Simultaneous measurement of PWB and chip package warpage using the projection Moire technique and automatic image segmentation

The shadow moire technique is a widely used method of measuring printed wiring board (PWB) warpage. It has a high resolution, high accuracy and is suitable for use in an online environment. The shortcoming of the shadow moire technique is that it cannot be used to measure PWBs populated with chip packages. In this paper, a warpage measurement system based on the projection moire technique is presented. The system can be used to measure bare PWBs as well as PWBs populated with chip packages. In order to use the projection moire system to accurately and separately determine the warpage of a PWB and chip packages in a PWB assembly, an automated image segmentation algorithm is developed and will be presented in this paper. The automated algorithm is based on active contour models (snakes) and can be used to detect chip package locations on a displacement image of a PWB assembly (PWBA). This paper will discuss the characteristics of the projection moire system and the characteristics of the automated algorithm. Warpage case studies of PWBs populated with plastic ball grid array (PBGA) chip packages will also be presented.