Are Intermetallics in Solder Joints Really Brittle?

In nearly all soldering processes, it is the intermetallic (IMC) layer that bonds the solder to the base metal. Thus, the IMC layer is necessary for any successful soldering operation used in the electronic industry. That is, a solder joint always has at least one IMC layer. While the IMC layer is needed, it is not static. It grows in subsequent reflows and during aging with time. Its growth without control could have adverse effect on the reliability. This is particularly true for flip-chip solder joints. The IMC, while necessary, seems to have also brought some problems. Thus, many packaging and solder experts believe that "the intermetallics are brittle and they can often embrittle the solder joints". On the other hand, not one really presents quantitative analysis to illustrate that the IMC is indeed brittle. No one really interprets what it means by "brittle." In this paper, we collect and review the physical properties of four commonly seen intermetallics: Cu₆Sn₅, Cu₃Sn, Ni₃Sn₄, and AuSn₄. These properties are compared with that of reference materials Cu and Sn3.5Ag solder. Based on the measured properties available, we analyze and evaluate whether these IMCs are indeed brittle. Based on the fracture of flip chip solder joints reported by others, we assess whether the fracture is caused by the "brittle" nature of the intermetallic as many believe or by something else.