Effect of intermetallic stabilization on the impact resistance of joints to BGA packages

To the extent that it provides a potential low energy route for crack propagation the intermetallic compound that forms at the interface between a tin-based solder and the substrate can play a critical role in determining the likelihood of joint failure in impact loading such as can occur when portable devices are accidentally dropped. While it has been clearly documented in the standard equilibrium phase diagrams the implication of the allotropic change in the Cu₆Sn₅ intermetallic from a close-packed hexagonal to monoclinic crystal structure at 186°C with an accompanying 2.15% increase in volume has been found to have implications for the mechanical integrity of the interfacial intermetallic that were not previously recognized. In this paper the authors report the effect of a trace addition of Ni in permanently stabilizing the high-temperature form of the Cu₆Sn₅ to room temperature on the response to high speed shear of Sn-0.7Cu spheres reflowed to a copper substrate. By reducing the incidence of incipient cracking in the intermetallic layer the stress required to initiate cracking and the energy required for crack propagation are significantly increased with consequent benefits for the impact resistance of the solder joint.