Formation of AuSn x intermetallic compounds (IMCs) in laser reflowed solder joints was investigated. The results showed that few IMCs formed at the solder/0.1um Au interface. Needlelike AuSn 4 IMCs were observed at the solder/0.5 um Au interface. In Sn-2.0Ag-0.75Cu-3.0Bi and Sn-3.5Ag-0.75Cu solder joints, when the laser input energy was increased, AuSn 4 IMCs changed from a layer to needlelike or dendritic distribution at the solder/0.9 um Au interface. As for the solder joints with 4.0 um thickness of Au surface finish on pads, AuSn 4, AuSn 2, AuSn IMCs, and Au 2Sn phases formed at the interface. Moreover, the content of AuSn x IMCs, such as, AuSn 4 and AuSn 2, which contained high Sn concentration, would become larger as the laser input energy increased. In the Sn-37Pb solder joints with 0.9 um or 4.0 um thickness of the Au surface finish on pads, AuSn 4 IMCs were in netlike distribution. The interspaces between them were filled with Pb-rich phases.
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