Minor Ga addition to effectively inhibit PdSn4 growth between Sn solder and Pd substrate

• Liquid-state and solid-state Sn–Ga/Pd interfacial reactions were examined.
• PdSn4 growth is greatly suppressed with 0.5wt.%Ga addition in solder.
• Inhibition of PdSn4 growth in solid-state reaction is due to the PdGa formation.
• Growth kinetics of PdSn4 was investigated with various Ga additions.

PdSn4 is the major reaction phase in the Sn-based or Sn–Ag–Cu solder joints with Pd substrate and it exhibits an extremely high growth rate. Ga is considered as a candidate alloying element in the Sn–Ag–Cu solders. This paper investigates the effects of Ga addition on the interfacial reactions between Pd and Sn–Ga (0.1wt.%–1wt.%Ga) solders by solid-state aging at 160, 180, and 200 °C and liquid-state aging at 250 °C. The most important finding is that minor Ga addition can effectively inhibit the fast PdSn4 growth. In the solid-state reaction, with only 0.1wt.%Ga addition, the PdSn4 growth was suppressed by ∼50%, compared with the pure Sn/Pd reaction. When the Ga content increased to 0.5wt.%, the PdSn4 growth was further reduced by over 90%. In addition, a thin PdGa phase layer was formed at the interface between PdSn4 and Pd, which was the main cause for the inhibition of PdSn4 growth. The growth kinetics was systematically explored. The PdSn4 growth had a higher activation energy for the higher Ga addition (>0.5wt.%). Furthermore, a similar reduction in the PdSn4 growth was observed in the liquid-state reaction, but it was not as strong as in the solid-state reactions. The PdGa phase was not formed in the liquid-state reaction and the growth inhibition could be attributed to the Ga doping in the PdSn4 lattice to retard the Sn diffusion.