Effects of surface finishes on the intermetallic growth and micro-structure evolution of the Sn3.5Ag0.7Cu lead-free solder joints

The growth rate and kinetics of the intermetallic formation, intermetallic grain morphology, and micro-structural evolution for the solder with I-Ag, OSP and ENIG finish were investigated. Comparing the intermetallic thickness growth rate in Sn3.5Ag0.7Cu solder joint on PCB Cu metallizations with I-Ag, OSP and ENIG finish, the OSP had the highest intermetallic growth rate followed by I-Ag and ENIG. The growth rate for I-Ag was a little bit lower than OSP finish. ENIG served as a good barrier to block the IMC formation. These results suggested that the PCB finishes had a big impact on the intermetallic growth rate. The activation energy was determined to be 30.83 kJ/mol and 32.67 kJ/mol for I-Ag and OSP finishes respectively. The growth mechanism of the intermetallic layer was compared in terms of IMC thickness and grain size evolution during isothermal ageing. Results reveal that the selection of solder alloys and PCB finishes played an important role in the morphology, microstructure evolution, growth of the intermetallics formed in the solder joint interface.