Selective interfacial reaction between Ni and eutectic BiSn lead-free solder
Journal
Chemistry of Materials
Journal Volume
13
Journal Issue
3
Pages
1051-1056
Date Issued
2001
Author(s)
Abstract
The chemical compatibility of the Ni surface with the eutectic BiSn lead-free solder was studied. According to the literature, the compound NiBi3 is both soft and brittle and has a very high growth rate when Ni is reacted with Bi. Therefore, it is highly undesirable for NiBi3 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 NiBi3 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 Ni3Sn4 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 Ni3Sn4 is considered the best possible outcome from the standpoint of electronic packaging applications. At lower temperatures (180, 240, and 300 °C), Ni3Sn4 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 Ni3Sn4 + 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 Ni3Sn4 formed near the interface had rounded edges. At locations away from the interface and inside the solder joint also existed a small amount of Ni3Sn4, but this Ni3Sn4 exhibited a faceted, needle-shaped morphology. It is believed that the Ni3Sn4 with rounded edges formed during reaction, while the needle-shaped Ni3Sn4 formed during the solidification of the solder.
Type
journal article