A simplified reflow soldering process model

Established models of temperature development during the reflow soldering process have used commercially available finite difference (FD) or computational fluid dynamics (CFD) modelling tools to create detailed representations of the product and reflow furnace. Such models have been shown to achieve a high degree of accuracy in predicting the temperatures a particular PCB design will achieve during reflow, but are complex to generate and analysis times are unacceptably long. With the move to adopt lead free soldering technology, and the consequently higher reflow process temperatures, optimisation of the reflow profile will gain a renewed emphasis, increasing the value of an effective process model. This paper will report on the development of a less complex approach to modelling of the process, using simplified representations of both the product and the process, in order to achieve an accuracy comparable with more detailed models. In this simplified model the product is represented using a two dimensional mesh of thermal conductances linking thermal masses whose values are calculated based on the averaged properties of the PCB material and attached components within the area of each of the elements. In order to establish an accurate representation of the specific reflow furnace being simulated, a reflow logger equipped with suitable sensors is used to make measurements of the temperature and level of thermal convection at each point along the length of the furnace for a small number of carefully chosen reflow profiles. The behaviour for any other reflow profile may then be predicted from these measurements, thereby avoiding the necessity of making detailed measurements of the furnace geometry and air flow rates.