|Ask the Experts|
August 28, 2020
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.
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.
Manager of Assembly Technology
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.
Senior Manufacturing Engineer
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.
Director of Reclaim Business
Alpha Assembly Solutions
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.
Technical Sales Manager
BLT Circuit Services Ltd
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.
Technical Support Engineer
This can be flux residue, dross, etc. - I would say that we can speculate about this until you have a lab analysis on this.
Engineering and Operations Management
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