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An in situ SEM/energy dispersive spectroscopy (EDS) analysis revealed that the dispersed Sn-rich phase of the 95Pb-5Sn solder in a 95Pb-5Sn/63Sn-37Pb composite solder bump dissolved under current stressing. The dissolution was not prominent until the current density reached a threshold value of between 3.3 × 104 and 4.2 × 104 A/cm2. Supersaturation over thermal solubility of Sn-in-Pb was detected with current stressing. A polarity dissolution model was proposed for the dissolution behavior of the Sn-rich phase under current stressing. The dissolution mechanism under current stressing was discussed in relation to electromigration and thermomigration behavior of SnPb solders.

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