Massive Spalling of Intermetallics in Solder Joints: a General Phenomenon that Can Occur in Multiple Solder-Substrate Systems

The massive spalling phenomenon refers to the detachment of intermetallic compound from the substrate or the under bump metallurgy (UBM) surface in a very large scale during reflow. In addition to the apparent difference in the scale of the spalling, the massive spalling differs from the regular spalling in that regular spalling is usually due to the exhaustion of the thin wettable layer of the UBM or substrate metallurgy, but the massive spalling can still occur even when the wettable layer is intact. The massive spalling can occur in both lead-free and high-Pb solder joints, and has been reported in several solder/substrate systems, including SnAgCu soldered over Ni substrate, SnZn over Cu, high-Pb PbSn over Cu, and high-Pb PbSn over Ni. In this study, a unified thermodynamic argument is proposed to explain this interesting phenomenon. According to this argument, two necessary conditions must be met in order to have the massive spalling. The first is that at least one of the reactive constituents of the solder must be present in a limited amount, and the second is that the soldering reaction has to be very sensitive to the concentration of this constituent. With the growth of the intermetallic, more and more atoms of this constituent are extracted out of the solder and incorporated into the intermetallic. As the concentration of this constituent decreases, the original intermetallic at the interface becomes a non-equilibrium phase, and the spalling of the original intermetallic occurs.