Failure analysis for micro-short circuit between two pins in printed circuit board assembly

Function failure was found successively in one type of smart remote controls when using a period of time. With preliminary investigation by electric circuit, low resistance was found between the neighboring pins on failure sample which connected with flexible printed board and rigid printed board, but the pins on good sample were insulated with the others. After peeling off the flexible printed board, black materials which looked like migration were found by stereomicroscope and metallography microscope. With the help of scanning electron microscope and energy dispersive spectroscope (SEM&EDS), it was detected that the materials mainly contained the elements of Cu (about 50 at.%) and S (about 30 at.%), and the two short pins were almost connected with the black materials. As a result, the black materials which connected to the two pins should be copper sulfide (CuS) and cuprous sulfide (Cu2S). The failure process was as the follows. Some active sulfide was on the surface of board, with the bias voltage and moisture, the copper on the pads of board was sulfurized and migrated with electrochemical reaction. The migration materials which connected to the neighboring pins cause micro-short circuit. However, there was no element of Cu on the clean solder mask, and the element of S could not come from the filling material barium sulfate (BaSO4) because barium sulfate were difficult to break down and the element of Ba had not been detected in the black materials. Finally, with further investigation for the manufacturing process of the assembly, the element of S was detected on copper foil after soldering with the solder paste which used in the failure sample, but the element of S could not be detected with the other type of solder paste. It could conclude that the active sulfide should come from some sulfur-containing materials in the flux of solder paste breaking down in soldering. When soldering, the solvent in the flux was difficult to volatilize due to the flexible printed circuit board (FPC) on the top. Some sulfur-containing materials in the solvent would decompose to active sulfide. So, in order to avoid the failure of micro-short, the solder paste without sulfur containing should be used in soldering.