Interfacial study of electroless Ni-Sn-P plating and Sn-3.5Ag solder after multiple reflows.
Date of Issue2009
School of Materials Science and Engineering
Electroless Ni-P plating has been a good candidate for Under Bump Metallization (UBM) in IC packaging due to its lower cost and slower chemical reaction with solder compared to Cu based UBM. However, during its reaction with Sn-based lead free solder, it faces the problem of rapid Ni out diffusion which can lead to the voids formation and brittle fracture. To enhance the property of electroless Ni-P alloy, alloyment of Sn into Ni-P to form Ni-Sn-P ternary alloy can be one of the options. Besides having good thermal stability and solderability, the presence of Sn in electroless Ni-Sn-P could influence the diffusion process during soldering reflow process. Electroless Ni-Sn-P film was prepared from alkaline citrate plating bath with hypophosphate ion as the reducing agent. Effect of sodium stannate as Sn source and plating time were investigated to maximize the quality of the plated surface. Multiple reflow soldering was performed to investigate the interfacial layer composition and growth under liquid state reaction. Tensile test was carried out to study the effect of the interfacial layer composition and growth on the mechanical property of the solder joint. Increasing stannate concentration and plating time resulted in smoother and more compact plated surface. There were two distinctive layers identified during multiple reflow of Ni-Sn-P/Sn-Ag solder joint. They were P-rich layer and Sn-rich layer which grew thicker as the reflow cycle increase. The presence of Sn in electroless nickel plating had affected the diffusion process in the solder joint. The changes could be observed by the presence of Sn in the P-rich layer and significant amount of P and Ni content in the Sn-rich layer during the reflow processes, as well as the voids location which was observed at different location from the voids in the Ni-P/Sn-Ag solder joint. In addition, Sn-rich layer was observed to have a slower growth rate compared to P-rich layer. This observation was on the opposite trend of the interfacial growth in electroless Ni-P/Sn-Ag solder joint. Slow growth of Sn-rich layer might affect the tensile strength of the solder joint, thus small drop in tensile strength was observed up to 40 cycles of reflow. As the stable mechanical property is required for solder joint in IC packaging, hence electroless Ni-Sn-P can be used as UBM in lead free solder joint application.
DRNTU::Engineering::Materials::Microelectronics and semiconductor materials::Thin films
Final Year Project (FYP)