Microstructure and intermetallic compounds in Sn-3Ag-3Bi-3In solder joints on Cu matrix

The requirements for lead-free electronics lead to the development of new Pb-free solders. The Sn-3.0-Ag-0.5Cu (SAC305) is one of them and is widely used in the lead-free solder market. But the melting point of it, 217°C, exceeds that of traditional Sn-37Pb solder. The lower melting temperature of lead-free solder are needed to meet the requirements of the manufacturing process that depend on conventional Sn-Pb based reflow equipment. Adding Bi can reducing the melting point, but it will affect the mechanical properties. Adding In can also reduce the melting point without reducing the mechanical strength of the joints. But In is rare and expensive. Therefore, there are Sn-Ag-Bi-In solders, four alloy solder systems, by adding the two kinds of elements into Sn-Ag based solder. Sn-3Ag-3Bi-3In (SABI333) is an alternative four alloy Pb-lead solder, with a melting temperature, 198°C, close to that of eutectic Sn-Pb solder. This study elucidates the microstructure and intermetallic compounds (IMCs) formed at the interfaces and in the bulk solder in SABI333 solder joints on Cu matrix. In the reactions between Sn-Bi-In solders and Cu matrix, the compounds formed at the interfaces were Cu₆ (Sn, In)₅. Whereas, in SABI333 solder joint, the compounds formed at the interfaces were Cu₆Sn₅ and Cu₃Sn, without the substitution of Sn by In. The compound formed in the bulk solder was identified as Cu₆Sn₅, which is totally different from the SAC305 and Sn-Bi-In solders. The quantitative test of exact compound composition of IMC formed at the interface and in the bulk solder joints was determined by electron microprobe analysis (EPMA).