Microstructure and shear strength of Sn37Pb/Cu solder joints subjected to isothermal aging

• The formation of Cu–Sn IMC is mainly controlled by the diffusion mechanism.
• The reflowed solder joint had better shear strength than the aged solder joints.
• The shear strength of all aged solder joints decreased with increasing aging time.

The effects of isothermal aging on the microstructure and shear strength of Sn37Pb/Cu solder joints were investigated. Single-lap shear solder joints of eutectic Sn37Pb solder were aged for 1–10 days at 120 °C and 170 °C, respectively, and then loaded to failure in shear with a constant loading speed of 5 × 10−3 mm/s. The growth of the interfacial Cu–Sn intermetallic compounds (IMC) layer (Cu6Sn5 + Cu3Sn) of Sn37Pb/Cu solder joints subjected to isothermal aging exhibited a linear function of the square root of aging time, indicating that the formation of Cu–Sn IMC was mainly controlled by the diffusion mechanism. And the diffusion coefficient (D) values of IMC layer were 1.07 × 10−17 and 3.72 × 10−17 m2/s for aged solder joints at 120 °C and 170 °C, respectively. Shear tests results revealed that as-reflowed solder joint had better shear strength than the aged solder joints and the shear strength of all aged solder joints decreased with increasing aging time. The presence of elongated dimple-like structures on the fracture surfaces of these as-reflowed or aged for short time solder joints were indicative of a ductile failure mode. As aging time further increased, the solder joints fractured in the mixed solder/IMC mode at the solder/IMC interface.