4. Electrochemical Behaviour of Solder Alloys

For a powered circuit, a metal corrosion–induced failure mechanism of particular concern is that of the formation of metal dendrites in the presence of contamination and moisture residues, which can lead to catastrophic failure. The propensities of two lead-free solders, SnAgCu (SAC) and Sn, to form dendrites have been assessed using electrochemical impedance (EI) technique and surface Insulation resistance (SIR) measurement, and benchmarked against the performance of conventional SnPb alloy. Two PCB finish materials (Cu and ENIG) have been also investigated for comparison. The dendrites formed from different metals were analysed using SEM-EDX equipment. The work has shown that dendrite formation occurs more readily with the SnPb solder than with the SAC and Sn solders due to the high corrosion rate of Pb in the Pb in SbPb alloy, and/or high solubility of Pb(OH)_2. In contrast, the low propensity to form dendrite for Cu finish board compared with ENIG finish suggests its low corrosion rate and low solubility of Cu(OH)₂. The good correlation of ionic resistance measured in the EI results and the propensities of metal to form dendrite indicates that metal corrosion rate and the solubility of metal play a major role in dendrite formation.