Estimation of Laser Blind via Hole Drilling Phenomena in Printed Wiring Board Using Two-Color Image Method and Its Analysis Based on Structural Equation Modeling

In this study, we aim to elucidate processing phenomena to establish a novel quality inspection method for laser blind via hole drilling of Printed Wiring Boards (PWBs), which are used in all kinds of electronic devices. First, we investigated how the copper foil, glass cloth, and epoxy resin that make up the PWB contribute to the light emitting phenomenon by creating and drilling samples with different configurations and observing the light emitting phenomenon with a hig h-speed camera and image two-color method. As a result, the light emitting was observed in the form of granular scattered objects and over a wide area. It was found that the granular objects were scattered glass emitting thermal radiation at high temperature and their temperature was over 1100 °C. Next, we quantitatively evaluated the causal relationship between the glass cloth thickness, the surface absorption rate, the drilled hole shape, and the light emitting phenomenon using structural equation modeling. As a result, it was found that the influence of the surface absorption rate on the drilled hole shape is relatively small compared to the thickness of the glass cloth and is negatively correlated with the light emitting time of the copper foil. The thickness of the glass cloth was also found to influence the area where light emitting can be observed. However, it was found that the shorter the drilling time, the more complex the processing phenomena became.