Effect of Sn grain orientation on microstructure and mechanical properties of pure Sn solder joints under thermoelectric coupling.

This paper investigates the microstructure and mechanical properties of Sn/Cu micro solder joints with varying β-Sn orientations under thermoelectric coupling conditions. The findings indicate that in single-crystal Sn/Cu solder joints, when the angle θ between the current direction and the c-axis of the β-Sn grain is ≤ 43.5°, significant dissolution occurs in the cathode Cu₆Sn₅ intermetallic compound (IMC) layer and the Cu substrate; large-sized Cu₆Sn₅ IMC forms at the anode. Conversely, for a solder joint with θ = 78.1°, only slight dissolution of the cathode Cu₆Sn₅ IMC occurs at the grain boundaries of Cu₆Sn₅, and small-sized Cu₆Sn₅ IMC forms at the anode. Thus, the smaller the θ angle, the more pronounced the asymmetric growth trend of IMC at the cathode and anode interfaces of the electromigration-affected solder joint. In contrast, no significant changes are observed in the growth of IMC at the interface of isothermal aging solder joints. Shear fractures in electromigration-affected solder joints consistently occur at the cathode interface. The dissolution of the cathode Cu₆Sn₅ IMC reduces the interfacial bonding strength, which is the primary cause of cathode fracture in the solder joint. A larger θ angle results in a smaller reduction in the shear strength of electromigration solder joints. The presence of large θ angle β-Sn grains suppresses the dissolution of the cathode interface Cu₆Sn₅ IMC, shifting the shear fracture location from the cathode Cu₆Sn₅/Cu interface to the brazing seam region. Consequently, the fracture mechanism transitions from brittle fracture, characterized by cleavage facets and tearing edges, to ductile fracture, dominated by parabolic dimples.