Influence Of Protective Atmosphere On The Microstructure And Properties Of Lead-free Solder Joints

Lead-based interconnects are banned across the world from many engineering applications due to their harmfulness to environments and health. SAC solder alloy systems have been considered as the most acceptable lead-free solder for microelectronic packaging applications. It has been reported in recent years that nitrogen inerting gas soldering can widen the process window of reflow and improve the reliability of solder joints. However, there is no clear conclusion about the concentration of oxygen and the results always varied under different conditions. In this thesis, SAC305 and SAC0307 solder alloys were used to instigate the effect of reflow process parameters (atmosphere concentration, peak reflow temperature, reflow cycles and solder pad finishes) on the microstructures and performance of solder joints.The experimental results indicate that reflow atmosphere, solder alloy and solder pad are there major parameters which affect the solder joint structure and performance. The influence of atmosphere on the solder joint of different solder alloys and different solder pad systems was different. 1000 ppm O2 in N2 was the most suitable situation for SAC305 solder joint fabricated on Sn-Pb pad while 200 ppm O2 in N2 was the best one for SAC0307 solder joint. There is less void and uneven IMC interface in the solder joints fabricated on Sn pad, which is better than that of the solder joints fabricated on Sn-Pb pad. Furthermore, reflow temperature and reflow cycles are another two factors affecting solder joint property. X-ray non-destructive testing results indicate that more voids could be found with increase of the oxygen concentration. Uneven layer of intermetallic compounds (Cu₆Sn₅) formed at the interface of the solder joints with some particle-like intermetallic compounds (Ag3Sn) inside the solder joint. We also find that Cu₃Sn layer could form between the Cu subatrate and Cu₆Sn₅ layer.