Thermal reliability of Cu sintering joints for high-temperature die attach

In this study, the thermal reliabilities of sintered Cu joints were investigated under aging and in thermal shock environments. Sintered Cu joints were maintained for 2000 h and 2000 cycles in high-temperature (200 °C) and thermal shock (from −45 to 175 °C) environments, respectively. Subsequently, the microstructural changes, Cu-oxide formation, and bonding strengths of the joints were evaluated. The initial shear strengths of the chips with bare copper and ENIG surfaces were 40.84 and 36.44 MPa, respectively, and after a 2000-h aging test, the strengths decreased to as 39.84 and 18.58 MPa, respectively. With an increase in the aging time under an aging environment of 200 °C, the internal porosity decreased owing to the formation of Cu2O and additional diffusion between the particles. Therefore, the shear strength of the chips with bare copper surfaces was maintained throughout the aging tests, whereas that of the electroless nickel immersion gold chip decreased owing to the formation of Kirkendall voids at the interface, which were attributed to the different diffusion rates of Cu and Au. In addition, Au3Cu, AuCu, and AuCu3 interfacial intermetallic compounds were formed between the Au and Cu layers after the aging test was performed at 200 °C for 2000 h. Copper oxide formed during the thermal shock test caused delamination at the interface because the coefficient of thermal expansion mismatched with that of pure Cu. Understanding this intrinsic Cu oxidation mechanism will widen the prospects for the application of Cu sintering in various devices.