Warpage measurements of printed circuit board during reflow process using strain gauges

The objective of this study is to propose an in-situ measurement of the warpage of the PCB during reflow process (or SMT process) using strain gauges. In the experiments, a full-field shadow moiré is used for measuring the out-of-plane deformations (or warpage) of a bi-material plate and the PCB with DIMM sockets during solder reflow heating. A finite element method (FEM) is used to analyze the thermally-induced deformation of both specimens in order to ensure the validity of the measurement. The conventional strain gauges are employed to measure the strains (even though they are in-plane strain data) in both specimens during the solder reflow process. The results indicate that the strain gauge measurement can be used to determine bending strains from the in-plane strain data on the top and bottom surfaces of the PCB occurring during the solder reflow. These bending strains can be converted into curvature data and global warpage which are in good consistency with the results from shadow moiré and FEM. Therefore, it was proved that the strain gauge measurement can provide a real-time and easy-to-use method for monitoring the PCB warpage with temperature variation during the reflow process.