Effective modeling of the reflow soldering process: use of a modeling tool for product and process design

The increasing component packing density and consequent reduction in feature size in printed circuit assemblies (PCA's) continues to place manufacturers under extreme pressures. The most significant demand is for improved first-off process yields because of the high cost and technical difficulty of rework processes and concerns over the reliability of reworked products. The dominant process for the production of PCA's is reflow soldering of a stencil/screen printed solder paste to form the interconnection between the component termination and the substrate. It is crucial for the manufacturer to ensure that each termination experiences a suitable thermal history throughout the reflow cycle. Despite the advances in processes to cope with complex product features, such as increasing the uniformity and amount of heat transfer in the process, ensuring right-the-first-time is still a problem leading to increased lead-times, reduced yields and the scrapping of assemblies used to establish the ideal process parameters for each particular product. This paper describes the utilization of a predictive model as a tool for the off-line determination of the most appropriate process and its specific set-up for a PCA. Results are also presented where PCA design is altered to improve thermal mass distribution.