Dross, the Scourge of the Wave Solder Machine
Dross in a wave solder machine is generated by two actions.
1). The formation of the oxide layer that covers the surface of the solder pot when it is exposed to the air.
2). The falling solder that breaks up the oxide layer, forcing oxide particles down into the body of the solder.
The broken oxide particles become wetted with solder and float to the surface forming the thick silvery slush we call “dross”. This is more than 90% clean solder and in the early days of wave soldering we pressed dross in a perforated spoon to force the free solder out and and returned it to the pot. Now there are machines that will do this and they can salvage a high percentage of the solder. Similarly the use of a “dross reducing agent” which is sprinkled over and mixed into the dross will return much of the free solder to the pot, leaving only a small amount of gray oxide floating on the surface which can easily be removed.
There is a common belief that contaminants in the solder increase the rate of dross production. This is a complex subject but at the temperatures normally used in wave soldering, (around 260C) and the degree of contaminants normally found in a wave solder machine, they have little or no effect. In fact of the 8 most common contaminants, small amounts of six of these, actually reduce the amount of dross formed. The addition of Phosphorus to the solder will reduce the amount of dross produced, and is the addition found in many “low dross” solders. However excessive amounts of Phosphorus can cause grittiness in the joints and dewetting, and it is generally not a recommended additive. The degree of turbulence in the wave is the prime factor in determining the rate of dross production and everything should be done to maintain this to a minimum.
A) Set the wave height as low as possible.
B) Keep the solder level in the pot as high as possible.
C) See that any “solder chutes” or guide plates in the pot are adjusted correctly.
D) Make sure there are no cracks of missing screws in the nozzle.
If the the oxide on the solder surface can be eliminated so is the formation of dross. In practice there are only two ways to achieve this.
- Maintain a layer of tinning oil on the solder surface. This will have to be replaced routinely, about every 40 operating hours, and many users find this increases the amount of maintenance on the pot. Some of the oil may find it’s way onto the PWBs which can be a problem if soldering with the No Clean process.
- The use of a nitrogen blanket over the entire solder surface, is probably the most practical method of eliminating dross. Many users claim that the reduction in maintenance and the lower solder consumption pays for the cost of the nitrogen. In addition it permits a reduction in the amount of flux applied when using the No Clean process and thus reduces the residues left on the soldered assembly.
It is extremely difficult to measure the rate of dross production accurately, unless carried out over several weeks There are so many variables that can affect the results. The excellent paper published in 1980 by the Tin Research Institute is probably the definite document on the subject, and says the following — When these considerations are taken together, the various and contradictory opinions as to the effect of particular impurities on dross formation are readily explained. For example — aluminum at the 01% level would produce 25% more oxide or oxidation than pure 60/40 alloy at 245C, while at about 265 C it would produce 15% less.