Components´ emissivity in reflow soldering process

Thermal processes encountered in surface mount assembly are very complex. Transferring heat between hot source (reflow oven) to assembly (printed circuit board, components and interconnecting alloy) is done through well known possibilities: convection, infrared radiation, conduction and phase changing. The convection and the IR radiation are the main ways to transfer heat from source to assembly. The conduction and the phase changing are secondary forms. They are very important to establish the uniform temperature distribution in an assembly. Modeling the four heat transfers in case of surface mount technology it is the necessarily to split the problem in two parts. First, the reflow oven must be modeled. Second, the assembly must be modeled. Since the model for the oven is known, the assembly could be different (in the same oven are passing through different equipped PCBs). The paper present some steps of assembly modeling in fact the capability of the assembly to take heat from the heat source(reflow oven) through IR radiation. For IR heat transfer the most important parameter is the emissivity of exposed surfaces. These surfaces are: heater sources´, the SMD components´, and the PCB´s solder mask. Supposing heater´s surface emissivity to be constant to know how the assembly will absorb the heat through IR, it´s necessary to know the emissivity of components´, surfaces (and the emissivity of exposed PCB). The components emissivity usual is established by measurements. The method, measurements (including the corrections) and conclusions are presented in the paper.