The comparison of electrochemical migration mechanism between electroless silver plating and silver electroplating

Abstract

As the electronic devices tend to be miniature and compact, the pitches both between the adjacent leads on printed circuit board (PCB) and between connectors’ terminals mating with PCB reduces to the level of one tenth millimeter. With lead-free legislation impacting PCB manufacturing, immersion silver finish is adapted as one of the top finish of copper trace on PCB due to its many advantages. At the same time, driven by cost pressure, the silver electroplating is also considered to replace gold plating on electrical contacts of connectors. Therefore, there is a potential risk for silver finish to form electrochemical migration (ECM) under certain potential bias in high humid environment. The effects of the silver plating processes on PCB and connectors, and the thickness, porosity, defects of silver finish, as well as the underplating and substrate materials will increase the complexity of ECM. In this study, possible chemical reactions happened on the silver electrodes were analyzed theoretically based on the analysis of plating processes and detection of porosity and defects. The time to failure caused by ECM was quantitatively calculated by the change of surface insulation resistance between the electrodes with potential bias. The failure mechanisms of ECM both on electroless silver plated PCB and on silver electroplated conductors were compared by element compositions and growth stages of migration dendrites, as well as polarization performance of different material combinations. The influencing factors on ECM, such as the potential bias, the thickness of silver plating, underplating nickel, substrate copper, were also discussed.