Selective Interfacial Reaction between Ni and Eutectic BiSn Lead-Free Solder

Abstract

The chemical compatibility of the Ni surface with the eutectic BiSn lead-free solder was studied. According to the literature, the compound NiBi 3 is both soft and brittle and has a very high growth rate when Ni is reacted with Bi. Therefore, it is highly undesirable for NiBi 3 to form in a solder joint. The objective of this article is to study the reactions between Ni and 58Bi−42Sn in detail and to clarify whether the formation of NiBi 3 will be an issue. In this study, Ni was reacted with the eutectic BiSn solder at 180, 240, 300, 360, and 420 °C for times ranging from 0.5 to 48 h. At all temperatures only Ni 3 Sn 4 was detected as the reaction product. None of the other Ni−Sn intermetallic compounds and none of the Ni−Bi intermetallic compounds were observed. The formation of only Ni 3 Sn 4 is considered the best possible outcome from the standpoint of electronic packaging applications. At lower temperatures (180, 240, and 300 °C), Ni 3 Sn 4 formed at the interface as a thin continuous layer, which was protective and resulted in the parabolic growth kinetics. However, reaction at a higher temperature (420 °C) produced at the interface a thick reaction zone, which was a two-phase mixture of Ni 3 Sn 4 + solder. This morphology, which was nonprotective, led to the linear growth kinetics. The growth rates at temperatures below 300 °C were quite slow and were considered beneficial for packaging applications. The Ni 3 Sn 4 formed near the interface had rounded edges. At locations away from the interface and inside the solder joint also existed a small amount of Ni 3 Sn 4, but this Ni 3 Sn 4 exhibited a faceted, needle-shaped morphology. It is believed that the Ni 3 Sn 4 with rounded edges formed during reaction, while the needle-shaped Ni 3 Sn 4 formed during the solidification of the solder.