Electrochemical migration behaviour of Cu, Sn, Ag and Sn63/Pb37

The Electrochemical Migration (ECM) behaviour of leaded surface finishes was compared to other surface finishes that are applied in the electronics manufacturing. The studied surface finishes were as follows: bare copper (bCu), immersion tin (iSn), immersion silver (iAg) and Sn63Pb prepared by HASL (Hot Air Solder Leveling). The results were evaluated by Water drop test with the calculation of the Mean Time to Failure (MTTF) and by the investigation of the composition of the dendrites. The results have shown some contradictions relating bare copper and HASL compared to the ECM ranking published previously (Harsányi and Inzelt in Microelectron Reliab 41:229–237, 2001; Yu et al. in J Mater Sci: Mater Electron 17(3):229–241, 2006). The copper and the HASL can change their places in the ECM ranking depending on the technological circumstances of the investigations. This impact can be caused by the composition of lead alloys (eutectic or not), the solubility parameters of the metal hydroxides; the oxidation state of the copper surface, etc. Further and theoretical explanations and the necessary fine adjustment of the migration models are discussed in the paper.