Wavelet - Based Printed Circuit Board Inspection System

An automated visual printed circuit board (PCB) inspection system proposed in this paper is designed specifically to detect various types of defects occurred during circuit printing process in manufactory. Two-dimensional HAAR wavelet transform is incorporated in the PCB inspection algorithm of the inspection system. During the inspection, two type of images are required; reference and tested PCB images. The wavelet transform is applied to the reference and tested PCB images. According to previously proposed wavelet-based PCB inspection algorithm, a reference comparison between the reference and tested PCB images has been done in wavelet domain by employing image difference operation. However, in practice, this operation also bring along the unwanted noise due to misalignment and uneven binarization. Thus, in this paper, for the real-time implementation, the image difference operation between the reference and tested PCB images is replaced with image subtraction. The output of the image subtraction operation can be differentiated as positive, negative, and zero images. By doing so, a noise elimination procedure is applied to positive and negative images, resulting noise free positive and negative images. Lastly, the noise free images are subjected to XOR logic operation to generate the output of the inspection system. During the manufacturing process, since etching usually contributes most destructive defects found on PCBs, it is important to perform the proposed automated visual PCB inspection after the circuit printing and before the etching process. Keywords—Automated visual inspection system, HAAR wavelet, image difference, image subtraction, noise elimination, printed circuit board.