Thermal and mechanical properties of micro Cu doped Sn58Bi solder paste for attaching LED lamps

The junction temperature is a key factor in determining the reliability of light emitting diode (LED) package, which can be lowered using high thermal conductive soldering material. The effects of Cu micro particles on the thermal and mechanical properties as well as microstructures of Sn58Bi solder paste were investigated in this paper. The results indicated that the thermal conductivity of Sn58Bi-5 wt% Cu solder layer is 26.60 W/(m·K), which is much higher than the Sn58Bi solder layer of 18.89 W/(m·K). According to the thermal simulation analysis, the temperature on chip is the highest area inside the whole LED package. The maximum junction temperature of soldered LED package decreases with 5 wt% Cu particles addition and then increases. The maximum junction temperature of LED package increases as the input power increases from 1 to 3 W. The lowest junction temperature is obtained in Sn58Bi-5 wt% Cu soldered package. There is no significant degradation in shear strength of 5 wt% Cu micro particles doped Sn58Bi solder joint when comparing with the pure Sn58Bi soldered ones. The shear strength of solder joint decreases approximately 50% when the content of added Cu micro particles increases from 5 to 15 wt%, and weak bonding presents in 20 wt% Cu doped solder joint. Comparing with the Sn58Bi solder, the solder layer with 5 wt% Cu micro particles exhibited more uniform microstructure and the number of coarse Bi-rich phases is reduced. The Cu micro particles facilitate the heat dissipation process of Sn58Bi solder layer by adding highly conductive cores in the layer. The 5 wt% Cu micro particles doped Sn58Bi solder exhibits remarkable thermal and mechanical performance in attaching LED packages.