The Pt–Al Cooperative Effect Improves the Wettability of Sn–Zn Solder via Oxide Film Structure Optimization and Solid–Liquid Interfacial Tension Reduction

The development of Sn–Zn lead-free solders is hindered by poor wettability caused by porous ZnO oxide films and high solid–liquid interfacial tension. This study introduces a Pt–Al coalloying strategy to achieve concurrent oxide film structure optimization and solid–liquid interfacial tension reduction. The results demonstrate that Al-alloying prompts chemical potential gradient-driven Al surface segregation, forming a dense amorphous Al-rich oxide layer at ZnO/matrix interfaces that inhibits oxygen permeation and reduces the oxide film thickness. Furthermore, Pt–Al coalloying enhances chemical potential gradients of Al between the bulk and surface (by generating preoxidized Al atoms with enhanced electropositivity while strengthening Al 3+ –O 2– bonding in the oxide film), which accelerates Al surface segregation and promotes oxide layer densification, ultimately promoting oxide film thickness reduction. Meanwhile, under conditions of Pt–Al coalloying, element Pt segregation at solid–liquid interfaces enhances the ionic characteristic of interfacial bonding, reducing solid–liquid interfacial tension through strengthened atomic interactions. As a result, the Pt–Al cooperative effect markedly improves Sn–Zn solder wettability, reducing the equilibrium contact angle on Cu substrates from 38.2° for the Sn–9Zn solder to an optimal 25.1° for the Sn–9Zn-0.02Al-0.1Pt solder. However, Pt–Al coalloying also compromises Sn/Zn interfacial stability, enabling oxygen ingress along grain boundaries and promoting ZnO block nucleation. Beyond critical Pt concentrations (0.25 wt %), Al segregation at ZnO/matrix interfaces becomes insufficient to inhibit ZnO block growth, resulting in the degraded wettability. Consequently, Pt–Al coalloying requires precise optimization to balance oxide film structure optimization and interfacial stability.