Thermally induced stresses play a very important role in controlling the structural reliability of microchip packages. To address this issue, the shadow moire 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 moire fringes by using digital image processing method from a original Moire fringe pattern is the key to accurate measurement of the warpage. The paper mainly concentrates on the method of effectively extracting the shadow moire 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 moire pattern. Then, the medial axis transformation and pruning algorithm applied to extract skeleton of moire 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 moire fringes. The three-dimensional delineation of detected object is resolved according to the moire fringes extracted from shadow moire pattern.

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

An algorithm was proposed, which could detect the interference-strias accurately and quickly. In this algorithm, the image was divided into some little areas at first. Then median filter and threshold method were carried in every little areas. At last, the position of interference-strias could be found by difference. The simulation demonstrates that the strategy is feasible.

Quick algorithm about how to detect strias in image

Warpage measurement of PWB board and BGA package is considered as the most popular but difficult issue in microelectronic fabrication. Ordinary measuring method cannot afford the requirement of direct observation, efficiency, immediacy, high-precision as well as low cost. Moire fringe is a kind of interference pattern by which we are able to measure the flatness of surface of an object. This paper mainly concentrates on how to extract the information of effective shadow moire fringes by using digital image processing technique. It presents some algorithms applied to moire fringe image processing, such as preliminary extraction algorithm of moire fringes, intensity enhancing algorithm of fringe, fringe thinning algorithm, removal of the forficate fringe etc. In the end, the three-dimensional image from data of the two-dimensional fringe matrix has been drawn. Experiment result shows the effect of methods for extracting effective moire fringes.

Ping Zhong

Papers

Research on moire fringe information extraction based on warpage measurement