Interfacial wetting enhancement driven by Cu₆Sn₅ morphology transformation in transient liquid phase sintering

Transient liquid phase sintering (TLPS) has attracted increasing attention for high-power electronic packaging due to its capability of achieving low-temperature interconnection and high-temperature service. However, its widespread application remains limited by the rapid Cu–Sn reaction during solder melting, which leads to premature formation of a rigid Cu₆Sn₅ skeleton, severely restricting solder wetting and necessitating high external pressure (>5 MPa) to achieve reliable bonding. Although previous studies have improved TLPS through particle design or pressure-assisted sintering, the kinetic competition between rapid intermetallic compound (IMC) formation and effective solder wetting remains unresolved. In this work, the morphology transformation of layer-type Cu₆Sn₅ to scallop-type during solder rewetting is revealed. This transformation creates a time window for active solder wetting before the rapid Cu–Sn reaction resume. Based on this mechanism, a novel Cu@Cu₆Sn₅-based preform is proposed to regulate solder wetting without relying on high external pressure. Compared to Cu-based solder joints, Cu@Cu₆Sn₅-based solder joints were successfully fabricated at an ultralow pressure (0.05 MPa). Cu@Cu₆Sn₅-based solder joints exhibit better wetting ability, the shear strength at room temperature and 300 °C Celsius is 54.5 MPa and 22.6 MPa, respectively. Moreover, the transformed Cu₆Sn₅ coating acts as a grain growth matrix, weakening Cu₆Sn₅ texture and enhancing joint mechanical reliability. These findings provide a new strategy for low-pressure TLPS and high-temperature interconnection applications.