Optimizing of Microbump Design for Stable Solder Joints in Thermocompression Bonding: A Simulation-Based Approach

Thermocompression bonding (TC bonding) is a crucial process in the manufacture of high-bandwidth memory (HBM), facilitating the electrical connection between memory dies through microbumps and pads. However, this process is susceptible to defects, such as nonwet, dewet, or solder bridge formation. Understanding the dynamic behavior of molten solder is paramount due to the rapid nature of both solder melting and nonconductive film (NCF) curing, often occurring within seconds at ramp rates as high as $100~^{\circ }$ C/s. In this study, we employed Surface Evolver software to simulate solder shape variations by manipulating solder height across a range of solder volumes and pad sizes. Through this simulation, we determined critical solder heights indicative of stable solder joint formation, including the height at which solder shape instability arises and the height at which applied force on the solder becomes zero. The fitting equations derived from these simulations allow for the calculation of microbump design parameters, facilitating stable solder joint formation without the need for complex Surface Evolver or finite element method (FEM) simulation. The validation of these equations against data from various literature sources demonstrates their efficacy in determining microbump design parameters. This work contributes to the advancement of reliable microbump design for TC bonding processes, offering practical insights for engineers and researchers in the field.