Development of a robust Sn-Cu based lead-free solder paste

During services and/or storage, solder joint is frequently exposed to operational conditions such as temperature, mechanical load, and electrical current. These conditions increase the demand for solder material which has high performance value in physical, mechanical, electrical and thermal stability. Thus, this research was motivated to develop Sn-Cu solder paste for robust solder joint through micro-alloying, composite and transient liquid phase soldering (TLPS) approaches. The aims of this study are to investigate the thermal properties, the phases that exist, the solderability, the microstructure evolution and also the shear strength of the new robust Sn-Cu based solder paste. The thermal stability of each solder paste has been investigated by determining the intermetallic compound (IMC) growth kinetic during isothermal aging. Isothermal aging was conducted for 24, 240 and 480 hours at a temperature of 75, 125, and 150 °C respectively. The robust solder paste was synthesized by using Sn-0.7Cu (SC), Sn-0.7Cu-0.05Ni (SCN), Sn-0.7Cu-0.05Ni-1TiO₂ (SCNT) and Sn-10Cu (SC10) TLPS bonding system. Findings reveal the reduction of about 1.5°C - 33.5°C in undercooling of solder paste with third element addition; Ni, TiO₂, and 10Cu. Result also reveals the improvement in contact angle for about 1.3°, 14.9°, and 9.5° for SCN, SCNT and SC10 solder respectively. The addition of Ni and TiO₂helps in refining the microstructure which had improved the mechanical properties. Furthermore, the IMC formation for TiO2 reinforced solder paste has been suppressed for 13.9%. The lowest growth rate was presented by SCNT (0.280, 1.390 and 2.800 0 ms-1 at aging temperature 75, 125 and 150°C respectively) and the SCNT solder also displays the highest activation energy which was 37.35 kJmol-1 compared to the other solders. The thickening of interfacial IMCs thickness occurred with prolong aging time has caused a decreasing in shear strength for all solder joints. Nevertheless, the highest strength was observed in SC10 TLPS. Overall, owing to excellent in solderability, thin IMC thickness, finer microstructure, and high shear strength have proven the SCNT solder paste composite a potential solder interconnect to be applied in typical electronic assemblies. In addition, SC10 TLPS could be established as a promising candidate for TLPS solder alloy for high power electronic assemblies.