Comprehensive Report on Low Temperature Solder Alloys for Portable Electronics

We present here the findings of Alpha’s Alloy Development Program on the next generation of low temperature alloys that can be used in reflow soldering temperatures from 170 to 200C. Approaches in alloy development, test methodologies and results are discussed. The alloy properties targeted for improvements include: Alloy strength, alloy ductility, microstructure stability, improvements in thermal cycling, high temperature creep and drop shock. We show how the use of micro-additives in eutectic Sn-Bi alloys improves these properties. Further, comprehensive reliability studies were undertaken for new low temperature alloys. Thermal Cycling was undertaken from -40°C to +125°C with a 10 minute dwell time. Compared to standard Sn-Bi systems, improvements in thermal fatigue resistance are discussed. The basic properties of the new alloys shown here were fully characterized and their use in SMT applications evaluated, especially in drop shock and temperature cycling tests. In summary, we present here a new generation of low temperature Pb-free alloys, which are capable of delivering high reliability performance at low soldering temperatures. Various alloy compositions were evaluated and alloy B showed superior performance versus the benchmark and other low temperature alloys. Overall, the new low temperature alloys show significant improvements in metallurgical properties, soldering properties for SMT assembly, and thermal and mechanical reliability.