Interfacial Reaction and Shear Behavior of Sn-19Bi-19In Solder Joints on an ENEPIG Surface Finish During Isothermal Aging

The interfacial reaction and shear behavior of Sn-19Bi-19In (SBI) solder joints on an electroless nickel/electroless palladium/immersion gold (ENEPIG) surface finish were investigated during isothermal aging at 85 °C for durations of up to 500 h and compared with those of the SBI joints on electroless nickel/immersion gold (ENIG) reported previously. A thin densely packed needle-like intermetallic compound (IMC) layer formed at the interface and grew with aging time. According to scanning electron microscopy (SEM) and electron probe microanalysis (EPMA), the IMC layer is suggested to be (Ni,Pd)(Sn,In)2, and Pd was concentrated at the tips of the needle-like IMCs after aging. Since the composition of the IMC layer is similar to that of the loosely stacked cuboid Ni(In0.3Sn0.7)2 IMC on ENIG, the difference in morphology is likely due to the participation of Pd in the formation of the (Ni,Pd)(Sn,In)2 IMC. Transmission electron microscopy (TEM) analysis of the interface at the initial stage of the reaction suggests that Au(In,Sn)2 and (Pd,Ni)(Sn,In)4 IMCs form prior to the (Ni,Pd)(Sn,In)2 IMC. In ball shear tests, the SBI/ENEPIG joints showed a gradual ductile-to-brittle transition with aging time, and the crack path tended to shift from the solder bulk toward the IMC/Ni3P interface. The maximum shear strength of the SBI/ENEPIG joints remained at 39.5–44.2 MPa on average, whereas that of the SBI/ENIG joints decreased with aging time. This difference in the maximum shear strength can be attributed to the difference in the IMC morphology between the two surface finishes.