Development of Warpage Measurement System to Simulate Convective Solder Reflow Process

In this paper, a warpage measurement system to simulate forced convective reflow is discussed. A warpage measurement system that can simulate convective reflow enables the real-time monitoring of printed wiring boards (PWBs), PWB assemblies (PWBAs), and chip package warpage during the reflow process. This paper will describe the two major parts of the warpage measurement system: the optical measurement part which utilizes the projection Moire method and advanced image processing, as well as the laboratory oven which is used to simulate forced convective reflow. This is the first system that allows PWB/PWBA/chip package warpage to be measured during a simulated convective reflow process. Also, this is the first system that employs automatic image segmentation to separately extract the warpage of the PWB and electronic components from the same measurement. The results will show that when compared to infrared heating which was previously used in this research area, convective heating minimizes thermal gradients on the PWB/PWBA sample. Thermal gradients on the PWB/PWBA sample have the inadvertent effect of inducing warpage into the sample and will interfere with the warpage measurement result. In the first design iteration presented in this paper, the system can simulate low ramp rate industrial convective reflow profiles and simultaneously measure the warpage of PWBAs. A computational fluid dynamics (CFD) model of the system was developed to determine how to increase the system's heating rate. The CFD model was used to perform a design of simulations (DOS) and regression analysis. The validated regression results will be used to predict oven design parameters to enable the next iteration of the convective system to simulate high ramp rate convective reflow profiles. This paper will show that the presented system is a powerful tool for measuring the warpage of PWBs, PWBAs, and chip packages.