We propose a kinetic model to describe a pancake-type void propagation in flip chip solder joints due to current crowding in electromigration. The divergence of the vacancy fluxes at the interface between the solder and Cu6Sn5 leads to void formation and propagation along the interface between them. Based on the continuity condition, the void growth velocity is calculated. The theoretical calculations are in reasonable agreement with the experimental results.

The most frequent cause of failure for wireless, handheld, and portable consumer electronic products is an accidental drop to the ground. The impact may cause interfacial fracture of ball-grid-array solder joints. Existing metrology, such as ball shear and ball pull tests, cannot characterize the impact-induced high speed fracture failure. In this study, a mini-impact tester was utilized to measure the impact toughness and to characterize the impact reliability of both eutectic SnPb and SnAgCu solder joints. The annealing effect at 150°C on the impact toughness was investigated, and the fractured surfaces were examined. The impact toughness of SnAgCu solder joints with the plating of electroless Ni/immersion Au (ENIG) became worse after annealing, decreasing from 10 or 11 mJ to 7 mJ. On the other hand, an improvement of the impact toughness of eutectic SnPb solder joints with ENIG was recorded after annealing, increasing from 6 or 10 to 15 mJ. Annealing has softened the bulk SnPb solder so that more plastic deformation can occur to absorb the impact energy.

Due to a significant change in electromigration behavior of eutectic SnPb at different temperatures, it is critical to understand it at the device temperature, i.e., 100°C. In the present study, the electromigration behavior of e-SnPb is investigated at 100°C and at different current densities, varying within the range of 5×103–5×104A∕cm2. The test samples are prepared by reflowing solder into V grooves etched on (001) Si wafers. The electromigration is observed at the current densities higher than 1×104A∕cm2. The dominant diffusing species at 100°C is found to be Pb. The threshold current density for electromigration to occur in e-SnPb comes out to be 5×103A∕cm2. We also notice Pb rich phase coarsening, which becomes significant at high current densities. The results are compared to that in preannealed samples. The effects of the changes in microstructure on the electrical reliability are also discussed. The interface diffusion is dominant when the grain size is small. Subsequently, as the grain size increases, it is reduced to the order of the lattice diffusion, and therefore the electromigration is impeded. The critical product for e-SnPb solder is estimated to be 150 experimentally, and is also compared to the theoretically calculated value of 8. The reason for the discrepancy is discussed.

The most frequent failure of wireless, handheld, and movable consumer electronic products is an accidental drop to the ground. The impact may cause interfacial fracture of wire-bonds or solder joints between a Si chip and its packaging module. Existing metrologies, such as ball shear, and pull test cannot well represent the shock reliability of the package. In this study, a micro-impact machine is utilized to test the impact reliability of three kinds of lead-free solders: 99Sn1Ag, 98.5Sn1Ag0.5Cu and 97.5Sn1Ag0.5Cu1In (hereafter called Sn1Ag, Sn1Ag0.5Cu, and Sn1Ag0.5Cu1In). The effect of thermal aging on the impact toughness is also evaluated in this study. We find a ductile-to-brittle transition in SnAg (Cu) solder joints after thermal aging. The impact toughness is enhanced by the thermal aging. This is a combination effect of the growth of intermetallic compound (IMC) at the interface provided strong bonding, and the softening of the solder bulk during the thermal aging absorbed more energy during plastic deformation.

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