Ask the Experts
November 20, 2018 - Updated
February 23, 2009 - Originally Posted

Problem with 'No Clean' Solder Flux Residue

We have a problem with no-clean solder flux residue. After processing the circuit boards we see a white residue. Could this residue be corrosive? What is this white residue? If all of the no-clean flux is not heat activated could it have a negative effect on the circuit board assembly? In addition, we need to conformal coat the boards. What may happen if we apply the conformal coating over this white residue?

C. S.

Expert Panel Responses

It is extremely likely that this white residue from a no-clean flux is perfectly safe. However, it is suggested that you communicate your findings and concerns to your flux supplier for verification. Questions to ask your supplier:
  1. Is the residue from this flux supposed to be white?
  2. If not, what circumstances would cause the residue to be white?
  3. If the residues do turn white, does this make the residues any more or less reliable than expected?
It is important to note that some fluxes do leave a white residue that is perfectly normal and safe. However, other factors that cause white residues -- such as partial removal of the residue in an inadequate cleaning process or even over-application and under-heating of the flux -- could be cause for concern in terms of long-term product reliability. Another potential cause of white residue can occur when fluxes are mixed together in a hand soldering operation, and this is generally safe provided that the hand soldering technique (especially the amount of flux applied and the completeness of heating the flux) is carefully controlled. The flux residue may indeed contain halides; your flux supplier can also help with this. But the presence of halides does not necessarily mean the residues are corrosive, nor does the lack of halides mean that the residues are certainly safe. Reliability of flux residues are not that straightforward and need to be tested chemically to answer the question regarding corrosion. As to your concern about conformal coating over this residue, this will become more clear after answering the questions above and also discussing with your flux supplier. Most no-clean fluxes can be coated with a conformal coating without any issues. Simple compatibility testing between the flux residue and coating material can be performed to verify that the coating and flux can be safely used together.

Brian Smith
General Manager - Electronic Assembly Americas
DEK International
Mr. Smith has been supporting customers in the electronics assembly industry since 1994. His expertise is focused on solder paste printing and reducing soldering defects. He holds a BS in Chemical Engineering and an MBA in Marketing. He has authored several papers in trade magazines and at industry conferences. He is an SMTA Certified Process Engineer.

What is really meant by No Clean? "No-Clean" solder paste is a misnomer. There is no such thing as a "No-Clean" solder paste, only a low residue solder paste, whose flux residue after reflow, may not have to be cleaned from the board, to ensure board functional reliability. No clean refers to a production process in which cleaning of the final product is deemed unnecessary. Therefore the process does not containing a cleaning step in the production line. With certain production processes such as those with high throughput and low product complexity and value substrates, i.e., a continuous flow type process, a strong argument can be made, that cleaning may not be necessary, nor desirable for production cost and product quality considerations. However, for production process producing lower throughput, high product complexity and value substrates, i.e., a job shop type process, cleaning is necessary for production cost and product quality considerations. The effect of climatic stress over time can crack the amberish / transparent resin layer which is normally formed during soldering and expose hygroscopic polar Activators underneath that layer to the atmosphere where moisture would be absorbed. This would result in one of the two failure mechanisms:
  • Contamination Induced Leakage Current
  • Electrochemical Migration
Regarding conformal coating related failures:
  • Resin Part of the Flux would result in insufficient adhesiion and subsequent delamination when exposed to climtaic changes
  • Activator Part of the Flux would result in unfavorable adhesion effects and danger of delamination due to hygroscopic properties
  • Tin-Organic Compounds would result in cross-linking toxins promoting insufficient adheand creeping underneath coatings
  • Chemical facilitators for component release impacts in wetting properties and seeds for delaminatuion
  • Oligomeric parts of the substrate and solder mask interaction results in faulty processes similar to chemical facilitators.

Umut Tosun
Application Technology Manager
Zestron America
Mr. Tosun has published numerous technical articles. As an active member of the SMTA and IPC organizations, Mr. Tosun has presented a variety of papers and studies on topics such as "Lead-Free Cleaning" and "Climatic Reliability".

White residues are the bane of the electronics industry. There are dozens of different possible sources -- the boards, the solvent and fluxes, the people, the processes, the way the fluxes and solder paste are stored, and even the weather. It is very hard to determine precisely what might be causing the problem. Since the industry switched primarily to lead-free soldering materials, problems with white residues have skyrocketed. These materials use different ingredients, operate at higher temperatures and respond to solvents differently than the older products. I suspect that your situation is in this category. The residues themselves usually (but not always) are salts, which are the "activators" in the fluxes. When these salts meet heat or other chemicals, white residues can result. These residues can corrode delicate circuits and, even more urgently, greatly facilitate dendrite growth. For additional thoughts on this, email and ask them for a reprint of a very good article in SMT Magazine about PCB cleaning, "Defluxing Deja vu" by By Michael Konrad, of Aqueous Technologies Corporation. But lead-free isn't the only source of residues. For example, if somebody uses the wrong flux, or a solder paste that has not been properly stored or is past its expiration date, white residues can result. If cleaning is not performed properly, white residues can result. Even improperly cured substrates can manifest white residues after reflow. So determining the true root cause can be tricky. But an educated guess would suggest yes, most likely the white residues are corrosive AND they will inhibit the binding of the conformal coating with the surface of the board. For more details and suggested diagnostics, visit the MicroCare website or other FAQs on the MicroCare web site.

Mike Jones
Vice President
Micro Care
Mr. Jones is an electronics cleaning and stencil printing specialist. Averaging over one hundred days a year on the road, Mike visits SMT production sites and circuit board repair facilities in every corner of the globe, helping engineers and technicians work through the complex trade-offs today's demanding electronics require.

The white rsidue is typically not Halites. I'm not a chemist but we believe the white to be a type of oxide formed during the soldering process. No Clean fluxes should probably be named, "Don't Clean" flux, as there can be actives trapped below the exterior portion of this material. Mild RMA that is not exposed, will most likely not effect majority of class 2 assemblies. We don't recommend Conformal coating over NC fluxes, due to the potential and adhesion issues that can occur, more importantly. If you do need to coat these, we generally use a water soluable flux, wash them and bake prior to coating them in high reliability applications. The volitales used in many coatings can cause NC shell to break down as well, so it's important to know full compatibility of the chemical systems involved and the ambient environment of the assemby as well. If cleaning is deemed necessary, we have cleaned NC fluxes with Mutiphase chemicals with good success. Important note, is that when cleaning or even running NC flux that you monitor activity on the PCB, to ensure you dont have exposed ionic activity, and decrease the life of the assembly. We do this with indicator solution to monitor pH across areas where more concnetration of flux exists.

Rodney Miller
Capital Equipment Operations Manager
Specialty Coating Systems
Rodney is currently Operations manager at SCS coatings, Global Leader in Parylene and Liquid Coating equipment. Rodney applies his BS in Computer Integrated Manufacturing from Purdue University, along with 20+ years of Electronic manufacturing and Equipment Assembly, to direct the Equipment business at SCS Coatings. "We provide unique, value added coating equipment solutions for our customers". Including conformal, spin and Parylene coating expertise.

There are different classes of no clean flux. Ones that are a low solids containing no rosin and ones that have a higher solids content that do contain rosin. Based on the residue that you see, it sounds as if you are using the low solids type flux. In these fluxes most solder companies use organic acids to activate the solder but at the soldering temperature these acids vaporize leaving very little residue. Some fluxes in the market today utilize activators that don't clean off completely which is probably what you are experiencing. These residues usually are white as you describe and are non corrosive and non conductive. As to conformal coating I would check with your supplier to see if they indeed can be coated over this residue.

Mike Scimeca
FCT Assembly
Mike Scimeca created FCT Assembly after the purchase of Fine Line Stencil, Inc., and consists of two major operations: stencil manufacturing and the manufacturing of electronic assembly products such as solder paste, flux and solder bar.

Just to be clear, no-clean does not mean that it does not have to be cleaned. It simply means that it leaves behind less residue than higher solids content fluxes. The residue is most likely lightly corrosive. The white residue is most likely unencapsulated metal salts. Conformal coating over the top of the residue is not recommend. The potential for electro-migration is increased when flux (no-clean or otherwise) is allowed to stay on the assembly. It only takes three key ingredients to grow dendrites (metal crystals) on a board. These three ingredients are voltage, conductive / corrosive material (flux) and moisture (humidity). The flux residue forms a conductive path connecting an anode to a cathode. The results are either electrical leakage (a temporary problem) or dendrite growth (a permanent problem). Use of lead-free alloys have increased visible residues on circuit assemblies. The increased heat associated with lead-free alloys causes the no-clean flux to polymerize before it has a chance to fully encapsulate the newly created oxidized material created during the reflow process. Increasingly, assemblers are removing no-clean flux residues as part of their normal assembly process. I would recommend the same for you.

Mike Konrad
Aqueous Technologies
Mr. Konrad has been in the electronic assembly equipment industry since 1985. He is founder and CEO of Aqueous Technologies Corporation, a manufacturer of automatic de-fluxing equipment, chemicals, and cleanliness testing systems.

This is white residue around all the SMT pads and on the bottom selective wave solder areas. This is flux that is reacting with moisture and may be conductive or is benign but we must prove it. Please contact me off line and we can discuss your conditions in detail. Check out and look at the articles on white residues. Just because they are white does not indicate they have halides.

Terry Munson
President/Senior Technical Consultant
Mr. Munson, President and Founder of Foresite, has extensive electronics industry experience applying Ion Chromatography analytical techniques to a wide spectrum of manufacturing applications.

Firstly determine if this is flux residue ornot. Try applying a small amount of flux to the TOP of a bare PCB and processthis through the wave - if no residue is present then your process is set upcorrectly and you have the correct preheat settings.Secondly pass the populated PCB with residue on through aglue profile so around 130-150C for 90 seconds IF this comes out clean then itwas probably over fluxing or wrong preheat settings used.If the board still comes out white then wash in IPA IFthis doesn't remove the white powder then you have Mineral salts(Talcum Powder)from undercured solder resist left on your PCB. If this is left on when coatingthe undercured resist will cause the coating to not wet correclty and give anorange peel effect.Hope this helps.

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.

Reader Comment
Is there a problem in utilizing Perylene coating over electronic PCBAs that were assembled with NO Clean flux soldering material?
Ehud Gitai

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