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Dross/Oxide problems with wave soldering machine

Robert Steltman


Dross/Oxide problems with wave soldering machine | 29 January, 2000

We have a Seho wave soldering machine and cannot seem to get rid of dross that is being generated within the solder pot, and is being pumped out in the wave. It is not the same kind of dross that forms on the surface of the solder pot(this is very much under control.) The stuff being pumped up is brownish in colour and seems to develop on the stainless steel surfaces in the pot. Is this an oxide from the stainless steel, or perhaps a bad batch of solder? I have drained the machine and cleaned out all the pumps etc, and am wondering if I should use new solder now. I am planning to use the Alpha vacculoy bars.

Any comments/help will be appreciated.

Many thanks in advance.

Robert Steltman

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Re: Dross/Oxide problems with wave soldering machine | 31 January, 2000

Robert: Several points:

1 Dross is a mixture of various things with no "fixed" composition: � The obvious things are metal oxides mixed with metal and this can vary in color with temperature and composition - remember some intermetallic oxides form at lower temperatures than the pure metals. � Of course, the main component of dross is tin oxides, black SnO and more commonly white SnO2. � On top of this is a host of gunges removed from the PCB, such as flux residues, metallic salts (of all colors), sticky labels, etc., generally, the organics rapidly carbonize. 2 When a heavy layer of dross gets build-up in the pot, the pump action of the motor pulls down bits of dross under molten solder and pumps up through wave nozzles. Crusty layers of dross build up on solder pot walls, and restrict or block solder flow. The pumping mechanical action grinds dross into fine powder and pushes it through the wave. May be that�s the reason the appearance of your pumped dross looks different from your surface dross. 3 Pumping dross is not good practice. Dross is very abrasive. That�s why wave solder equipment manufacturers don't want dross anywhere, but at the top of the solder pot, because it will damage your pump, nozzles, and screens. That�s why they encourage PM on their machines. 4 The first thing, I�d suggest is removing and cleaning the pumping mechanism. Next, if you still have dross through the wave, remove plates and nozzles and give them good scrubbing to remove crusty layers of dross from metal surfaces. Then, talk to your equipment supplier technicians, sales guys are useless here (some cynics might say they�re useless everywhere, but that�s another thread ;-)), to understand where the "hidden dross reservoirs" in your machine are located. 5 Stainless steel should not be developing any oxides, tarnish, or dross. But then again, I�ve seen tarnished "SS" which turned-out to be something other than SS. So, what is your equipment supplier telling you about your theory of dross formation on the SS they supplied you? 6 Chemical additives can be used to "de-dross" a solder pot. Use your wave solder machine supplier to recommend sources, because the different types of de-drosser formulations work differently with some pumps, nozzles, and chimney walls. Beware! Some of these products cause more harm than good. 7 An ounce of prevention is worth a pound of cure. Urk ... � Use dross-free metal to start with. Dross requires oxide nuclei to form and, if the solder is free from oxides, dross formation will be slowed down. - In the archives, Chrys talks (writes � whatever ;-)) about an inclusion test. Some may poo-poo the test as "unscientific." It�s not scientific. It�s a relative measure. - Alpha, SMG, and AIM are among the suppliers of dross-free metal bar. � Never return a drossy solder to the bath. � Keep solder paste out of your wave pot. � Skim the dross, being aware that this task is probably the most hazardous in your shop after routine wave solder machine maintenance. Remember with the high tin oxide content in dross, you are depleting your bath of tin as you skim. Not a good thing to do. � Occasionally, reduce the temperature of the solder bath to exactly 186�C and keep it there, with the pumps turned off, for several hours, then skim the solder surface. This will help keep the oxides down, as the intermetallics, which oxidize readily, will be partially removed. � Run your machine to minimize dross generation. � Run a tight PM program. Take care of the troops. 8 Have you analyzed a sample of your fine, brown dross (didn�t Robert Cray do a song about that?;-)) from your pot recently? It may give you direction on troubleshooting its source. 9 Have you analyzed a solder sample from your pot recently? It may confirm/deny the existence of impurities in the pot.

Good luck

Dave F

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