Solder Dross

16 January 2009

Every company that is operating a wave/selective soldering system has it, but what is it and how can you reduce it or dispose of it?

Dross is 85-90% solder so it is valuable to the company. During wave soldering in air, oxides form on the surface of the molten solder. They are displaced on the surface of the wave by boards being processed forcing the solder and oxides to mix on the bath surface and just below the surface of the static pot. The rate of dross generation depends on solder temperature, agitation, alloy type/purity and other contaminates/additives. Much of what appears to be dross is, in fact, small globules of solder contained by a thin film of oxide. The more turbulent the solder surface, the more dross is produced. Depending on the flux being used in the process the dross can be like sludge or more like a powder. Analysis of the dross when separated from the solder shows the remainder to be oxides of tin and lead.

As the assembly passes over the solder, the various metals on the board will dissolve in to the molten tin. The actual amount of metal concerned is very small, but a tiny amount of metallic contamination can affect the action of the solder wave, and can be reflected in the appearance of the solder joint. Generally speaking, as copper is the most common metal soldered it will be the most often experienced contamination in the solder. The actual solder in the dross however will have the same alloy content and levels of contamination as in the solder pot so it does have value and can be sold back to the supplier. The amount of solder in the dross will affect the price paid back for the scrap and also the metal value at that point in time.

The dross on the surface of the static bath protects against further oxidation. Therefore, it should not be removed more frequently than necessary. Only if it interferes with the wave action, restricts the control of the solder level or it is likely to cause a flood as the wave is turned on. Once per day is usually satisfactory provided the correct level of solder in the pot may be monitored and is not allowed to drop. If the solder level drops it will affect the solder wave height directly. During de-drossing the amount of solder in the dross can be controlled by the operator’s methods of removal. Care can significantly reduce the amount of good alloy removed from the bath. However staff are often not given time to de-dross the bath in a way to reduce the waste.

Remember a mask should always used when clearing dross from the wave, and place in a closed container normally supplied free from the solder vendor. This avoids the possibility of small lead dust particles getting into the air. Consider the use of a surfactant to get the solder out of the dross. Dross can of course be sold back to the solder vendor for purification and re-use in other applications.

With lead-free solder the levels of dross can be higher but can be maintained at acceptable levels with the correct selection of the original alloy. The surface of the solder and the characteristics will be different with lead-free solder, one example of this is copper. In a lead-free bath copper levels may be between 0.5-0.8% to start with increasing during production. In a tin/lead bath this would be considered well above maximum contamination levels.

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