|Ask the Experts|
February 19, 2021
Cleaning No-Clean Solder Paste
We are building small lots of circuit board assemblies using no-clean solder paste. We prefer to remove the no-clean residue and thus after the reflow process we are cleaning the boards by hand using a brush and no-clean cleaner. This is not working very well and we are getting a white residue on the boards.
What would you suggest is the best way to clean small lots of circuit board assemblies that were assembled using no-clean solder paste?
|Expert Panel Responses|
I find this a politically charged question. A flux material is being used which does not need to be cleaned off, yet the requester prefers to remove the no-clean residues. I'm curious as to what is a no-clean cleaner?
In any case, cleaning of no-clean flux residues has always left a film residue on the board as the cleaning solution only partially dissolved the residual film and the light reflection off the surface creates a whitish appearance.
I would offer two pieces of advice, first, if the flux residues are to be removed then use a flux which is easily removed, such as a water soluble flux. Second what is the reasoning behind removing the no-clean flux residues? Has the user experienced any failures for this action or is this a preference of appearance?
If it is a failure, then change the flux, if it is appearance then training is in order and leaving the benign residues on the boards would be of no concern to the functionality of the product. Billions of electronic products have been produced using these materials and are still functioning without going through any cleaning process.
If the residues have to come off completely for whatever reason, I then suggest contacting the paste manufacturer and getting recommendations from them as to what solvent would dissolve the residual materials. I would also keep in mind that some of these chemicals may no longer be available and if available would be difficult to dispose.
There is plenty of information on this condition on the web and searches should be able to provide more information.
Vice President, Technical Director
The removal of no-clean flux residues requires four things:
Cleaning must take place at elevated temperatures upward of 150 degrees F. Mechanical energy in the form of impact pressure and small fluid droplet size are required for complete impingement.
Most importantly, a vigorous rinse process must be implemented to remove the defluxing chemical. These processes are seldom successfully accomplished in a manual cleaning environment.
The most popular automated defluxing configurations are batch-format spray-in-air. Some batch format cleaners provide real-time cleanliness testing that helps ensure that all of the assemblies being cleaned meet your cleanliness standard.
Most machines also provide automatic wash, rinse, and dry functions. Batch format defluxing systems may be operate with either small assembly quantities or large assembly quantities.
Cleaning boards that have been soldered with no-clean paste can be a challenge. To some extent, it depends on the solder paste that you are using, but assuming that you cannot easily change solder paste formulas, here is what I would recommend:
If you have any sort of in-line cleaning equipment or if you have the capability to add cleaning chemistry to a batch cleaner, I would advise that you contact Kyzen or Zestron for a good cleaning product recommendation.
If you tell them which paste you are using, they can recommend a cleaning material that will work specifically for your chosen paste and your cleaning equipment.
If you are cleaning at the bench-top level, I would recommend that you contact Techspray or Chemtronics for a product recommendation that will work with the paste that you are using.
If you have some flexibility on which paste you can use, you may find that some no-clean pastes clean better in an aqueous system than others do. You can contact your paste vendor (or multiple paste vendors) for a sample of a paste that may be better from a residue removal standpoint.
General Manager - Electronic Assembly Americas
We use Zestron products, but if you are hand cleaning, this is not recommended.
The type of NC flux and the PCBA topography needs to be understood. Are these Handsoldered NC, SMT/PTH. When "disturbing" NC fluxes, especially Mild RMA's can create more of an issue later and will appear worse than if you left the NC flux alone and will degrade over long haul.
SIR or conductivity checking is recommended, to ensure the cleanliness and full removal of any ionic residuals.
There are several aerosol types out there, but you would be best to contact your flux manufacturer directly, as they can align proper removal chemistry for your application.
Capital Equipment Operations Manager
Specialty Coating Systems
The causes of the white residue in this case could be related to one of the following:
Application Technology Manager
Your problem is a combination of how good your solvent is at removing this particular flux and the fact that you are probably just spreading it around as opposed to removing it.
I would suggest, if your needs do not justify a proper cleaning system, that you immerse them in a small tank full of solvent and brush repeatedly rinsing in a tank of clean material.
You may have to heat the solvent or use ultrasonics if you can to increase the solvent action. If this fails then look for a better solvent!
Principal Engineer - CMA Lab
A question back. If it is truly no-clean, then why clean it? I would suggest talking with the manufacturer of the "no clean" product, since it seems to require cleaning? Or am I confused?!
We have been cleaning no clean fluxes for 9 years and it requires some good cleaning energy, flux cleaner and a brush will not work as you know.
We have been successful using saponified wash (heated) with steam (DI) water rinsing. Please contact me and we can discuss your cleaning challenges.
President/Senior Technical Consultant
The no-clean flux must be compatible with the no-clean cleaner. The white spots suggest otherwise.
Talk to your no-clean paste and/or no-clean cleaner supplier. One or both can probably suggest a more compatible chemistry.
CEO & Managing Partner
In general, there are only two good ways to clean complete circuit boards in low volumes: a small vapor degreaser using a low-boiling point solvent like Vertrel, or a small ultrasonic cleaner using an aqueous-based fluid.
Small vapor degreasers work great. They boil and condense low-boiling solvents to clean quickly, easily and inexpensively. Cleaning with a vapor degreaser can easily get into and around SMT and BGA parts with extremely tight clearances. They are speedy, with cleaning cycles measured in minutes.
Drying is simple because the solvents easily evaporate even from complex shapes, deep holes and blind vias. They use tiny amounts of electricity and no water, so they are very gentle on the environment. For more details, see http://www.microcare.com/faqdetails.aspx?faqid=149.
Ultrasonic cleaners such as those made by JetClean, JNJ, SmartSonic and other companies use aqueuous solvents and sound waves to knock contamination from SMT assemblies.
Cleaning cycles can be longer than vapor degreasers, and rinsing can sometimes be a problem, requiring multiple dip tanks to get the boards truly clean. Drying also can be a problem because water-based cleaners can be difficult to remove from circuit boards.
But the solvents are inexpensive and the cleaning machines are simple and usually small. Do not use flammable solvents like alcohol in ultrasonic cleaners due to the risk of fire. For more details, see: http://www.microcare.com/faqdetails.aspx?faqid=58.
Your third choice is to clean the boards manually. This can be time-consuming and, as you have already discovered, prone to residues unless the cleaning process is very, very thorough. While MicroCare does not normally recommend manual cleaning of entire circuit boards, if the quantity is small and the boards themselves not too large, you could use the MicroCare Trigger Grip system to clean these boards.
I would recommend PowerClean solvent for this application because it is the strongest, nonflammable cleaner in the family and it's also the least expensive. For more details, visit http://www.microcare.com/p-64-powerclean.aspx to see about PowerClean and then http://www.microcare.com/pc-37-7-solventmiser-trigger-grip.aspx to learn about the Trigger Grip. You also can see a useful video at http://www.youtube.com/watch?v=S67kUPb7Pyk.
Maybe I missed it in this thread, but I am not clear why you would clean boards if using a no-clean paste. The goal of moving to no-clean is to eliminate cleaning. A lot energy was put into how to clean, when the solution should be don't clean.M.S.
1st question is: Why clean off the no-clean flux? It is designed to be left in place. Is your Quality Control dept. misinterpreting the requirement for cleaning "white residue"?Jerry Wiatrowski, Jerry Wiatrowski, USA
The white residue from no clean flux is not required to be cleaned. Ref IPC-A-610E Para 10.6 (Note:the white residue from "cleanable" flux must be removed)
Aerospace and military customers usually require contract manufacturers to use J-STD-001 as a solder/workmanship requirement. This standard requires to use ROL0 and ROL1 fluxes in the process.Dean Edwards, APT Electronics Inc, USA
ROL0 and ROL1 fluxes are no-clean fluxes. But, then the customer requires ionic contamination testing. No-clean fluxes will not pass an ionic contamination test. Hence the cleaning requirement.
We run a no clean shop because 95% of our customers are fine with it, and it saves a lot of production costs and floor space. But when we get into the Aerospace or military market, some customers require cleaning. Thus, a small portion of our production must be cleaned. It seemed easier to install a batch cleaning process for this exceptional requirement, rather than trying to manage different solder pastes and hand soldering cored wires.Jack Lucas, Ametek Programmable Power, USA
Saponifiers work on the no clean SMT residues in the range of 5-10%, using Chemistries from Kyzon or Zestron, at 140-150F.
Unfortunately, batch cleaners always seem to have low impingement areas, due to "shadowing" and inconsistent distances to spray nozzles, and in these low impingement areas, the cleaning process leaves a white residue after cleaning. This residue is in deep recess areas, like between fine pitch TSSOP leads. We have been able to remove all residue with extended wash cycles, up to 18 minutes, in an 8-year old batch cleaner. More modern cleaners would probably work better.
Likely, in our shop, some of the white residue has escaped our cleaning process from time to time, and we have never experienced a problem with it. We have received neither customer complaints, nor field returns because of it. I need to do an X-Ray fluorescence on the white residue to confirm, but I expect it is non-ionic and quite benign, depending on the no clean solder paste used.
Water soluble solder pastes are designed to be easily cleaned, leaving little or no measurable residue. No clean pastes are designed to give high electrical reliability, and are compatible with conformal coatings. If your goal is no residue, a water soluble paste may be the best solution.
Director of Reclaim Business
Alpha Assembly Solutions
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