and 20μιη. N=4 die stacks could be produced using a collective bonding approach enabling to achieve an electrical yield of about ~80 to 90% for both 40 and 20μm DC's at all levels of the stack. Finally, a Finite Element Model (FEM) was built to understand the impact of the BL in terms of mechanical stress at the μbump level. It was shown that the BL does not significantly contribute to a reduction in the level of thermo-mechanical stresses. However the Cu plastic deformation is reduced in presence of the BL indicating that the risk of bump failure is lower.