Impact of electric field at rough copper lines on failure time due to electrochemical migration in PCBs

The impact of the electric field between rough copper lines on the failure time for electrochemical migration (ECM) in printed circuit boards is analyzed by means of modeling. The understanding and assessment of the failure time and thus the reliability becomes more and more important as the dimensions in the printed circuit boards decrease and the applied voltages increase. Once the epoxy/glass fiber interface is degraded in the printed circuit board, the additional time to form either Conducting Anodic Filaments (CAF) or dendrites depends also on the electric field. This electric field, as the main driving force, was computed on virtually constructed rough surfaces to take the real copper/prepreg interface into account before the onset of corrosion. It was shown, that roughness peaks increase the local maximum electric field, which follows a E=Ud0.63-relation that is underestimated in the state-of-the-art approach. Furthermore, it was found that the material dependent parameters can be related to the tortuosity of the possible short circuit path and the ion mobility in the material. The enhanced model for the determination of the failure time will facilitate the fitting and interpretation of future testing procedures.