Vapour phase reflow soldering with vertical assembly stacking to improve productivity

• Vertical stacking of assemblies were introduced in vapour phase soldering. • 20 % process time reduction and 360 % yield improvement was achieved. • A slight quality decrease was recorded (13–18 % shear strength reduction to ref.). • Vertical stacking improves solder wetting from the bottom towards the top. • Both transferred heat, time above liquidus, and heating intensity affects IMC growth. During vapour phase soldering, the lateral size of the specimen tray determines the number of assemblies that can be soldered at once. Our research aimed to find out how we can increase the productivity of an industrial-grade oven, improve the technology for a more energy-efficient production without significant retrofitting modifications, and validate the results by investigating the soldering quality on surface-mounted components. The productivity of the oven tripled by stacking the soldered assemblies. Furthermore, a tray with reduced thermal mass was applied, which shortened the cycle time by 20 %. Comparative tests were performed between several vertical configurations and the default setup of the oven. The resulting thermal profiles were analysed, and the quality of the solder joints was assessed by wetting and shear tests, as well as their microstructure on cross-sections. A new model of the intermetallic layer growth in the liquid phase was established, which showed that not only the transferred heat energy and the time above liquidus values, but also the heating rate affects the intermetallic layer growth in the liquid phase of the solder alloy during the vapour phase soldering process. The application of the stacking and the modified tray increased the productivity by 360 % with only a minor decrease in the shear strength of the solder joint (by 13–18 %), mainly due to increased void formation and reduced wetting; however, the overall product quality still fit the requirements of commercial electronics.