The wetting ability of low-melting-point lead-free Sn-based solder alloys on copper substrates is a critical factor influencing the reliability and performance of electronic assemblies. This review examines the fundamental principles of wetting, the characteristics of Sn-based solder alloys, particularly those with low melting points, and their interactions with copper substrates. The surface energy of both the solder and the intermetallic compounds plays an important role in the wetting characteristics. The effects of solder composition, substrate surface preparation and environmental conditions on wetting behaviour are discussed, along with recent advancements in solder technology aimed at enhancing wetting performance.