Current status on optimizing solder wettability from substrate surface treatment, alloying solder, flux, and process environments

Abstract

With the rapid advancement of electronic packaging toward high integration and miniaturization, the wettability of Sn-based solders has become increasingly critical, as it directly governs the formation of reliable metallurgical bonds and the service performance of micro-joints. This review systematically summarizes the fundamental principles and evaluation methods of solder wettability, focusing on contact angle, the core indicator regulated by the interfacial tension equilibrium described by Young's equation, along with spreading area, wetting force, and wetting time (the latter quantified via the wetting balance method). Moreover, the optimization strategies for wettability are elaborated from multiple dimensions: substrate surface treatment, including roughness control, texture design via directional microstructures to guide anisotropic spreading, and coating technology to modify surface energy and inhibit oxidation; solder alloying, which involves incorporating trace elements such as rare earths, nanoparticles, or active metals to reduce surface tension, promote interfacial reactions, and regulate intermetallic compound formation; flux formulation, where inorganic and organic fluxes function in chemical oxide removal and interfacial energy reduction; and process environment control, including temperature regulation to lower viscosity, protective atmosphere to suppress oxidation, and ultrasound-assisted technology to mechanically disrupt oxide films via the cavitation effect. These strategies, integrating substrate, solder, and process factors, provide a comprehensive theoretical framework to address the wettability limitations of lead-free solders, offering crucial technical support for the development of high-performance electronic packaging.