Abstract
Solder joint with Sn58Bi/Sn3.0Ag0.5Cu/Cu superposition structure was designed and fabricated by two-step soldering process. Sn, Ag, and Cu atoms diffused from the Sn3.0Ag0.5Cu bulk into the molten Sn58Bi solder paste during the second soldering process. The Sn3.0Ag0.5Cu bulk in the composite solder joint increased the concentration and grain size of β-Sn in the Sn58Bi bulk. Moreover, a large amount of tiny Bi-rich particles were found in the Sn58Bi solder bulk. The formation and growth of the β-Sn phases in the Sn58Bi solder bulk was affected by the microstructure of the Sn3.0Ag0.5Cu bulk. Additionally, the β-Sn dendritic in Sn58Bi bulk grew along the Sn3.0Ag0.5Cu bulk like the sunlight. The composite solder joint showed more ductile features than the traditional Sn58Bi eutectic solder joint due to its microstructural transformation occurred during the second soldering process. The Sn3.0Ag0.5Cu bulk with superposition structure works as a barrier to propagate cracks and effectively suppresses the brittle failure of the solder joint.
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