Electrochemical migration on lead-free soldering of PCBs

Abstract

It is well known that in printed circuits boards assembled by SMT technology may occur Electrochemical Migration (ECM). This phenomenon appears mainly because the new packaging has the terminals very close. Also the Electrochemical Migration may become a potential reliability problem in electronic soldering when lead free technology is used in soldering electronic devices. Electrochemical Migration is an electrochemical process where metal on an insulating material, in a humid environment and under an applied electric field, leaves its initial location in ionic form and redeposit. In a PCB two adjacent terminals may behave as electrodes so the dendrites grow from cathode to anode. It can show different morphologies with the different migration elements involved depending on the solder paste composition or PCB surface finishing. A structure with a comb shape printed on FR4 substrate was used in the experiments. The distance between the fingers in the structure was 102 or 254 micron, in order to simulate a real distance between dispositive terminals. The factors considered during the experiments were surface finishing (ENIG or HASL), solder paste composition, distance between terminals (102 or 254 micron) and applied voltage (2 or 3 V). Al the experiments were done two times. It was observed that the solder paste and the surface finishing don't influences the ECM process. Tin was the main metal that migrates. All the results obtained in these study agrees with the literature.