Practical Notes on Dross Formation

Wave soldering is a process of thermal balance rather than of fixed settings. The solder pot, the preheaters, the fluxer and the conveyor interact, and a change in any one of them shows up somewhere else. Two symptoms dominate production problems: joints that do not fill, and a pot that is slowly contaminated. Both are manageable if the parameters that drive them are understood and monitored.

Alloy, Temperature and the Operating Window

The classical eutectic solder for wave soldering is a tin-lead mixture of 63 percent tin and 37 percent lead, which melts at 183 degrees Celsius. The pot temperature must be comfortably above that, and it must be uniform across the bath, because a board passing over the wave sees the average temperature of the solder it touches. Historically 250 degrees was treated as standard, and improvements in flux chemistry, together with the addition of preheating stages, have pushed the common range down to 230 to 245 degrees.

Lower pot temperatures reduce the thermal shock seen by components and laminate, slow the formation of dross and reduce the amount of flux that is consumed. They also make it easier to keep flux available at the exit of the wave, which reduces bridging and solder balls. The limit is the thermal mass of the assembly: where components and planes vary in mass, the heat must still be sufficient to bring every lead and pad above the wetting temperature for long enough to form a proper joint.

Why the Alloy Composition Drifts

The composition of the bath changes with use. Tin is consumed in the joint and in the dross that forms on the surface, while gold and copper dissolving from the boards accumulate in the solder. Copper from plated through holes and gold from surface finishes dissolve faster than they are removed, and above a certain concentration the alloy loses fluidity. The visible symptom is a rough, grainy joint surface.

The standards that govern this are worth quoting in purchasing and process documents. A minimum tin content of 61.5 percent is a typical specification for a 63/37 bath, and the permissible dross and impurity limits are defined in the applicable electronics assembly standards. Solder purity is therefore a process parameter rather than a purchasing detail, and sampling and analysis at regular intervals, plus after any addition of pure tin, keep the composition inside those limits.

Composition and temperature are the two parameters that most directly determine joint quality, but the wave itself has to be stable as well.

Controlling Dross Formation

Dross is unavoidable when solder is exposed to air, and the wave exposes a large surface. A covering layer of dross actually slows further oxidation, but when the layer is disturbed it entangles with the wave and produces an unstable, turbulent flow. A turbulent wave entrains oxide particles, which end up in the joint and appear as a rough, granular surface.

The traditional remedy is to skim the surface, but skimming frequently exposes fresh metal and creates more dross, and it consumes solder. The better approach is to keep the pot full, fitted with a low-level sensor and an alarm so the level never falls far enough for surface dross to be drawn into the pump, and to remove dross only at intervals that the process data supports. Adding solder regularly to maintain the level is part of normal operation, not a corrective action.

Flux and Preheat Interaction

Flux has to be active at the point where the board meets the wave, and that depends on the preheat profile. Too little preheat and the flux is still liquid and evaporating when the wave arrives, which produces spattering and incomplete wetting. Too much preheat and the flux is exhausted before the wave, which produces poor wetting and a dull joint.

The correct profile activates the flux and drives off most of its solvent without destroying its activity, leaving the board at the temperature where the wave can complete the joint quickly. Profiling with thermocouples attached to representative joints, rather than relying on the air temperature above the preheaters, is the only reliable way to confirm that the profile reaches the intended values on the board.

Machine Setup and Wave Geometry

Wave height and contact width determine how long each joint is in contact with the solder. Chip waves, used for high-density assemblies, present a narrower, shallower wave and produce more dross because they expose more solder surface to air. The trade-off is between covering dense areas and keeping the wave stable, and it is resolved by profiling rather than by a rule of thumb.

Conveyor speed and angle complete the picture. A steeper angle reduces bridging but also shortens contact time, and a shallow angle does the opposite. Where a board has both fine-pitch surface mount parts and heavy through-hole connectors, the compromise is usually found by adjusting angle and preheat rather than by changing the pot temperature.

Monitoring closes the loop and turns an unstable process into a controlled one.

Monitoring and Analysis

Routine sampling of the pot is the backbone of process control. The samples show tin content, copper and gold accumulation and the level of contamination that affects fluidity. Combined with a log of alloy additions and dross removal, the data shows when the bath needs to be partially replaced rather than merely topped up. Keeping the pot full and adding pure tin when the tin content falls is cheaper than replacing the alloy.

Incoming solder should be purchased against a specification that sets the maximum permissible dross content and the minimum tin content. The comparison between lead-free and leaded solder matters here as well, because lead-free alloys operate at higher temperatures and dissolve copper faster, so the analysis interval has to be shorter.

Appearance as a Diagnostic

Joint appearance is a fast diagnostic. A bright, smooth fillet suggests correct temperature and clean alloy. A rough, granular surface points to dross entrained in the joint or to low tin content. Dull joints can indicate low tin, contamination from the components themselves, or disturbance during solidification caused by vibration or mechanical shock while the joint is still liquid.

Because appearance alone cannot distinguish those causes, any suspicious batch should be checked against pot analysis and the conveyor profile. Combining the three sources of information – appearance, analysis and profile – identifies the cause rather than the symptom, and the quality characteristics of the board and its solder mask behaviour both influence how forgiving the process is.

FAQ

What pot temperature should be used for leaded solder? The common range is 230 to 245 degrees Celsius, above the 183 degree melting point with enough margin for thermal mass. Higher temperatures increase oxidation and thermal stress without improving wetting.

How often should the solder bath be analysed? Regular intervals defined by the process control plan, and always after a significant addition of pure tin. Heavier boards with gold or copper finishes require shorter intervals because those metals dissolve into the bath.

Is skimming dross removal the right approach? Only at a controlled interval. Frequent skimming exposes fresh solder and generates more dross. Keeping the pot full and preventing surface dross from reaching the pump is more effective.