Method of extracting high-resolution digital Moiré fringe in warpage measurement

Abstract

Thermally induced stresses play a very important role in controlling the structural reliability of microchip packages. To address this issue, the shadow moiré interferometry is developed, which has been widely used in the field of observation for its real-time and high resolution. But how to extract the moiré fringes by using digital image processing method from a original Moiré fringe pattern is the key to accurate measurement of the warpage. The paper mainly concentrates on the method of effectively extracting the shadow moiré fringes from interference pattern produced by warpage of the PWB board and BGA package. Firstly, the low passing filtering and the dynamic threshold binarization proposed in this paper are applied to preprocess the moiré pattern. Then, the medial axis transformation and pruning algorithm applied to extract skeleton of moiré fringe have been put forward. Because the forficate fringes and noise fringes are main factors to affect precision of measurement, a effective main fringe extracting algorithm is proposed, which not only identifies and removes the forficate fringes and noise fringes from the main fringes, but also can connect the crack fringes which are hardly avoided after thinning operation. Finally, the information of warpage can be obtained from the location and orientation of moiré fringes. The three-dimensional delineation of detected object is resolved according to the moiré fringes extracted from shadow moiré pattern.