Effects of Pd concentration on the interfacial reaction and mechanical reliability of the Sn–Pd/Ni system

The liquid–solid reaction between Sn–xPd alloy and Ni (x = 0.05–1 wt.%) and the resulting mechanical reliability of the system were examined in this study. The reactions strongly depended on the Pd concentration and the reaction time. When the Pd concentration was low (i.e., x = 0.05 wt.%), the reaction product was only Ni3Sn4. In contrast, when the Pd concentration was high (i.e., x ≥ 0.2 wt.%), the reaction product became a dual-layer structure of (Pd,Ni)Sn4–Ni3Sn4. Between 0.05 wt.% and 0.2 wt.% (e.g., x = 0.1 wt.%), discontinuous (Pd,Ni)Sn4 grains scattered over the Ni3Sn4 layer developed. Interestingly, the (Pd,Ni)Sn4 grains were gradually dispersed in the molten Sn–Pd alloy, leaving the Ni3Sn4 at the interface, as the reaction time increased. These Pd-dependent reactions were dictated by thermodynamics and can be rationalized using the Pd–Ni–Sn isotherm. Furthermore, the results of the high-speed-ball-shear (HSBS) test indicated that the mechanical strength of the Sn–Pd/Ni joints dramatically degraded by over one third due to the formation of (Pd,Ni)Sn4 at the interface. The implication is that the Pd concentration in Sn–Pd solder joints should be reduced to a level below 0.2 wt.% to prevent the creation of an undesired microstructure.

► Strong Pd concentration effects on the reaction and mechanical reliability of the Sn–Pd/Ni system. ► A slight increase in the Pd concentration (0.05 wt.% → 0.2 wt.%) translated the reaction product(s) from Ni3Sn4 to a dual-layer of (Pd,Ni)Sn4–Ni3Sn4. ► Formation of the (Pd,Ni)Sn4–Ni3Sn4 dual-layer structure significantly degraded the mechanical reliability.