Guide · Solder and flux
How to clean flux residue
When flux residue can stay and when it must come off, by flux type, plus a step-by-step cleanup with isopropyl alcohol and how to dry the board safely.
Short answer
First decide whether the residue has to come off. SparkFun says no-clean residue can stay but water-soluble residue must be removed,[1] and Kester says rosin residue is cosmetic unless the board runs above 200 °F (about 93 °C).[2] When you do clean, power down, scrub with isopropyl alcohol and a small stiff brush, blot while the area is still wet, and let the board dry completely before power.
Do you need to clean at all?
| Flux you used | Clean it off? | Who says so |
|---|---|---|
| No-clean | Not required. Clean it for looks or to inspect joints | SparkFun[1] |
| Rosin | Usually optional. Required if the board runs above 200 °F, where the residue melts and becomes conductive | Kester[2] |
| Water-soluble | Yes, always | SparkFun[1] |
| Unknown | Treat it as must-clean | Our advice |
No-clean residue is made to stay. Kester describes the residue of its 275 no-clean wire as extremely clear without cleaning.[3]
There are still good reasons to clean off an optional residue:
- Inspection. Residue can make a joint hard to judge, especially under a microscope.
- Sticky boards. Tacky residue collects dust and lint.
- Rework. A clean area is easier to see and to solder again.
What you need
- Isopropyl alcohol, the solvent SparkFun suggests for no-clean residue.[1] For other fluxes, use the cleaner the flux maker's data sheet names.
- A small stiff brush, such as a short-bristle flux brush or an old toothbrush kept for the bench.
- Lint-free wipes or paper towel for blotting.
- Gloves and safety glasses, to keep solvent and dissolved residue off your skin and out of your eyes.
How to clean flux residue
- Power down. Unplug the board. If it sits in a device with a lithium battery, disconnect the battery before you start: iFixit warns that a charged lithium-ion battery may catch fire if it is damaged.[4]
- Let it cool. Never put solvent on a board or a tool that is still hot.
- Work one small area at a time. Wet the residue with alcohol and scrub with the brush until it lifts.
- Blot while it is still wet. Press a wipe onto the area to soak up the dissolved residue before it can dry back onto the board as a film.
- Repeat with fresh solvent until no tacky or cloudy film remains. A dirty brush only moves residue around.
- Dry it completely. Let the alcohol evaporate, and check under chips and connectors, where liquid can sit unseen.
- Inspect the joints under good light or magnification before you power up.
For water-soluble flux, follow the cleaning process on the flux maker's data sheet. As the name says, its residue is designed to be washed off.
If a white or cloudy film appears
Clean again with fresh alcohol and a clean brush, and blot while the area is still wet. If the film stays, check the flux maker's data sheet for a dedicated cleaner. Do not scrape at it with metal tools near fine pads.
When cleaning is not worth the risk
Cleaning an optional residue is not free. Liquid can run under parts, into connectors and into switches, where it is hard to dry. If the flux is no-clean and the board works, leaving the residue is a sound choice: SparkFun says cleaning is not required.[1]
The same goes for rosin on a board that stays cool in use. Kester's view is that removing it is cosmetic.[2] Clean it when you need to see the joints, not out of habit.
Choosing a flux that saves you the job
If you dislike cleaning, choose a flux whose residue can stay. A no-clean core in your solder wire plus a no-clean gel for rework is a simple setup that leaves nothing you must remove. The flux types guide explains the three flux types and how to read a J-STD-004 code.
Next
Questions
Do I need to clean no-clean flux off the board?
No. SparkFun says cleaning and flux removal are not required with no-clean flux. Clean it with isopropyl alcohol only if you want the board to look clean or need to inspect the joints.[1]
What is the best solvent for flux residue?
For no-clean residue, SparkFun suggests isopropyl alcohol. For other fluxes, use the cleaner named on the flux maker's data sheet, and for water-soluble flux follow the maker's washing process.[1]
Why must water-soluble flux residue be removed?
Water-soluble flux is designed to be washed off after soldering, and SparkFun says to remove any residue that remains. If you cannot clean a board thoroughly, use a no-clean flux instead.[1]
Is rosin flux residue harmful to the board?
Normally not. Kester says rosin residue is non-conductive and non-corrosive and removing it is cosmetic. The exception is a board that runs above 200 °F (about 93 °C), where the residue melts, becomes conductive and must be removed.[2]
How long should a board dry after cleaning with alcohol?
Until no liquid remains anywhere, including under chips and inside connectors. Alcohol is flammable and a wet board can short, so check before you apply power rather than going by the clock.