Solder Pot Maintenance: Dross Control and Alloy Contamination

A solder pot is a chemical reactor that runs unattended for months, and the alloy inside it changes every shift. Copper dissolves from boards and plated holes, iron and zinc arrive as plating and component contamination, and oxide skins off as dross that carries good alloy away with it. Managing those three effects is what keeps wave soldering repeatable.

How Dross Forms and What It Costs

Dross is tin oxide that forms wherever molten alloy meets air. On a SnCu0.7 or SAC305 pot running at 250 to 260 degrees Celsius, 0.5 to 1.5 kg of dross per eight-hour shift is normal on an open pot, and each kilogram removed takes roughly 0.7 kg of usable alloy with it because the oxide skin traps metal. Over a year that is a significant alloy cost and a steady drift in pot composition.

Dross formation accelerates with turbulence and with nozzle pressure, so a wide wave running flat at low flow generates less oxide than a narrow, fast wave. Pot geometry matters too: a deep pot with a small surface area exposes less metal to air than a shallow pot with the same volume.

Pot Temperature and Dross Rate

Oxide growth roughly doubles for every 10 degrees Celsius of added temperature above 250 degrees. Dropping a pot from 265 to 255 degrees Celsius reduces dross noticeably, but it also raises the risk of incomplete hole fill on thick boards, so the setting must be chosen against the fill requirement rather than against dross alone. Measure fill on a thermal coupon before lowering temperature.

Temperature also controls how fast copper dissolves from board surfaces. A pot running 10 degrees hot will pick up copper faster, so the temperature set point and the alloy analysis interval are linked. Log pot temperature every shift and compare against the analysis trend rather than treating them as separate records.

Skimming Practice and Nitrogen Blanketing

Skim dross on a schedule, typically every two to four hours, and always before sampling the alloy, because the oxide blanket insulates a surface that would otherwise oxidize continuously. Skim with a tool that lifts oxide rather than stirring it back into the melt, and never add fresh alloy on top of an unskimmed pot, since that traps oxide below the surface.

Nitrogen blanketing reduces oxide formation by keeping the surface oxygen below roughly 1000 ppm, and it typically cuts dross by half or more on an open pot. The gas has to cover the wave and the static surface, not just the nozzle, and a leaking enclosure defeats the effect entirely. Where nitrogen is not used, a reducing cover flux can do part of the job at the cost of residue.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/PCB-assembly-on-bare-boards.jpg" alt="Operator skimming dross from the surface of a wave soldering pot” />

Sampling the Pot for Alloy Analysis

Take an alloy sample from the working depth, not from the surface, and from the same location every time so the numbers are comparable. Use a clean, preheated ladle and pour a coupon that fills the spectrograph’s window, then cool it in air rather than quenching, since quenching segregates the phases and skews the reading. Mark the coupon with the pot number and date.

Sampling frequency follows throughput. A pot running more than twenty million joints a month should be analyzed monthly, while a pot used intermittently can go quarterly. Every analysis should be taken while the pot is at operating temperature and after skimming, otherwise the coupon represents the oxide layer rather than the alloy being pumped to the board.

Copper, Iron and Antimony Limits

Copper is the contaminant that matters most. In SAC305 the copper level starts near 0.5 percent and rises toward the specification limit of about 1.0 percent; above that, joints become dull, fillets lose wetting and the liquidus temperature climbs so the alloy starts to freeze in the wave. Iron limits are typically 0.10 percent, antimony 0.30 percent, zinc 0.005 percent and aluminum 0.005 percent.

Assay each element separately rather than relying on a single tin or silver reading, because the failure modes differ. Excess gold at 0.2 percent embrittles joints, zinc and aluminum at very low levels cause poor wetting and gritty surfaces, and cadmium and copper raise the melting range. Every out-of-limit element has a different corrective action.

What Alloy Contamination Does to Joints

High copper shows up first as dull, grainy fillets, then as incomplete hole fill on thick boards as the alloy thickens near its liquidus. Zinc and aluminum contamination produce a rough, matte surface and de-wetting on plated surfaces. These are not cosmetic findings: each one changes the metallurgy of every joint produced after the contamination appeared.

The damage accumulates rather than appearing suddenly, so joint quality should be tied to the alloy analysis record. When a fillet defect appears across a whole board rather than at one location, the pot is the first suspect, before nozzle setting or flux chemistry.

Adding Alloy Versus Replacing the Pot Charge

Adding fresh alloy on a schedule is the normal way to hold a pot at specification, because it dilutes the contaminants in proportion to the alloy added. That dilution works only if the added alloy is analyzed too, and it cannot correct a pot that is already past the copper limit; at that point the charge has to be drained and replaced.

Replacement is expensive and rare, but the decision is arithmetic rather than judgement: if the copper level exceeds about 1.0 percent and the pot is still climbing, dilution cannot keep up with the dissolution rate. Plan the change during a shutdown so the pot charge and the nozzles can be serviced at the same time.

Dross Handling and Safe Disposal

Dross is a hazardous waste that still contains a large fraction of usable tin, and it must be kept dry and segregated from general production waste. Store it in a covered, grounded metal container well away from water, because wet dross reacts with molten alloy in a later charge and produces violent spatter. Record the weight removed so the alloy loss can be reconciled with consumption.

Never return dross to the pot and never use a wet tool in the melt. Both mistakes cause spatter or add oxide and contamination back into the charge, which is exactly the outcome the skimming schedule exists to prevent.

Records That Keep Wave Soldering in Control

Keep four records together: dross weight per shift, pot temperature log, alloy analysis certificate and the joint quality results for the same period. Read as a set, they explain most drift: rising dross with stable temperature points to turbulence, a rising copper level with rising temperature points to the dissolution rate, and a quality drop without either points to flux or nozzle wear.

Trend the copper level rather than comparing it to a limit alone, because the rate of change predicts when a charge will need replacement. A pot rising 0.05 percent copper per month has a very different planning horizon from one rising 0.01 percent, even when both are inside specification today.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

What copper level should trigger action on a SAC305 pot? Treat roughly 0.9 percent as the action point and 1.0 percent as out of specification. Above that the liquidus rises and hole fill on thick boards suffers.

How much dross is normal on an open pot? Roughly 0.5 to 1.5 kg per eight-hour shift at 250 to 260 degrees Celsius. Nitrogen blanketing typically halves it, and every 10 degrees hotter can roughly double the rate.

Can a pot be recovered by adding fresh alloy? Only while contamination is modest. Fresh alloy dilutes proportionally, so it cannot correct a pot already past the specification limit or still climbing.