The Pt-Al Cooperative Effect Improves the Wettability of Sn-Zn Solder via Oxide Film Structure Optimization and...

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 Al³⁺-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.