Optimisation of Reflow Profile of Surface Mount Assembly Using Taguchi Design of Experiments

Abstract

A major source of concern in the electronic manufacturing industry is the reliability of solder joints produced by lead-free solders pastes. Research has shown that solder joint produced with a thin layer of intermetallic compounds (IMC) during the reflow process has a higher reliability. The focus of this paper is on the development of a deeper understanding of the lead-free reflow soldering process and to optimise the reflow profile using Taguchi Design of Experiments (DOE). The optimum parameters are those that yield lower IMC layers. The experiment was designed to consider the effect of the following reflow soldering parameters: soak temperature, time above liquidus, soak time and time to peak temperature. The eight (8) reflow profiles studied were derived from the Taguchi DOE which also included three factorial interactions. A stencil with three (3) holes of diameters 3.1 mm, 3.7 mm and 4.3 mm was used in the solder printing process, after which the samples were reflowed. The samples were then thermal cycled, cross-sectioned and finally the IMC image captured and analysed using an optical electron microscope. The analysis of variance (ANOVA) carried out on the results indicate the soak time to be the most significant factor in achieving a solder joint with a thin IMC layer. The next most significant factor was the time above liquidus, and the most significant interaction was found to be the interaction between the soak temperature and time above liquidus. The results indicate a soak time of 60 seconds and time above liquidus of 60 seconds is preferred to achieve a thin IMC layer. The recommended time to peak temperature is 240 seconds and the soak temperature is 150°C. The results can be very useful to researchers and scientists in the surface mount technology (SMT) field, providing an understanding of the effect of the four reflow parameters studied. In addition, the DOE methodology used in this research can be adopted by other process improvement engineers or scientists involved in the optimisation of a process or product.