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Dross "Grabber" separates good solder from dross.

Virtually every wave soldering machine on the market features a method of managing and removing dross from the system before build-up can reach the boards being processed--before solder defects can be formed. To avoid removing all dross, manual removal is most often employed, since a light blanket of dross can inhibit oxidation of the solder in the pot. Nitrogen purging of the solder pot helps slow down or minimize dross formation, but does not prevent dross formation entirely. Dross is continually formed on top of molten solder whenever the wave or selective soldering machine is in operation--at any time air can reach the hot solder and react with the exposed molten alloy. Eventually, excess dross builds up and must be removed, either manually or through a dross management system.

The problem with dross is not the powdery film on the surface of the pot, but the good solder that mixes with the dross and is either thrown away during skimming/removal of dross or, worse, mixed with the dross (forming nuclei) and circulated within the system and inside the wave. In addition to lead and tin oxides, dross can contain other contaminants. The lower density of dross allows the material to float on top of the solder and accumulate. When dross is inadvertently drawn into the process, through pumps, nozzles or the wave, solderability problems can result, such as contaminating and compromising solder joints or deposition on the board, which causes bridging and other defects.

Past research has shown that dross is formed not only by metal oxides and contaminants of chemical agents, but also from non-oxidized solder. During oxidation, nuclei of solder are formed and are consequently enveloped by oxides. These clusters form in the dross, and, when the dross is manually scooped away, a considerable amount of usable solder, along with metal oxides and contaminants, are removed. This process results in the disposal and loss of a large quantity of metals, of which, a considerable amount is still usable. This loss is undesirable from both environmental and economic viewpoints. Generally, 50 to 75 percent of the material thrown away as dross is good solder.

Loss of usable materials through dross removal varies with the type of machine. Disposal can get expensive, especially concerning hazardous waste disposal and solder replacement. The increased cost of lead-free solders and higher temperatures required by such solders, which can result in more rapid dross formation, can become an operational nightmare. Innovations are driven by need--perhaps losses of solder through dross were not significant enough in the past to drive such a system or, perhaps, the higher costs of lead-free solders, higher losses and tighter budgets forced the issue of saving all usable materials in the wave soldering process.

The Dross Grabber, a system of rotating cutting blades, chops up dross as it is automatically removed from the solder pot by the dross management configuration. This module pulverizes dross in a separate compartment, using rotating cutters that slice into the nuclei of good solder and dross--destroying the oxide envelope and allowing the usable solder to rejoin the reserve of solder in the pot. The remaining dross is powdered and collected for reprocessing or disposal.

Since good solder is automatically separated from dross, the system reduces dross volume by up to 70 percent, and the dross removed consists solely of oxides and waste--not good, usable solder. By adjusting the frequency, which is user-programmable, the cutting system can control the fineness of the powder.

The system does not remove all dross from the wave soldering system. Rather, the machine works in conjunction with the unit's dross management system, as a means of separating good solder from dross that has already been removed from the system. The machine's purpose is to keep solder/dross clusters out of the solder circulation and to conserve good solder. The product also eliminates the need for regular solder pot maintenance and extends the interval between necessary maintenance cycles.

The system cannot be retrofitted or installed to machines other than those belonging to the manufacturer. For more information, contact Vitronics Soltec, Stratham, NH; www.vitronics-soltec.com.
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Title Annotation:Tech Watch
Publication:Circuits Assembly
Geographic Code:1USA
Date:Jun 1, 2002
Words:684
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