|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.
Technical Sales Manager
BLT Circuit Services Ltd
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:
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.
Senior Manufacturing Engineer
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.
Sr Field Applications Support Engineer
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.
Technical Support Engineer
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.
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.
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.
Vice President, Technical Director
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.
Technical Expert Sales Support
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.
Engineering and Operations Management
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.
Director of Product Management
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.
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 1Ike Sedberry, ISEDS
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