Introduction
During wave soldering in air, dross will be generated. The amount of dross depends on nozzle width in relation to the width of the boards that are soldered, the solder temperature, the wave height and the level in the solderpot. In a practical case, given below, the question was: "What is the solderpot efficiency one can expect?"
The case
“The average dross weight is 10.36 Kg /day. This is very surprising, given that we are only putting in 12 Kg to 15 Kg of new bars per day. Please comment on the ratio of dross too new bar”.
Solderpot efficiency
The question about the ratio of dross to new bar is impossible to answer without detailed information. Even when one does not solder at all, but let the waves run idle, one will create dross. In that case the entire solder one has to add is just for the replacement of the dross that is removed and the dross ratio is in that case 100%, or 0%efficiency.On the other hand if one has a large consumption of solder going to the joints on the PCB, the ratio will decrease. According to the book Soldering in Electronics from R.J. Klein Wassink, chapter 9.1.2.1, this ratio will in standard production with tin-lead solder be around 40%, having an efficiency of 60%. This is just an example, so do not use these figures as 'standard'. It is often difficult to evaluate if a process runs well or not without a reference of comparable situations. In the same chapter of this book one will find that a dross formation of about 1 kg/h (for a 400-mm wide nozzle) is not exceptional in a normal process running at 250°C.The amount of dross can be minimized by reducing the wave height(s), the fall height (by keeping the solder level in control) and by a reduction of the solder temperature. Some customers are using a nozzle which is specially designed to the (standard)width of their product, e.g. 260 mm instead of 400 mm, so that less solder is pumped around and so less dross will be formed. If the machine will not be used for some longer time, e.g. during lunchtime, it is advised to switch off the solder pump(s) to reduce the dross formation.
Dross removal
Regarding the dross formation: It is of utmost importance that the dross is frequently removed from the dross compartment, because dross creates dross. For automatic dross removal a dross-grabber can be used if applicable. If the dross is not removed regularly, it will be picked up by the solder flow going from the wave to the pump compartment. If that happens, the dross can also accumulate in the under compartment of the solderpot, from where it will come into the nozzle, and from there eventually onto the PCB's. This is a very unpleasant situation, because one never knows when the accumulated dross will be 'washed out' from the under compartment. Therefore, the strong advice to remove the dross frequently to reduce the dross formation and to avoid that dross can be pumped around.