A Novel Projection Moire´ System for Measuring PWBA Warpage During Simulated Optimized Convective Reflow Process

Abstract

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. A shortcoming of the shadow moire´ technique is that it cannot be used to measure PWBs populated with chip packages. In this paper, a novel 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 determine the warpage of PWBs and chip packages separately, an automated chip package detection algorithm based on active contours is utilized. Unlike the shadow moire´ technique which uses a glass grating, the projection moire´ technique uses a virtual grating. The virtual grating sizes can be adjusted, making it versatile for measuring various PWB and chip package sizes. Without the glass grating, which is a substantial heat inertia, the PWB/PWBA/chip package sample can be heated more evenly during the thermal process. The projection moire´ system described in this paper can also be used to measure PWB/PWBA/chip package warpage during convective reflow processes. In this paper, the characteristics of the projection moire´ warpage measurement system will be described. In addition, the system will be used to measure the warpage of a PWB and plastic ball grid array (PBGA) packages during a Lee optimized convective reflow process. It is concluded that this projection moire´ warpage measurement system is a powerful tool to study the warpage of populated PWBs during convective reflow processes.