Soldering profile optimization for vapour phase reflow technology

Vapour Phase Soldering (VPS) is an alternative method for widespread reflow soldering technologies (e.g. infrared or convection type). VPS has emerged in the last few years after the introduction of Galden fluid. The vapour of Galden is used as the heat transfer medium. The main advantages of VPS are the lower peak temperature, nitrogen/oxygen-free inert atmosphere, the improvement in solder wetting and the reduction of the profiling time. One disadvantage of VPS is the non-controllable, linear soldering profile achieved by common VPS soldering method. With controlled dipping of solder goods into the vapour space, non-linear soldering profiles can be obtained. This method is called Soft-VPS (S-VPS) and was developed and patented by IBL. With this method, custom temperature gradients and profiles can be realized, reducing voids, tombstoning and avoiding high thermal stress on the boards. This paper presents thermal profiling of an experimental VPS station from the aspect of this method, with a successful optimization of the process. The optimized profiles for low melting point (138 °C) lead-free Sn-Bi solder paste were inspected. Soldering results of 0603 chip size resistors were then evaluated with resistance measurements and x-ray inspection.