Voltage injection and readout method for PCB (printed circuit board) testing

Abstract

The main objectives of PCB (Printed Circuit Board) testing are to find short and open circuits before attaching components to the PCB. An electrostatic imaging technique was described earlier by Zentai at al. in the SPIE-NDE 2003 conference and has been accepted as a US patent. The main goal of that application was to test the PCBs quickly and efficiently without attaching test probes to each of the traces. It was achieved with the proposed technique. However, we still had to use test probe matrix for generating test signals in the traces and the detected signal was greatly reduced in reference to the test signal because of capacitive sharing of charges caused by the insulator layer placed between the PCB and the TFT sensor array. A new idea has been developed for applying voltages to the traces with an addressing matrix, different from the pin addressing one. This matrix is similar to the readout matrix that the previous method referenced to, but instead of readout circuits, driver circuits are connected to the pixels to apply voltages to them. The printed circuit board is laid on top of the array, but rather than using an insulator foil, a directional conducting foam (rubber) layer can be applied between the excitation matrix (array) and the PCB. We get direct coupling of the matrix pixels to the PCB traces and no connection between neighboring pixels (traces) using the directional conductive layer, which conducts only in z direction and not in x (and y) direction. Therefore, by addressing each pixel separately, which is easy to do by software, we get an addressable voltage (pulse) injector matrix. The same directional conducting foam coupling can also be used for reading out the image of traces. Because the capacitive coupling is eliminated, the detected signal increases and so the sensitivity.