Optimization of a reflow soldering process based on the heating factor

Purpose - This paper aims to establish a method to optimize reflow profiles and achieve high reliability of solder joints on the basis of the heating factor, Q(eta), a measure of the reflow profile related to reliability of reflow processed products. Design/methodology/approach - The focus of the paper is on how to realize the optimal range of Q,7, since there is no need to pay particular attention to the shape of a reflow profile when performing a heating factor-based optimization. The coldest point on the printed circuit board assembly (PCA), which experiences the minimum heating factor (Q(eta)(min)) during the reflow process, was used to set the lower limit of the optimal range (Q(eta)(L)). If Q(eta)(min) approaches Q(eta)(L) and the temperature differences across the PCA is minimized, then the solder joints on the PCA will all experience heating factors within the optimal range, ensuring high quality reflow soldering. Establishing an initial reflow profile may be performed using profiling software. The resultant oven recipe may then be used as the reference recipe by which to apply the heating factor-based optimization. A combinatorial parameter, H-t, is defined to represent the temperature settings of all the top heating zones within the heating section of the reflow oven. The relative difference between Ht and each top heating zone temperature setting is derived from the reference recipe, and H-t is then adjusted to achieve Q(eta)(L) for Q(eta)(min). This is achieved by using a least squares estimation method to build a regression model for Q(eta)(min) versus H-t. Findings - Experiments and regression analysis have demonstrated that Q(eta)(min) varied linearly with H-t, with alpha denoting its slope. With a measured Q(eta)(min) in response to the reference setup after the first run of a PCA, H-t should be increased by ((Q(eta)(L)-Q(eta)(min))/alpha) to attain Q(eta)(L) in the second run. Thereby, a suitable reflow process recipe can be obtained with only two reflow runs where Q(eta)(min) is close to Q(eta)(L). Research limitations/implications - The optimal range of heating factor for lead-free solder pastes is currently unknown, and the method to establish the required oven recipe for achieving a required reflow profile requires further exploration. Practical implications - Provides a methodology for reducing the risk of process-related reliability issues in lead-free soldering. Originality/value - Q(eta)(L) can be fairly quickly achieved for Q(eta)(min) with the approach established in this paper, facilitating the formation of solder joints with high reliability during the reflow soldering process.