|Ask the Experts|
December 7, 2017
Best Way to Eliminate Dross?
What is the best way to reduce the dross in tin/lead solder pot?
P. H. K.
|Expert Panel Responses|
The best way to reduce dross in a tin/lead solder pot is a question people have been trying to answer for years. There is a variety of dross inhibitors that can be used on the surface of the solder pot, but be careful in selecting the correct material for your process. One needs to understand the chemistry used and whether or not it will be detrimental to the machine and the people working the system. Basically what needs to be done is prevent the solder from oxidizing, which is preventing the solder from being exposed to the air. So covering it with a dross inhibitor is one way to protect the pot, yet the solder coming off the wave generator would be exposed to the air and that will create some dross. The other would be to solder in a complete inert environment, such as nitrogen. A nitrogen blanket can be used to protect the solder coming through the wave generator as well, which will reduce the dross going into the solder pot. Another way is to maintain the correct solder volume in the wave and once the dross blanket forms, leave it alone. The more you remove the dross the more dross you will create, so keep the solder volume high to prevent the dross from being drawn down into the pump and you should be all set. I'm sure there will be many inputs regarding this comment, and I'm sure many of us have seen many ways to prevent the dross including peanut oil, and synthetic oil from the old oil injected wave solder systems. I'm positive this is not the way to go and there is plenty of history to back up that thought. Good Luck.
Vice President, Technical Director
There are several processes to reduce dross contamination from a tin/lead solder pot. Dross is formed when molten solder on the surface of a solder pot or bath comes in contact with oxygen forming tin oxides. A thin film of dross in a solder pot is desirable as it slows down further oxidation of the solder as long as it is not detrimental to the integrity of a solder wave. Atmospheric conditions, humidity, soldering temperature, agitation of the solder and quantity of flux contribute to the generation of dross. The most common methods of minimizing dross are:
Dross is an expensive and wasteful result of melting metals that are prone to oxidation and then adding turbulence such as causing them to flow over a solder wave. With the advent of lead free solders with their higher percentage of tin and higher operating temperatures which accelerates dross generation along with the much higher prices for lead free solders the reduction of dross has become a necessity if one is to stay competitive. Oils, powders and dross squeezers have been used for years. These are half steps and fall into the category of "better than nothing" They moderately reduce dross but they leave in oxides and dross laden solder which just accelerates the generation of more dross and which can reduce solder joint quality. The newer and far more effective method is to use a material that works right on the pot and actually breaks the organometallic bond between the metal and the oxygen thus returning the metal to the solder pot as clean oxide free solder while at the same time liberating the oxygen back into the air. One such material is the newly patented Molten Solder Surfactant or MS2 from P. KAY Metal Inc. in L A California (http://www.pkaymetal.com/ ). This material breaks down the afore mentioned bond and reduces solder consumption by as much as 85%. This method of dross elimination has been adopted by a large number of leading EMS companies and is now in use on hundreds of wave solder machines globally.
President and Founder
The majority of Dross is created when solder comes in contact with Oxygen. The best way to reduce Dross creation is to limit the solders exposure to Oxygen. The industry accepted method of doing this is to introduce a blanket of inert gas, (N2) Nitrogen, over the solder. This should be done not only over the pot, but especially in areas of turbulence, where oxygen can be forced into the solder, such as around pump shafts and impellers. When cleaning Dross from surface of the solder pot, a thin layer of Dross should be left on the surface. This will also act to limit the solders exposure to oxygen.
Global Director of Sales
There are several ways to reduce dross formation in the wave soldering process. The use of a dross reduction chemicals can be used and have shown acceptable results. These chemicals are placed over the solder in the solder pot and form a layer over the molten solder to reduce metal oxide formation. The chemicals will need to be replaced periodically. The other method is to use an nitrogen retrofit kit or an inerting cover. This system can reduce dross effectively and also improve wetting of the solder into the PTH. Another benefit is one can convert to a less active flux chemistry and to reduce the volume of flux required per board. Both alternatives are good and need to be evaluated for your production requirements and overall cost of ownership.
Global Segment Manager
Air Products & Chemicals, Inc.
Keep the temperature of the pot as low as possible and turn off the wave whenever possible. The addition of dross reducing fluid with help reduce the amount of oxygen in contact with the solder at the top of the bath.
Regional Sales Manager
OK International Inc.
Dross is produced by movement of molten solder in air producing tin and lead oxides. Reduction in exposure to air by using a nitrogen blanket will greatly reduce this, as will making sure the pumps are switched off when the machine is not in use.
Principal Engineer - CMA Lab
When targeting to reduce dross in any kind of solder pot, it is important to reduce the cause of dross as much as possible. Dross is just oxidized solder; therefore, the easiest remedy is to remove exposure to air. This can be done by using an inert gas such as nitrogen, however, that can become a costly way. Another method is to use an oil coating to cover the exposed solder, what acts as a barrier to prevent dross forming. Limiting the frequency of de-drossing can also help. This method tends to work best on static pots. If you have a dynamic pot, such as a wave or selective soldering machine, then reducing turbulences by correctly setting up the machine will offer the best improvement. Finally, you can also use phosphor doped solder which reduces dross formation through a reaction process. This material can usually be purchased through your current supplier. In case, this do not work then it may be worth consulting the equipment supplier or asking your solder supplier to come in and help.
One of the most overlooked aspect of dross contamination is the quality of the solder bar to start with. If you use poor quality (CHEAP) dross laden solder bar then you will inevitably produce more dross. It is a complete false economy to buy cheap. Use Phosphor inhibitors work very well. Be wary some other inhibitors may well be hygroscopic in nature or be oil based that can contaminate the PCB so see what works for you and check the MSDS sheet thoroughly. Fit a simple switch to the machine so the Solder Pump runs only when the PCB hits the wave and then turns off. Depleted solder pallet material will also cause lots of spongy dross to occur so watch and clean the wave pallets regularly. Hope it helps
Technical Sales Manager
BLT Circuit Services Ltd
The first thing that I would do is start using nitrogen – that will be the main agent that will reduce oxidation because the metal exposure to the oxygen will be limited. There are also chemicals that you can add in the solder pot to prevent dross. There are also few machine parameters that have a significant contribution to dross formation: how much time is the pump running (is it running when not needed), how high is the solder temperature, what are the parameters of the pump (higher turbulence in the pot creates higher dross formation potential). Last but not least, are the boards that you are running introducing any external contamination (oxidation from components leads and/or PCBs) in considerable quantity?
Engineering and Operations Management
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