Modeling the Thermal Behavior of Solder Paste Inside Reflow Ovens

Abstract

The development of a mathematical model of the soldering process of actual pastes as used in surface mount technology (SMT) lines is described in this paper. The coupled heat transfer processes between the solder paste and the flux including changes in solder paste properties are considered in the model. Specifically, the loss of solvents in the vehicle system, melting, solidification and further single phase cooling of the solder paste are contemplated in the model. Experiments were conducted with the objective of validating the predictions of the solder paste temperature profile and of the loss of weight due to flux extraction. Results are shown in this paper for typical eutectic paste 63%Sn-37%Pb and experimental data is in good agreement with the numerical predictions. Simulations using the lead-free solder paste systems 96.5%Sn-3.5%Ag and 42%Sn-58%Bi are also reported in this paper. The proposed model is suitable for incorporation into existing three dimensional heat transfer models of PCBs for simulations in ovens with similar characteristics as those used in actual manufacturing applications.