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How to Clean Flux Off PCB: A Comprehensive Guide

How to clean flux off PCB ( printed circuit board) after soldering has been a matter of debate among most designers. While some technicians prefer or advise the use of Isopropyl alcohol (IPS), some fear it for its hygroscopic nature. Instead, they recommend the use of a specialized PCB flux cleaner for this purpose.

We have included all the possible solutions for post-cleaning soldered PCBs and how to use them. You will also learn why you need to clean your circuit board after soldering with flux. 

Why is it Essential to Remove Flux?

The purpose of flux in soldering is to remove oxide film that forms on the PCB surface as you solder. However, once you finish soldering, you should clean off the flux residue on your circuit board for the following reasons:

Improve the Circuit Board Reliability

Flux residues left on a PCB will lead to reliability issues. PCBs with flux residues are usually prone to problems such as dendrite growth, shorting, electrochemical migration, and parasitic leakage. Cleaning fluff residue off your circuit board improves the functionality and performance of the board. 

Improve the Aesthetic Value of the PCB

PCB assemblies with uncleaned flux have greasy residues around the solder joint. This may be a concern if the quality controller notices such residues on your circuit board. 

Flux residues also cause poor wetting and delamination. To improve the appearance of your circuit board, you need to eliminate the flux residues. 

Prevent Corrosion of the PCB and its Components

The flux residues that remain on the PCB can be acidic. If the flux stays on the board for a long time, it can erode the component of the circuit board by drawing ambient moisture from the air. Though there are other ways to prevent your PCB components from corrosion, cleaning off flux is the most reliable. 

Other ways to prevent corrosion include placing your PCB in an enclosure or applying the coating. Either way will prevent residues of the flux from trapping ambient moisture. 

The downside to the coating is that you’ll have to strip the layer off when you need to work on the PCB again. As for the enclosure, some moisture may still find its way and settle on the flux leading to corrosion.

Prevent Dendritic Growth

Flux residue contains ion contamination that facilitates the growth of dendrites when exposed to air. Dendrites are conductive, which can cause current leakage, making your circuit board less effective.

No-clean flux contains minimal ion contamination, so it may not be a concern if you use it for soldering. Otherwise, always clean off flux from your circuit board after soldering. 

How to Remove Flux Residue After Soldering the PCB

There are three categories of flux used for soldering: rosin flux, water-soluble flux, and no clean flux. Since no-clean flux doesn’t require cleaning, we’ll guide you on cleaning rosin and water-soluble flux. 

Removing Rosin Flux After Soldering

Soldering with rosin flux

You can clean off rosin flux residues after soldering in two ways:

  1. Brushing with a clean brush
  2. Soaking the board

Brushing With a Clean Brush

Requirements:

How to do it:

i. Wet the brush you are using by immersing it in isopropyl alcohol. You can gently shake your hand while holding the wet brush to drop excess solvent. 

ii. Gently scrub the circuit board with the wet brush. As you scrub, the Isopropyl alcohol solution will dissolve the flux residue. 

iii. You can repeat steps 1 and 2 until you clean away all the flux residues. But be very careful not to apply too much pressure during scrubbing and break the soldering point.

iv. Once you remove all the flux, gently wipe your clean PCB with a dry rag or toilet paper. Don’t use a material that could leave lint or fiber on the PCB. 

v. Give the PCB some time to dry out and blow the remaining flux dust, if any, with canned air. Isopropyl alcohol solution evaporates in seconds, so you don’t need to let it dry for long. 

NB: You can use many solvents in place of Isopropyl alcohol, including any other alcohol that is more than 90% proof. However, most designers prefer Isopropyl alcohol solution because it is readily available, cheap, and contains fewer chemicals. 

Removing Flux Rosin Residues by Soaking the Circuit Board

Though Isopropyl alcohol is safe if handled properly, scrubbing the circuit board may damage some components. Therefore, some technicians may prefer solvents that enable them to submerge the PCB. A good example of such solvents is poly clens. 

Requirements:

  • A container where you can immerse your PCB
  • Poly clens solution

How to do it:

Submerge the circuit board into the poly clients and stir for 30 seconds. Once you have stirred it enough, rinse it with distilled water, then you can dry it with a heat gun. There is no scrubbing, so it takes a shorter time. 

Removing Water Soluble Flux

Water soluble flux is the best if you need excellent soldering. However, its residues are highly corrosive and should be cleaned immediately after soldering.

Thankfully, you can remove these residues by simply washing them with hot water. You can add a saponifier to facilitate the cleaning process. Saponifier is alkaline and will react with the acidic residue of the flux to form soap and glycerine. 

Can I Use Acetone to Remove Flux Residues From the PCB?

Yes, but acetone is more assertive and aggressive than isopropyl alcohol. It can dissolve some of your PCB components that tolerate Isopropyl alcohol solution. Therefore you can only use it when you are sure your PCB silkscreening, solder mask, and all other PCB components are compatible with acetone. 

PCB components

PCB components

Conclusion

Removing flux off your PCB is an important finishing step after soldering. It makes your PCB look professional and different from the amateur made. In addition to improving the aesthetic look, cleaning flux residues off your circuit board increases PCB reliability and prevents corrosion and dendrite growth. Hopefully, our comprehensive guide on how to clean the flux off your PCB is helpful. 

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Hommer Zhao
Hi, I am Hommer Zhao, the founder of WellPCB. So far, we have more than 4,000 customers worldwide. If you have any questions, you can feel free to contact me! Your happiness as a customer is my priority!

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