Managing Dross in Soldering Processes

Estimated reading time: 8 minutes

Wave, selective and related soldering processes involve relatively large volumes of molten solder that are exposed to the atmosphere. Turbulence, oxygen exposure, temperature and other factors affect dross formation in wave soldering processes. Keith Sweatman and Keith Howell, both of Nihon Superior, advise operators on how to reduce dross.

  Figure 1: Dross from wave soldering. Dross: An Inevitable By-Product of the Soldering Process

Managing the effects of oxygen, which makes up 21% of the atmosphere, is a top challenge when soldering. Many of the features of solders, fluxes, PCB finishes, component finishes, and soldering equipment are designed to deal with oxygen. Oxidation can occur in all soldering operations and must always be managed, but it is a particular cost and performance problem with relatively large volumes of molten solder that are exposed to the atmosphere. Those processes include wave, dip, and selective soldering as well as hot air solder leveling (HASL).

What is dross exactly? Generally, dross describes worthless material generated as a result of the oxidation of the molten solder. Although, as any scrap dealer knows, the dross generated by the electronics industry is usually far from worthless. Although it results from oxidation of the solder surface, metal oxides are typically only 20-30% by weight of the sludgy material that is skimmed off a solder bath. The metal oxides tend to adhere to molten solder and--with the agitation generated by the soldering process--break up and disperse into a network that traps molten solder. Once this action is understood, methods to reduce lost solder in dross can be implemented. This article uses the wave soldering process for examples; however, the observations also apply to varying degrees to the other processes (selective soldering, HASL, etc). The tendency to form dross is exacerbated by any movement or agitation, which increases the area of molten solder exposed to the air. This means that wave soldering is typically the heaviest generator of dross.

Minimize Turbulence

Equipment manufacturers can do much in soldering machine design to reduce agitation of the solder surface. Less agitation means reduced oxidation and solder entrapment. Wave width and flow rate can be minimized and weirs used to minimize the distance that the overflowing solder has to fall to return to the solder bath. However, the increasing demands of thick, heavy PCBs are driving moves to wider, faster flowing, and more turbulent waves. Greater reliance has to be placed on atmosphere control. It is worthwhile to use board detection systems to minimize the time the wave is operating: the pump is switched on only as a board approaches and switched off as soon as there are no more approaching boards.

While the dross-generating characteristics of a wave solder machine are largely determined by its design, there are some operator actions that can minimize dross formation. A major determinant of the amount of dross generated is the extent of turbulence in the area where the overflowing solder re-enters the solder bath. That can be kept to a minimum by ensuring the solder is always kept topped up to the maximum level specified by the manufacturer. For machines with bar or wire feeding systems controlled by level detection, adding solder is handled automatically. Where that is not an option, operators should be trained to top up regularly rather than waiting until the end of the shift.

Shut Oxygen Out

Another method for controlling dross generation is to prevent or minimize contact between atmospheric oxygen and the molten solder surface. Options from soldering equipment manufacturers include:

• Fully enclosed systems, in which the complete wave soldering process is carried out in a nitrogen atmosphere; 
• Hood systems, which enclose the solder area; 
• Or boundary systems, in which just the surface of the wave is inerted with nitrogen.

Oxygen levels can be maintained below 50 ppm (virtually no oxidation of the solder wave) but even levels of 1,000 ppm greatly reduce dross volume.

Keep Solder Temperature to a Minimum

As with any chemical reaction, the rate at which solder oxidizes increases with temperature. Use the lowest possible temperature for soldering, and reduce the solder bath temperature if no boards are going to be run for an extended period. A caution for lead-free solders is that the standby temperature should not be reduced to the point that there is likely to be precipitation of the intermetallic phase--it can be slow to redissolve when the bath is reheated.

Use a Low-drossing Solder Alloy

In addition to machine design and operation factors, solder composition can also impact the tendency to generate dross. The change from tin/lead to lead-free has had major implications in that regard. Being much higher in tin content (typically >95%) and applied at temperatures up to approximately 30°C higher than typical tin/lead solder, lead-free solders experience more potential drossing. There is some evidence that the silver that is a constituent of many widely-used lead-free solders accelerates oxidation and dross generation.

Phosphorus, typically added at a level of about 100 ppm, has long been used to control drossing in tin/lead solder. It is presumed to work as an oxygen scavenger, migrating to the surface of the solder, reacting preferentially with it, and preventing it from oxidizing the solder. The phosphorus is consumed in that process, so if anti-drossing action is to be maintained, more phosphorus must be added to the solder bath regularly, typically as pellets of phosphor/tin.When the problem of heavy drossing with the tin/silver/copper (SAC) lead-free solders was noticed, makers added phosphorus at around the 100 ppm level, based upon experience with tin/lead. This addition was seldom mentioned in the product specification. However, the effect of added phosphorus soon became apparent, as solder pots constructed with the 316 or 304 grades of stainless steel were eroded and often perforated, spilling solder. It is well known in brazing technology that a phosphorus addition acts as a flux. In SAC solder, it breaks through the protective oxide film that makes stainless steel “stainless,” so that the underlying alloy is wetted by the solder. Once wetted, iron (Fe), even if alloyed with nickel (Ni) and chromium (Cr), dissolves quickly in a molten high-tin alloy. This problem forced SAC alloy users to expensive upgrades to solder pot, pump, and nozzle assemblies constructed of exotic titanium-containing materials, or with special ceramic coatings or surface treatments. While the equipment upgrades largely solved the machine damage problem, phosphorus as dross reducer still creates a problem when the solder user wants to take advantage of microalloying with nickel. If phosphorus is added to a nickel-containing solder, it reacts with the nickel to form a nickel-phosphide that effectively takes the nickel out of action.

Use of nickel turns the otherwise unusable tin/0.7% copper (SnCu) alloy into a highly efficient and user-friendly solder.* Extensive research revealed that effective oxidation control without the negative effects experienced with phosphorus could be achieved by adding less than 100 ppm germanium. That combination of tin-copper-nickel-germanium has a dross rate typically half that of SAC alloys and can be used in untreated stainless steel solder pots.

Recovering Value from Dross

If, despite all the available measures for dross minimization, you still end up with a large amount of dross, there are steps to recover the up to 70% of skimmings that is actually clean, unoxidized solder. There are several methods of extracting dross, but since these involve handling hot molten metal, suitable safety equipment must be worn and great care taken.

The solder can be released from the oxide by a suitable flux or chemical, liquid or crystalline. This separation can be done on the solder bath or in a separate heated pot. If done on the solder pot, the dross is raked back to one side and the liquid or crystals applied and worked in with a spatula. The clean solder should drop back into the pot, leaving only a black powder or sludge to be removed. If done in a separate pot, the recovered solder can be poured into molds that produce solder blocks for use later in the solder bath. Again, if a nickel microalloyed solder is being used, care must be taken to avoid phosphorus-containing chemicals to separate the solder from the oxides. There is also commercially available equipment that applies high pressure to the dross to squeeze the solder out with or without a fluxing addition.

In a world that is becoming increasingly aware of the need to minimize waste and recycle, even the metal oxides in the black powder or sludge have value. When a sufficient amount has been accumulated, it can be sold to a recycler for smelting back to metal, re-entering the cycle that provides the electronics industry with the solder it needs.

An Overall Strategy for Dross Management

The best starting point in a dross management program is a solder alloy that incorporates an effective antioxidant with no detrimental side effects. The next step is to choose soldering equipment with oxidation minimization built-in. This can be protection for the molten solder’s surface, nitrogen atmosphere over the molten solder, blanketing materials on the solder bath, solder exposure minimization, or combinations of these methods. Care must then be taken to operate the soldering equipment in a way that minimizes any remaining opportunities for oxidation. Finally, the cost of solder lost in dross can be minimized via chemicals and/or equipment that separate and recover as much unoxidized solder from the oxides as possible. Even the residue from the recovery process should not be discarded, but sold to companies that recycle the material. These efforts combine to keep soldering’s net effect on the environment to a minimum.

* Tetsuro Nishimura, Nihon Superior, discovered and patented the use of nickel for this purpose. He also is credited with research that led to germanium-based oxidation control.

Keith Sweatman and Keith Howell, Nihon Superior Co. Ltd., may be contacted at www.nihonsuperior.co.jp/english/.