| Due to increasing environment and health concerns over the use of lead, Pb-free electronic manufacture tends to be unreversed in future, and with the development of high density packaging and assembly technology, Sn-Pb solder alloys must be replayed by other alloys that with perfect comprehensive properties. Ag-Cu alloy system is recommended as the best candidate of Sn-Pb solders, but there are still some problems with it because of high melting point, low wet ability, unstable reliability of joints, etc. It is necessary to optimize Sn-Ag-Cu system by adding microelements either in respect to theory or experiments. The reliability of joints is greatly affected by the interfacial reaction; therefore it is very important to study on the interfacial behavior and microstructure of the joints both in the process of welding and process of aging. What's more, as one way of research, CALPHAD (calculation of phase diagram) technique plays a great part in developing new solder alloys as well as designing the ideal interface between solders and substrates.According to the need of electronic assembly industry, this paper aims to develop some certain Pb-free solder alloy with lower melting point, then to study and assess the new alloy by means of theoretical calculation and experimental work.A new solder alloy with the composition of Sn-3.0Ag-0.5Cu-4.5Bi was primarily developed by means of thermodynamic calculation method through Thermo-Calc software. Sn-Ag-Cu eutectic system was further optimized by adding the forth element Bi; the melting point was reduced by about 5 degree compared to the former system and its melting temperature range agreed with the practical restriction. |
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