Dross in wave soldering: causes, measurement and how to reduce it
Dross is the film of oxides that forms on the surface of the solder bath. In wave and selective machines it is the main source of alloy consumption after the soldered boards themselves: reducing it lowers process costs and improves joint quality. This guide explains where it comes from and how to keep it under control.
Dickmann Srl · updated
What dross is
When molten alloy is in contact with air, the tin oxidises. The oxides float on the bath and trap a significant amount of still-good metallic alloy: this is why dross weighs much more than the oxides it is made of, and removing it badly means throwing away alloy.
In wave soldering, formation is more intense than in a still bath: the wave continuously renews the surface in contact with the air and the turbulence folds oxides and air into the alloy.
The main causes
| Cause | Why dross increases | What to do |
|---|---|---|
| Contact with air | More exposed surface and more turbulence mean more oxidation | Nitrogen atmosphere, wave adjusted without unnecessary turbulence |
| Bath temperature | Oxidation increases with temperature | Work at the lowest temperature that gives good joints |
| Wave running idle | The surface is renewed even without boards | Stop or lower the wave during breaks |
| Alloy | Different alloys oxidise differently | Alloys with germanium, such as 99SnCuNiGe, limit oxidation |
| Dross removal | Vigorous skimming stirs oxides and alloy together | Remove dross gently, letting the alloy flow back |
How to measure it
The simplest way is to weigh the dross removed at each shift or each day and relate it to the machine's operating hours or the number of boards produced. A consistent log shows the effect of every change, for example a lower temperature, nitrogen or a different alloy, and lets you spot an anomaly immediately.
Alloy, nitrogen and temperature
A nitrogen atmosphere around the wave or the selective nozzle markedly reduces oxidation. The alloy matters too: 99SnCuNiGe contains germanium, which limits oxidation of the bath surface. For a comparison of the alloys see the guide SAC305, SAC0307 or 99SnCuNiGe; for typical temperatures, the melting temperature chart.
Dross recovery
Dross contains a lot of recoverable alloy: it should not be disposed of as general waste. Dickmann takes back dross and other process residues for recovery, and analyses customers' baths on a sample of at least 250 grams. See our analysis and recovery services and the wave soldering bars for topping up.