Up to 70 percent of dross is bound-up metal that would otherwise be available for soldering.
During wave soldering, printed circuit boards pass over the crest of a continuously flowing fountain of molten solder. It’s a quick way to solder many joints at once, but the process is not without drawbacks, and one of the biggest is dross.
Dross is the oxide scum that forms on the surface of molten solder. How much dross is generated during production depends on the temperature and agitation of the solder bath. The more turbulent the solder surface, the more dross gets produced.
To ensure a quality wave-soldering process, dross must be removed periodically from the surface of the molten solder. This can be a dangerous task. Moreover, good solder gets bound up with the metal oxides in dross and is therefore unavailable for soldering. In fact, this bound-up metal can comprise as much as 70 percent of dross. In a typical wave soldering machine, this unusable metal can amount to more than 3 pounds per hour.
“In the best case scenario, a wave solder machine generates 40 percent dross, meaning that for every 10 pounds of solder you put into the machine, 4 pounds of dross is coming out,” explains Larry Kay, president of P. Kay Metal Inc. (Los Angeles). In some of the worst cases we’ve seen, that figure is more than 80 percent
To put that into perspective, consider a high-volume electronics assembler running two-shifts per day. If the company consumes 1,000 pounds of solder per month, it will generate 400 pounds of dross per month. If tin-lead solder costs $3.50 per pound, the company is wasting $1,400 per month. “Multiply that figure by 3 if it’s lead-free solder,” adds Kay.
Electronics assemblers have developed several ways to reduce the formation of dross. Oil or wax can be poured on top of the bath to keep oxygen from the surface. Marbles can be floated on the bath to limit the surface area exposed to the air. Or, the wave-soldering process can be done in a nitrogen-based inert atmosphere.
However, all of these methods only slow or inhibit the formation of dross. They don’t eliminate it. A new material developed by P. Kay Metal does.
MS2 molten solder surfactant is a nontoxic, nonvolatile organic liquid that is poured onto the surface of the solder bath. The material does not mix with the metal, but forms a thin floating layer that covers the entire surface of the bath, except the wave, which it does not disturb. The surfactant does not produce fumes or odor, and it does not leave residue on boards or components.
Formulations are available for both standard and lead-free processes. Approximately 200 to 300 milliliters is poured onto the bath initially, and a smaller amount is usually added once or twice per shift. A single-shift assembly operation will consume approximately 1 liter of MS2 per week, says Kay.
The surfactant prevents dross from forming on the surface of the solder bath. In addition, any dross generated by the exposed solder wave is immediately converted back into usable metal, so no dross accumulates. As solder is pumped through the system, the surfactant keeps removing metal oxides, cleansing and purifying the bath. This lowers surface tension and enhances wetting. The molten solder fills holes better, and solder-related defects are reduced.
As the surfactant converts dross back into usable metal, it becomes more viscous. This thicker material is easily removed with a skimming tool, which is supplied with the surfactant. The small volume of spent, nonhazardous material can then be shipped back to the manufacturer for recycling.