CO2 Soldering
Reflow soldering in toaster oven or on a skillet is popular among hobbyists and small-scale manufacturers. The process is simple and the equipment necessary is inexpensive and readily available. The results are very good if several simple rules are followed. One of these rules is to use freshly made PCBs – the shelf life of hot air leveled tin PCB finish is 6 months. A PCB which was used close to the end or past its shelf life will exhibit poor solder wetting resulting in bad joints.
Poor wetting can be overcome by increasing soak time and/or rising reflow temperature to give solder flux more time to act on oxide layer formed on the PCB finish during storage and reflow. This approach worked well in good old times of leaded solder, however, modern lead free solder reflow temperature is already quite high and increasing it may not be possible. In this case, soldering in inert atmosphere could be better option. In this article I will explain the process that I have started using some time ago – with great success.
Conceptually, soldering in inert atmosphere is easy: substituting air (which contains oxygen) in the oven with inert gas eliminates the cause of oxidation. The best gas for this purpose is nitrogen; it is non-reactive at soldering temperatures and being the major component of air it is the cheapest of all industrial gases. Being air-like it is also easy to use – all that is needed is a constant low volume flow into the oven during soldering cycle.
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