Spalling of Cu3Sn intermetallics in high-lead 95Pb5Sn solder bumps on Cu under bump metallization during solid-state annealing

We report the spalling of Cu3Sn intermetallics in high-lead 95Pb5Sn solder bumps on Cu under bump metallization (UBM) during solid state annealing. Upon reflow, the Cu3Sn intermetallics formed on Cu UBM. However, after solid state annealing at 170 °C, the Cu3Sn intermetallics spalled off from Cu UBM and the Pb phase filled the gap between the Cu3Sn intermetallics and Cu UBM. This is primarily explained by the loss of chemical adhesion between the Cu3Sn intermetallics and Cu UBM due to no additional chemical reaction. Thermodynamic principles are used to interpret the spalling phenomenon and the analysis showed that the interfacial free energy without spalling is greater than that with spalling after solid-state annealing. Spalling of the Cu3Sn intermetallics initiated at an open interface such as the edge of Cu UBM and finally extended to the flat interface at a slower rate.