The effect of thermal conductivity of board to laser condition in laser soldering the case of paste solder

Recently, a high accuracy soldering method is required. Then, laser soldering has attracted attention. In laser soldering, it is difficult to detect an optimal laser condition depending on production conditions, because the amount of heat transfer from solder through copper wiring is different in circuit boards and soldering positions. Laser condition should be adjusted according to the situation. Therefore, the method to detect the optimal laser condition depending on positions of the circuit board is required. Then, the objective of this study is to investigate the appropriate laser conditions depending on the soldering position on the circuit board. In this paper, we discuss the effect of the thermal conductivity of the board on the appropriate laser condition for soldering experimentally and analytically. In the experiment, we compare the required minimum output laser power to melt the solder by employing boards of three kinds of thermal conductivities, and discuss the relationship between the required minimum output laser power to melt the solder and the irradiation time. Moreover, to investigate the temperature distribution of the board, we performed transient CFD (Computational Fluid Dynamics) analyses. From the experimental results, the effect of the thermal conductivity of the board is large, and more laser power is required to melt the solder when the thermal conductivity of the board is higher. Additionally, the required minimum total amount of heat to melt the solder becomes greater with increasing in the irradiation time. The analysis results show the temperature distributions of three types of boards are different. Therefore, the thermal conductivity of the board has large effect on the laser condition for appropriate soldering.