Finite element simulation of solid-liquid interdiffusion bonding process: Understanding process dependent thermomechanical stress

Solid-liquid interdiffusion (SLID) bonding finds a wide variety of potential applications toward die-attach, hermetic encapsulation of microelectromechanical systems (MEMS) devices and 3-D heterogeneous integration. Unlike soft soldering technique, the solidification of intermetallic compound (IMC) formation in SLID bonding occurs during the process isothermally, making it difficult to predict and mitigate the sources of process-dependent thermomechanical stresses. Literature reports two dominant factors for the built-in stress in SLID bonds: volume shrinkage (due to IMC formation) and coefficient of thermal expansion (CTE) mismatch. This work provides a detailed investigation of the Cu-Sn SLID bonding process by finite element (FE) simulations. Specifically, the FE simulation of the SLID bonding process is divided into three steps: ramp-up, hold-time, and ramp-down stages to understand the stresses formed due to each individual step. Plastic material properties for Cu as well as temperature-dependent material parameters for different entities are assigned. Process-dependent thermomechanical stresses formed during the ramp-up and hold-time steps (IMC formation) were found not to be significant. The hold-time step is governed by the reaction and diffusion kinetics, which determines the bond line quality including defects, such as voids. The ramp-down step is the dominant phase influencing the final stress formations in the bonds. The results show an average of >30% decrease in the stress levels in Cu3Sn layer (IMC) when the bonding temperature is brought down from 320 °C to 200 °C, thus demonstrating the importance of low-temperature SLID process.