Regulation of interfacial intermetallic compounds growth and void defect formation in Sn-9Zn-0.02Al/Cu solder joint by balancing interfacial elemental diffusion: An effect of Pt alloying

While micro-alloying with Al has resolved poor wettability of Sn-Zn (SZ) solder alloys, suppressing interfacial void defects in Sn-Zn-Al (SZA)/Cu joints during service remains a critical reliability challenge. Previous studies on Pt-alloying SZA alloys demonstrated enhanced mechanical reliability but lacked mechanistic insights into the role of Pt in regulating Cu diffusion (the key influence factor of void formation) and failed to establish dynamic correlations between intermetallic compound (IMC) growth and void formation. Building on this foundation, this study integrated aging experiments, atomic-scale microstructure observation, and density functional theory (DFT) calculations to reveal dual-path suppression mechanism of Pt. The addition of 0.25 wt% Pt increased the Cu₅Zn₈ layer thickness by 19 % (from 7.02 to 8.38 μm) and reduced void area by 76 % (from 25.8 to 6.1 μm²) after 240 h aging. A three-stage model elucidates the relationship between Cu₅Zn₈ growth and void formation: (I) Zn-sufficient rapid growth with balanced diffusion, (II) Zn-depletion transition initiating localized voids (<80 h), and (III) Cu-dominated void expansion via vacancy extensions (>80 h). Crucially, Pt suppresses Cu diffusion through grain boundary blocking and elevates intragranular diffusion barriers through lattice distortion and crystallographic orientation optimization. These atomic-scale mechanisms relieve the Cu/Zn interdiffusion imbalance, offering a transformative strategy for designing high-performance solder joints in high-density microelectronics.