Performance of lead-free solder joints under dynamic mechanical loading

The performance of Sn/Ag/Cu solder with different component lead coating materials (Ni/Pd, 85Sn/15Pb and 98Sn/2Bi) under dynamic mechanical loading was studied in this work using a free fall drop test. PCB substrates with organic solderability preservative (OSP) and immersion gold over electroless nickel (Ni/Au) surface finishes were included in the investigation. Tests were carried out before and after thermal cycling between 0°C and 100°C for 3040 cycles. For comparison, lead pull tests were also carried out. It is found from this study that in the as-assembled state, the 98Sn/2Bi coating had the best performance in the drop and pull tests, followed by the 85Sn/15Pb coating. After thermal cycling, a decrease of ∼40-50% in the drop test results were seen when compared with the as-assembled state. Failure modes from the drop tests were investigated and the intermetallic compounds (IMC) layer was examined using SEM/EDS to study the impact of IMC on drop test failures.