Study on the wettability, intermetallic compound growth, voids formation and mechanical properties of Cu/Sn joints with changes in substrate roughness for electronic packaging

The study investigates the effects of roughness changes on the wettability, intermetallic compound growth, voids formation, and shear mechanics of Cu/Sn joints by preparing microtextures with different intervals on the Cu surface using femtosecond laser. The study found that the roughness of the substrate increased from 0.03 μm to 0.09 μm ~ 0.16 μm after laser treatment, and the corresponding wetting angle decreased from 49.75° to 30° ~ 40° due to the formation of periodic grooves after ultrafast laser processing, which created energy barriers that reduced the spreading ability and wettability of liquid metal. During the variation of surface roughness, the thickness of the intermetallic compounds (IMCs) is the greatest at the maximum roughness level both before and after thermal ageing. After 168 h of thermal treatment, a significant number of voids are generated at the joint with the maximum roughness. The joint with a roughness of 0.16 μm exhibits the fastest growth rate of IMCs thickness, while the joint with the maximum roughness shows the lowest shear strength. Additionally, as the roughness decreases, the fracture mode transitions from ductile fracture to brittle fracture. The reasons for the aforementioned phenomena are that the changes in roughness lead to variations in nucleation at the interface during the liquid-state process, which in turn affect the diffusion channels at the interface and ultimately cause unbalanced diffusion of copper and tin.