After soldering, every printed circuit board (PCB) is left with flux residue — a sticky, resin-based film that helped the solder flow but now sits on the board as a contaminant. Left uncleaned, this residue can attract dust, trap moisture, and interfere with the long-term reliability of the assembly. An ultrasonic cleaner for PCB flux removal has become one of the most widely used methods for clearing this residue quickly and consistently, without the manual scrubbing or harsh solvents that older cleaning methods relied on.
This guide walks through what PCB flux is, how ultrasonic PCB cleaning works, a step-by-step cleaning process, and the key factors that determine good results.
Ultrasonic PCB cleaning uses high-frequency sound waves passed through a cleaning solution to generate microscopic cavitation bubbles. These bubbles collapse against the board’s surface and lift flux residue from components, pads, and tight gaps that are difficult to reach by hand — typically without the abrasive contact of manual cleaning.
Flux is a chemical compound applied during soldering to remove oxidation from metal surfaces and help solder bond properly. Once soldering is complete, the flux has done its job — but the residue it leaves behind can still cause problems if it isn’t cleaned off.
This is why PCB flux removal is treated as a standard step in electronics manufacturing rather than an optional one.
An ultrasonic cleaner works by generating high-frequency sound waves — typically in the tens of kilohertz range — through a liquid cleaning solution. These waves create and collapse millions of tiny vacuum bubbles in a process called cavitation. As the bubbles collapse against the surface of the PCB, they help dislodge flux residue from pads, component leads, and the narrow gaps beneath surface-mounted parts.
Because the cleaning action happens throughout the liquid, an ultrasonic cleaner for PCBs can reach areas that are difficult to access with a brush or wipe, which is one of the main reasons this method is preferred for boards with dense component placement.
Check boards for any components that are sensitive to liquid immersion or are not rated for ultrasonic cleaning. Group boards with similar cleaning requirements together where possible.
Choose a solution suited to the type of flux used during soldering (see the next section for guidance).
Place PCBs in the basket with adequate spacing so the cleaning solution and ultrasonic waves can circulate evenly around each board. Avoid stacking boards directly on top of one another.
Configure the machine according to the flux type and board sensitivity. These settings are covered in detail below.
Start the cycle and allow the cavitation process to work through the set duration without interrupting it partway.
Rinse with clean, deionized water (or a dedicated rinse stage, where the equipment supports it) to remove any remaining solution residue.
Dry boards completely before further handling, testing, or packaging. Trapped moisture in connectors or under components should be avoided.
Solution selection matters because different flux types — rosin-based, water-soluble, and no-clean fluxes — respond differently to cleaning agents. Using an unsuitable solution can leave residue behind or, in some cases, react poorly with certain board materials.
| Flux Type | Recommended Solution Approach | Notes |
| Rosin-based flux | Solvent-based or saponifier cleaning solutions | Requires a solution capable of breaking down rosin residue |
| Water-soluble flux | Deionized water-based solutions | Should be cleaned promptly, as residues can be more reactive if left too long |
| No-clean flux | Mild aqueous or specialty solutions, only if cleaning is required | Not all no-clean flux residues need removal; confirm with your process requirements |
Always confirm solution compatibility with your board’s laminate, components, and any conformal coatings before running a full batch.
Consistent results depend on how well frequency, temperature, cleaning time, and ultrasonic power are matched to the boards being cleaned.
| Factor | What It Affects | General Guidance |
| Frequency | Bubble size and cleaning intensity | Higher frequencies produce finer cavitation, often preferred for delicate or densely populated boards |
| Temperature | Solution effectiveness and cleaning speed | Should stay within the range recommended for the specific cleaning solution used |
| Cleaning Time | Thoroughness of residue removal | Set according to flux type and soil level; excessive time is unnecessary and should be avoided |
| Ultrasonic Power | Strength of cavitation action | Should be matched to component sensitivity to avoid stress on fine leads or delicate parts |
These settings are typically established through testing on representative boards rather than applied as a single fixed formula across all products.
Compared with manual wiping, brushing, or spray cleaning, ultrasonic cleaning offers a few practical advantages for PCB flux removal.
That said, ultrasonic cleaning is not automatically the right fit for every board — some components or assemblies may require alternative methods, which is why an initial compatibility check matters.
The right ultrasonic PCB cleaning machine depends largely on three factors: board size, production volume, and the specific cleaning requirements of the flux and components involved.
This is where working with an established PCB cleaning machine manufacturer helps — matching the right tank size, frequency configuration, and control features to your specific boards rather than using a generic setup. Hisashi Industries designs industrial ultrasonic cleaning equipment for electronics and PCB applications, with configurations built around the production scale and board types manufacturers actually work with. If you’re evaluating equipment for flux cleaning from circuit boards, it’s worth reviewing your board specifications and volume needs against the available system options before making a decision.
Most standard PCBs tolerate ultrasonic cleaning well, but boards with certain sensitive components, electrolytic capacitors, or unsealed parts may require evaluation before cleaning. Checking component ratings beforehand is recommended.
Cycle time varies based on flux type, soil level, and equipment settings. It’s generally determined through initial testing on representative boards rather than a single fixed duration.
Ultrasonic cleaning can remove no-clean flux residue when required, though not all no-clean fluxes need to be cleaned off. This depends on the product’s end-use requirements.
Temperature should stay within the range recommended for the specific cleaning solution in use, as going outside that range can reduce cleaning effectiveness or affect the solution itself.
Higher production volumes generally benefit from multi-stage ultrasonic systems with separate cleaning, rinsing, and drying stages, rather than a single-tank setup designed for lower-volume use.
Removing flux from PCBs is a necessary step for long-term board reliability, and an ultrasonic cleaner offers a consistent, repeatable way to do it — reaching areas that manual methods often miss. Getting the right results comes down to matching frequency, temperature, time, and solution to your specific boards and flux type.
If you’re looking to set up or upgrade your PCB cleaning process, explore Hisashi Industries’ range of industrial ultrasonic cleaning systems at hisashiultrasonic.com to find a configuration suited to your board size and production volume.
Have questions about ultrasonic PCB cleaning or need help choosing the right system for your production line? Get in touch with the Hisashi Industries team.