Properties of Sn-Ag-Bi-In Solder Joints

The lead-free soldering technology has been developed all over the world while IEEE RoHS prohibits the use of lead contained solder in 2006. Sn-Ag-Cu solder of which melting point is 219°C has the highest solder joints reliability of leadfree solder materials. This melting point is much higher than that of the conventional solder of 183°C. So reflow process for low heat-resistant components on Print Circuit Board needs lower melting point solder. Sn-Zn-Bi solder with low melting point of 197°C has a high barrier to apply to electric products due to lower reliability at high temperature, in high humidity and after reheating a joint comparing to conventional solder. Adding both bismuth and indium into Sn-Ag solder alloy is effective especially for decrease of melting point of Sn-Ag solder and Sn-Ag-Bi-In solder which had a melting point of 206°C was developed. In this paper, we mentioned design of the solder alloy and soldering properties of Sn-Ag-Bi-In to the point of appearance and microstructure of solder joints concerning about the influences of temperature, humidity and heat story on joints reliability. As the results a corrugated pattern appeared on the joint surface after 1000 cycles at -40°C/125°C and after 1000-hour at 85°C/85%RH. But the solder jont strength of Sn-Ag-Bi-In is comparable to that of Sn-Pb eutectic solder in each test. And no significant deterioration of Sn-Ag-Bi-In solder joint with heat story was also observed. Therefore we clarified the Sn-Ag-Bi-In solder had the same reliability as conventional solder and could be useful to expand the practical use of lead-free solder for a lot kinds of products.