Study on the optimization and analysis of the Mixed Reflow soldering with lead and lead-free solder in the high density assembly

With the continuous improvement of the packing density, and the continuous developments of the assembly material and lead-free craft, Electronic industry switches to a transitional phase of the lead to lead-free-mixed lead and lead-free solder joints. Due to that the melting point of lead-free solder is 217°C, which is nearly 34°C higher than lead solder 183°C. If the solder curve is used to implement welding, the lead-free pins are not molten while the component is being sent into cooling area, which must lead to delamination between solders and solder pastes. And if the solder-free curve is used, crystal grains of the solder pasts will be thick due to the high temperature. That will make the welding poor. In order to avoid the defects above in the mixed assembly, a set of process parameters are needed to determine a correct reflow curve. To direct at a high density mixed assembly, a component equivalent model is established using Ansys software. The reflow process is simulated under ERSA reflow-oven with 12 zones. L9 (34) orthogonal tests are designed to analysis the four main process parameters, which include Conveyor belt speed A, Preheating temperature B, Heat preservation temperature C, Reflow temperature D, on the influence of the five key indexes including heating rate V_up, cooling rate V_down, temperature difference ΔT, peak temperature T_peak, time t₁ above 217°C. According to the temperature distribution, process parameters are adjusted. Finally, the optimal process parameters are determined. The results show that when the conveyor belt speed is set to 690mm/min, preheating temperature 160°C, and heat preservation temperature 170°C, reflow temperature 245°C, the reflow curve can meet the five key indexes, which can ensure the welding quality of the mixed assembly.