Abstract: The schematic diagram of PCBs (printed circuit boards) is analyzed based on fault diagnosis, and the diagnostic information flow model (DIFM) of PCBs is built. The observing relations between tests and faults are emphasized and described clearly with DIFM. Based on DIFM, fault diagnosis of PCBs is studied, and efficiency of the model is verified. Moreover, DIFM also can be used for handling with some multiple faults.