Ask the Experts
November 3, 2023 - Updated
January 13, 2016 - Originally Posted

Dust contamination after selective soldering

We have discovered an extremely fine, low density "dust" of dross on PCB surfaces after selective soldering. The dross is difficult to see without careful examination. Is there an effective way to clean, or remove this? Our batch washing machine is unable to remove it.

We are using SN100C alloy and a no-clean flux applied with drop jet. The dross dust appears in the vicinity of solder joints, but not immediately adjacent to them. It seems that the dross may be carried to the PCB via the nitrogen blanket surrounding the nozzle. We de-dross the pot once at the beginning of each 8-hour shift, and de-dross the pump assembly once per week.


Expert Panel Responses

Dross dust can adhere to solder mask when the solder mask is softened through heating. The solder mask hardens as it cools and the dross dust can be very difficult to remove. There are a few possible solutions to this issue.

1. Try baking the circuit board before selective soldering. This will harden the solder mask a little and then during selective soldering the solder mask will soften less. I suggest baking at 150C for about 1 hour if the components on the board can handle it.

2. This next suggestion goes against recommended practice but might help. Try to overspray flux around the vicinity of the solder joint onto the circuit board. Flux helps to coat and cool the surface of the circuit board essentially protecting the board from adhesion of dross dust. Drop jet flux systems are designed to spray flux in a tight area, so a spray program would have to be created to spray around the solder joint. Alternately the flux could be hand sprayed onto the board before it is run through selective solder.

3. Try using a surfactant based cleaning chemical in the batch washer. Surfactants can wet under the dross particles and might help to lift them off of the circuit boards.

4. Reduce the solder pot temperature by 10-20 deg C which will help reduce the rate of oxidation of the solder. This could reduce the quantity of dross particles that are generated.

5. Reduce the flow rate of the nitrogen gas. If the nitrogen gas is spreading the particles around and carrying them to the board surface, then reducing the flow rate may help.

Tony Lentz
Field Applications
FCT Assembly
Tony has worked in the electronics industry since 1994. He worked as a process engineer at a circuit board manufacturer for 5 years. Since 1999, Tony has worked for FCT Companies as a laboratory manager, facility manager, and most recently a field application engineer. He has extensive experience doing research and development, quality control, and technical service with products used to manufacture and assemble printed circuit boards. He holds B.S. and M.B.S. degrees in Chemistry.

There are a couple of things here.

First, is what you are seeing truly "dross" or is it just flux residue? If you are using a low solids flux (what the marketing people have designated a "no clean" flux) then it's possible you are simply seeing a benign residue.

Since you are using a nitrogen blanket, the chances are reduced that it is truly dross.

Second, you note that, "The dross is difficult to see without careful examination." To me this throws up a flag on how you are inspecting.

If you are using industry standard practice and inspecting according to the criteria in IPC-A-610, visual examination with the unaided eye is acceptable. Magnification, if required after determining that the residue is a failure mechanism, is limited to 4x. If you have to examine very carefully to see the residue, it may not be a functional issue. Extra cleaning steps for a benign residue will cost more and be one more step which may lead to a failure.

I know this doesn't answer your question about how to remove the residue, but it may help you determine that the residue is benign and you may not have to remove the residue.

Kris Roberson
Manager of Assembly Technology
Kris Roberson has experience as a machine operator, machine and engineering technician and process engineer for companies including Motorola, and US Robotics. Kris is certified as an Master Instructor in IPC-7711 / 7721, IPC A-610 and IPC J-STD 001.

Correct, the wash won't remove this type of residue. Try to answer these questions; when did the issue begin (was there a process or material change)? Has the defect worsened with time?

I see that you are maintaining the solder pot by de-drossing pot & pump. The next step will be to conduct a solder pot content analysis. There is a possibility that the pot is contaminated with other metals which are creating the problem you are experiencing.

Edithel Marietti
Senior Manufacturing Engineer
Northrop Grumman
Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.

Are you using a dross recycling system? Our testing shows that the alloy produced from an in house dross recovery system contains 15 to 20% dross by volume. This accumulates dross much quicker than if you use bar solder from a supplier who meets J-STD-006 with their bar solder.

Mitch Holtzer
Director of Reclaim Business
Alpha Assembly Solutions
I've been in the soldering materials/applications industry for 25 years. Since joining Alpha, Ive been the global product manager for preforms, wave soldering flux, solder paste and more recently the Director of the soldering materials reclaim business.

I would try to add additional Germanium as this may have depleted or even Tin/P as this will treat the Solder by improving the flow while reducing the Dross.

One question does it appear to be stuck to the resist at all as surprised if it is dust that the cleaner cant wash it off hence is it adhering to the resist?

If it is have you checked for undercured solder resist? A picture of the offending dust would be great if possible.

Greg York
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.

I would first verify that the dust is indeed metallic. It could also be flux residue or PCB dust that can build up in the vacuum fixtures that "puff" off when the vacuum is released.

Kay Parker
Technical Support Engineer
Indium Corporation
Kay Parker is a Technical Support Engineer based at Indium Corporation's headquarters in Clinton, N.Y. In this role she provides guidance and recommendations to customers related to process steps, equipment, techniques, and materials. She is also responsible for servicing the company's existing accounts and retaining new business.

This can be flux residue, dross, etc. - I would say that we can speculate about this until you have a lab analysis on this.

Few questions:
  • If you are using a no clean flux why do you clean it?
  • What type of solvents are you using in the batch washing system?
  • How did you determine that the de-dross every 8 hrs. is sufficient to ensure clean solder pot?

Georgian Simion
Engineering and Operations Management
Independent Consultant
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at

A rapid change in process performance could indicate a change in the equipment settings or process settings. Some selective systems incorporate designs to mitigate dross from re-emerging onto the PCB. Below are a few process/machine settings that can have an impact if your system does not include these features:
  1. What is the contaminate? As per other posts, this could be a product of contaminated solder or the substance may be appearing as dross dust but is something else.
  2. What is the solder temperature? If it is dross in powder form, this may can indicate an overheating of the solder. Check both the solder at the tip and compare to the machine setting to confirm that machine is calibrated properly. For SN100C, we usually to not recommend above 310C as a solder temp setting.
  3. Are there any other process indicators besides the dross dust? If the solder temperature calibration is off you would may see other changes in the process.
  4. Is the Nitrogen purity and Nitrogen flow rate measured and controlled by the selective system? If not, Low N2 purity or high flow rate through the diffusers can create/exacerbate this issue. An excessive flow rate of the N2 on some systems can push the dross powder to the surface if it does not have a mitigation system. Low N2 purity or low flow rate increases the oxidation and again would likely have other process indicators such as points on the ends of leads or even increased bridging.

Jonathan Wol
Pillarhouse USA, Inc.
Jonathan is president of Pillarhouse USA, a director of the Selective Soldering Academy and has 20+ years of selective soldering experience.
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