Influence Of Argon Reflow On The Microstructure And Properties Of Lead-free Solder Joints

With human’s increasing attention to the environment protection, the transition of electronic assembly technology from tin-lead to lead-free is a matter of necessity. Sn-Ag-Cu solder alloy systems have been considered as the most potential substitute for traditional Sn-Pb solder alloys. It has been accepted that, in protective atmosphere, the peak reflow temperature can be reduced and the wetting performance of lead-free solder joints can be improved. Ar is the most plentiful rare inert gas. To date, however, few researches on using Ar as reflow atmosphere have been reported.This thesis investigates the influence of Ar reflow atmosphere on the microstructure and properties of lead-free solder alloys. During the lead-free reflow soldering, we investigate the influence of different reflow atmosphere, peak reflow temperature and solder pad surface finishes on the microstructures and properties of lead-free solder joints.Experimental results indicate that protective atmosphere of Ar played an important role in affecting solder joint quality. The solder joints fabricated in Ar atmosphere have a better appearance and a better wetting behavior than those of the solder joints fabricated in air. In Ar atmosphere, the solder joints fabricated at the peak reflow temperature of 235°C have the same or even better wetting performance in comparison with the solder joints fabricated at 245°C in air. Furthermore, less void was found in the solder joints fabricated in Ar atmosphere.Interfacial microstructures of the solder joints fabricated in Ar atmosphere also demonstrated noticeable advantages. It shows that Ar atmosphere could greatly improve the wetting performance and reliability of the solder joints in comparison with the solder joints fabricated in air atmosphere. After 1000 cycles of TCT(-55°C to 125°C), the quantity of cracks formed in the solder joints fabricated in Ar atmosphere was much less than that in air. After 2000 cycles of TCT, the cracks grew even larger to affect the mechanical performance and solder joint quality. Tin whisker growth was found after 2000 hours of TH storage at 85°C /85% RH. The density and length of tin whiskers of the samples formed in Ar atmosphere were much lower than those of the samples formed in air. Ar atmosphere could restrain the tin whisker growth.