Ask the Experts
March 28, 2022 - Updated
April 18, 2018 - Originally Posted

Application Using No-Clean and Water Soluble Fluxes

If we use a no-clean solder paste for SMT, is it acceptable to use a water-soluble solder wire for subsequent processes (soldering through hole components by hand, not touching up the original SMT)?

We have some difficult assemblies where water-soluble wire just works better for hole fill.


Expert Panel Responses

Wouldn't be advisable as the No Clean flux residue stands a very good chance of going white when it comes into contact with the Water used for cleaning the Water Soluble from the assembly.

The Water Wash flux residue needs to be cleaned 100% or you could face reliability issues if you leave any of the Water Wash flux on the assembly.

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.

You'd need to be absolutely sure that you removed all of the water soluble flux residue, as any remaining residue would be a high reliability risk.

The challenge would be to do so without damaging the no-clean residues. Partially removed no-clean residues may represent a higher risk for creating ionic pathways that can lead to latent failure (dendritic growth), and will also present a cosmetic issue.

An alternative approach would be to optimize the through-hole soldering process to reduce/eliminate the fill problems. Some ideas:
  • Ensure that the soldering stations have adequate heat transfer capability
  • Ensure that the tips used are not too small for the application
  • Ensure that operators are trained in proper technique to ensure good heating
  • Ensure that inspectors are not over-aggressive in applying the required criteria
  • If possible, ensure that ground & power planes have adequate thermal relief
  • Consider pre-heating difficult assemblies to approx. 50 degrees C
  • Consult with your solder wire supplier on flux formulation and flux percentage for your application

Fritz Byle
Process Engineer
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

One rule I follow is; "never mix flux chemistries". The activity of the organic flux used on water-soluble wire is much higher than no-clean. You need be 100% sure that all water-soluble was removed or else those same residues will react in the solder joint and create reliability issues in your product.

One alternative is to use the intrusive reflow method. In the intrusive reflow process you deposit solder paste via your stencil around the through-hole. This allows you to place the through-hole component before reflow and to have the whole assembly soldered in one reflow pass. If you find that this process still not enough you may try to add solder slugs (preforms) in order to increase the solder volume.

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.

Using Water soluble flux to touch up a board that has been soldered with no clean solder paste and or flux is not good practice. The no clean flux will leave a rosin or resin-based residue on the board. When heating the board to repair it with a water-soluble flux you will soften the no clean residue allow the organic water-soluble flux to be absorbed by the no clean flux.
  • It is best to always use the same classification and family flux when building a board. No Clean (ROL0) with NC (ROL0). WS (ORH1) with WS (ORH1).
  • If required to use two different families and classifications all residues but be washed after each process. In example, the no clean residues would need to be removed prior to doing repairs with WS flux in this case. The board will require a second cleaning after the repair is completed. This is not a practical or safe process.

Michael Kaminsky
Sr Field Applications Support Engineer
Kester Inc.
Mr. Kaminsky has 30+ years of circuit board soldering assembly experience along with a patent for wave solder VOC flux process.

If you clean off all the residues, no-clean and water-soluble, there would likely not be any issue.

If you are going to clean with water only, for the sake of removing the water-soluble flux, you will need to understand what water exposure will do to your no-clean flux residue, both in terms of cosmetics and electrical reliability.

If the water-soluble flux is making contact with the no-clean flux residue, and you are specifically not removing the no-clean flux residue, you will need to understand what the mingling of the fluxes does to the electrical reliability of the no-clean flux residue.

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.

You could use a water soluble wire solder for subsequent processes but the flux residue must be washed off after processing or it may cause issues like corrosion or dendritic growth. The no clean solder paste flux could be adversely affected by the water wash process.

No clean fluxes tend to turn white in color with exposure to water due to their insolubility in water. Some of the no clean flux ingredients could become "unsafe" to leave on the circuit board due to being partially washed with water.

It would be best to solder with a higher activity no clean wire solder, or possibly use a no clean gel flux in addition to the no clean wire solder to aid in soldering. Soldering materials suppliers can recommend compatible combinations of fluxes to use for these type of applications.

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.

Use of water-clean flux defeats the purpose of no-clean and also jeopardizes reliability. The water-soluble flux has to be cleaned to prevent subsequent corrosion. Exposing the no-clean residue to water may result in corrosion and also a messy looking assembly. Many no-cleans, when exposed to water, will result in a white residue.

There are cleaning solutions that will rid the board of no-clean flux deposits. First, the cleaner must be tested and qualified for your assembly. Reliability tests should be done to verify that no corrosion will occur. Low headroom components on test boards should be removed after cleaning to verify that the cleaning process was adequate to flush flux residues from under components.

Of course, all this negates the use of no-clean in the first place. My recommendation would be to find a flux-core no-clean wire that is aggressive enough for your application and compatible with your original assembly no-clean solder paste and/or wave solder flux and meets your reliability requirements. Take full advantage of the benefits of no-clean and make it work for you.

Gary Freedman
Colab Engineering
A thirty year veteran of electronics assembly with major OEMs including Digital Equipment Corp., Compaq and Hewlett-Packard. President of Colab Engineering, LLC; a consulting agency specializing in electronics manufacturing, root-cause analysis and manufacturing improvement. Holder of six U.S. process patents. Authored several sections and chapters on circuit assembly for industry handbooks. Wrote a treatise on laser soldering for Laser Institute of America's LIA Handbook of Laser Materials Processing. Diverse background includes significant stints and contributions in electrochemistry, photovoltaics, silicon crystal growth and laser processing prior to entering the world of PCAs. Member of SMTA. Member of the Technical Journal Committee of the Surface Mount Technology Association.

Flux chemistry has been continuously changing over the years to accommodate different surface conditions and oxides. Surface mount processes require the solder paste, the flux and the viscosity agents to work together to secure the components placed in the paste itself and to prepare all the exposed metal surfaces, including the solder spheres, the component terminations and the board land areas. As can be seen the flux is being asked to do many things and above and beyond all of this, we are also asking the residues to be benign and not impact the functional operation of the product.

Now the question is whether it is ok to use water soluble flux residues during the manually soldering of through hole components with the hopes of not touching the SMT components.

I would encourage all people who are building electronic products to follow the requirements of J-STD-001 Requirements for Soldered electrical and Electronic Assemblies, as defined in paragraph 3.3 Flux and 3.3.1 Flux Application for compatibility issues. Many times the various materials and chemistries are not compatible and the results are detrimental to the overall performance of the product.

The use of Water Soluble flux or an Organic Acid (OR) flux with an activator is not allowed for Class 3 types of products. The reasoning being the inability to clean off the residues from the surfaces of the board, which if not removed could lead to dendritic growth which will impact the functional operation of the product. Secondly the manual cleaning process is not quite as robust as going through a process control cleaning process and will more likely just spread the flux activators over a larger portion of the board.

If the manual cleaning medium or solvent does come into contact with the residues from the Surface Mount process, it will change the physical appearance of those residues to a whitish color, and this will end up being cosmetically unacceptable.

To conclude, verify the fluxes are compatible per the requirements of J-STD-001, verify the Objective Evidence of Material Compatibility per Appendix C of J-STD-001. Don't depend upon manual cleaning to remove all the OR flux residues from the surfaces of the board, and review the product for any areas where flux can be entrapped and be unable to be removed such as beneath components.

Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

From my experience this is acceptable however a water soluble flux cored wire needs to be effectively cleaned. It must not remain on the PCB. These fluxes are very active and corrosion will be the result if these residues are allowed to stay.

You do not indicate in your post if you are currently washing your "no-clean" solder paste flux residues. If you are great! Processing these boards after your subsequent processing will also be required.

An important note is that these water-soluble flux residues do not clean well using common bench top cleaning solvents, they are a poor match. Active/corrosive components of the flux will still remain on the PCB if these solvents are used.

The water-soluble flux core residues need to be cleaned using an aqueous wash media and process to insure no active flux residues are remaining. Don't hesitate to contact me if you have any further questions.

Charlie Pitarys
Technical Expert Sales Support
Kyzen Corporation
Charlie Pitarys has over thirty years of industry experience and has been with KYZEN for twenty-one years. Charlie is a former Marine and a retired Sargent First Class in the Army Reserves. His previous employers include Hollis and Electrovert. Charlie continues to use his expertise on cleaning processes and machine mechanics to help KYZEN customers and partners improve their cleaning operations.

The process you describe can be done but it would make sense to not wash the no clean solder residue. Breaking the surface of no clean flux residue by washing is a risk that can lead to latent failures.

I would recommend to use a water soluble SMT process and continue with the TH the same way should all the components be fit to the cleaning process.

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

This process is NOT recommended. While there's not enough information to know all of the liabilities, there are several areas of concern. 1) how is the WS residue being removed as it must be completely removed. Spot cleaning of WS flux residue is not recommended as it is not adequate to remove the highly active WS residue.

If being put through a washer, either in-line or batch, the NC residue can be cross contaminated by the WS residue in the wash and absorb ionic species, changing the residue electrochemical properties.

Also, exposure of NC residue to water can turn it white and can also change other properties. Cleaning all residue, both NC and WS with a saponified wash could be implemented successfully, but would require extensive process validation to ensure cleanliness requirements are met and maintained.

I would recommend speaking to your wire vendor to see if a more aggressive/higher flux percentage No Clean wire is available or using a No Clean flux to augment/compliment the flux in the wire. I would also speak to your soldering iron vendor. Many hand soldering issues can be addressed with tip design and process modification.

Tim O'Neill
Director of Product Management
Timothy O'Neill is the Director of Product Management for AIM Solder. AIM Solder is a leading global manufacturer of assembly materials for the electronics industry. Mr. O’Neill has 25 years of industry experience is a Certified IPC Specialist.

Mr. O’Neill’s responsibilities include developing product and technical information; he is a technical writer and presenter for industry trade publications and events and has been recognized as a Speaker of Distinction by the SMTA.

If you plan on washing the water-soluble PTH flux anyway, ie, washing every board in an in-line or submersible washing process, then why not use water-soluble flux for the SMT portion also? You are not saving any money and certainly not gaining any other benefit using the no-clean.

Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics
Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.

Reader Comment
Obviously there are a number cautions to be addressed if you attempt to mix fluxes. The simple answer is NO- it is not acceptable to mix flux technologies on the same board...even in class 1
Ike Sedberry, ISEDS

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