This study demonstrates the achievement of robust Cu-Cu bonding strength through thermocompression bonding (TCB) under a formic acid (FA) atmosphere. When subjected to sintering at 300 °C for 1 min under FA, sintering joints exhibit an average shear strength of 50.9 MPa. This strength further increases to an average of 131 MPa when the sintering duration is extended to 20 min at the same temperature under FA. Molecular dynamics simulations are employed to model the sintering behavior of copper particles of various sizes and thus understand the diffusion mechanism. The analysis of mean square displacement (MSD) and radial distribution function from these simulations suggests that the presence of small particles aids in the sintering of large ones. A copper paste, formulated by mixing micron-sized copper particles with organic solvents, is utilized in a series of experiments to explore different sintering methodologies aimed at enhancing the mechanical integrity of the sintering joints while simultaneously addressing issues associated with copper particle oxidation. Innovative strategies, including redox processes, are applied to improve the shear strength of the sintering joints and to minimize the detrimental effects of oxidation on the copper particles. Results indicate that preoxidation, which was used to form a nano surface structure, and using an FA atmosphere, remarkably enhance the shear strength of the Cu-Cu joints created via TCB. The findings of this research are pivotal for the advancement of rapid Cu-Cu bonding techniques using micron-scale copper pastes and can have profound implications for the development of future electronic packaging and interconnection technologies.

With the development of high-density integrated chips, low-k dielectric materials are used in the back end of line (BEOL) to reduce signal delay. However, due to the application of fine-pitch packages with high-hardness copper pillars, BEOL is susceptible to chip package interaction (CPI), which leads to reliability issues such as the delamination of interlayer dielectric (ILD) layers. In order to improve package reliability, the effect of CPI at multi-scale needs to be explored in terms of package integration. In this paper, the stress of BEOL in the flip-chip chip-scale packaging (FCCSP) model during thermal cycling is investigated by using the finite-element-based sub-model approach. A three-dimensional (3D) multi-level finite element model is established based on the FCCSP. The wiring layers were treated by the equivalent homogenization method to ensure high prediction accuracy. The stress distribution of the BEOL around the critical bump was analyzed. The cracking risk of the interface layer of the BEOL was assessed by pre-cracking at a dangerous location. In addition, the effects of the epoxy molding compound (EMC) thickness, polyimide (PI) opening, and coefficient of thermal expansion (CTE) of the underfill on cracking were investigated. The simulation results show that the first principal stress of BEOL is higher at high-temperature moments than at low-temperature moments, and mainly concentrated near the PI opening. Compared with the oxide layer, the low-k layer has a higher risk of cracking. A smaller EMC thickness, lower CTE of the underfill, and larger PI opening help to reduce the risk of cracking in the BEOL.