Size Effect of IMC Growth in Liquid-Solid Reflow Reaction at SAC305/Cu Interface

With the advancement of 3D packaging, the dramatic reduction in solder joint dimensions significantly impacts interface reactions, the formation of intermetallic compounds (IMCs) at interfaces, and the evolution of grains within the solder. Notably, the reaction of Sn from the solder with Cu from the substrate forms brittle IMCs at the interface, critically affecting the reliability of solder joints. This study investigates the growth of IMCs in SAC305/Cu solder joints of varying sizes under different reflow time. Furthermore, it simulates the temperature field during the reflow process to explore the impact of solder size on the growth behavior of interface IMCs. Results demonstrate that as solder joint size increases from 300 μm to 600 μm, the thickness of the interface IMC layer decreases, with Cu
6
Sn
5
being the predominant IMC. At 60 seconds of reflowing, with the increase in solder size, the most of Cu
6
Sn
5
grain morphology transformed scallop shape to roof shape. Additionally, the most of Cu
6
Sn
5
grain transformed ridge shape to scallop shape with the increase of reflow time. Kinetic analysis of IMC thickness growth reveals that with decreasing solder joint size, the growth mechanism of interface IMCs transitions from bulk diffusion-dominated to grain boundary diffusion-dominated. Simulations of the temperature field show that, within the initial one second of reflow soldering, solder joint temperature exhibits a pronounced size effect. Smaller joints achieve higher temperatures under identical reflow time, providing a greater nucleation driving force, resulting in smaller, more numerous Cu
6
Sn
5
grains in the early stages of reflowing. Therefore, small size solder joints quickly complete the transition from ridge shaped to scallop shaped.