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With miniaturization and heterogeneous integration in packaging, there has been a drive toward developing lower temperature solders. Sn-58Bi eutectic solder provides an attractive alternative with its meting point at 138°C. Due to the lower melting temperature of Sn-Bi solder, Bi coarsening may occur even at room temperature. In this paper, the microstructural evolution in Sn-58Bi joints was observed during room temperature storage. Room temperature aging induced the dissolution and coarsening of Bi phases in the solder matrix, especially in the primary Sn phases and Sn-Bi dendrites. The mechanical properties of individual Sn-rich and Bi-rich phases were measured by nanoindentation. The results showed that the Sn-rich phases had higher Young's modulus and hardness than Bi-rich phases in aged solder joints due to solution strengthening. Bi phases were more compliant and had lower hardness than Sn.

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