Effect of Hot Bar Shape on Temperature Consistency

Abstract

Pulse heated reflow soldering is widely used as junction methods of electronic devices such as ACF, HSC, FPC, PCB, TCP, and FFC. Hot bars are the devices used in pulsed heated reflow method to heat and melt the solder materials between devices and wire becoming a junction. Therefore the performance is determined by the welding properties. The welding time, temperature and induced pressure are important factors affecting welding property. And the temperature has the significant effect due to electricity and mechanical property of solder being determined by welding temperature. However, the consistency of temperature is hard to achieve due to complex coupling of driving current, materials property, heat generation, heat conduction and hot bar shape. In this work, different shapes of hot bat are designed to obtain the temperature consistency. Due to the welding requirement, the heating process is a two steps process. And the hot bar is heated by electric current from power supply with feedback control. For assembly purpose, the hot bar is designed as a rectangular plate with circular holes near the other side respect to the heating edge to fasten the hot bar on the welding machine. And the hot bar with polygon hole in the center part would induce the uniform temperature distribution. Four dimensions of the hot bar determine the shape and investigate the temperature consistency: polygon’s sizes and its positions. Some experiments are made to measure the temperature distribution. The results show the shape has significant effect on temperature consistency. Changing the polygon hole positions will alter the temperature distribution. The central part of heating edge will hotter than both ending part initially. And in some cases, during the heating process, the both ending part will have cooler, hotter, and cooler temperature that central part with respect to heating time. The polygon hole sizes have also significant effect on the temperature distribution. They have the similar temperature distribution phenomena as positions effect. Therefore, from the experiments and analysis, the hot bar could have more uniform temperature distribution by shape design. And it is also found the temperature distribution would induce thermal residual stress and deform the hot bar.