The leakage current phenomenon of a commercial ten-multilayer printed circuit board (PCB) occurred. Preliminary examination shows that there was a short circuit between the PIN_2 and POUT_4 networks of the PCB measured by a high-resistance meter, after a high-temperature aging treatment. In this case, microstructure and morphology of several layers for PCB sample were observed by optical microscopy, scanning electron microscopy and three-dimensional imaging x-ray microscopy. The chemical composition of the layer was determined by energy-dispersive spectroscopy. After observation and analysis, the results show that the leakage current failure cause of the PCB was that the metal powder impurities were found between the layers of PIN_2 and POUT_4 network, which caused the drop in the electric resistance between the two networks, resulting in the occurrence of the leakage current. The chemical composition of the impurity was only composed of copper. [ABSTRACT FROM AUTHOR]