What is flux cleaning (deflux)?

What Is PCB Cleaning (Flux Cleaning)? Its Purpose and Role

PCB cleaning is the process of removing flux residue, ionic contamination, solder balls, oils, and other contaminants remaining on a printed circuit board after soldering in order to help maintain the quality and reliability of electronic equipment.
Are PCB cleaning and flux cleaning different? The terms are often used almost interchangeably, but PCB cleaning can cover not only flux residue but also fingerprints, oils, dust, and various contaminants introduced during manufacturing.

As explained on our What Is Soldering Flux? page, soldering flux plays an important role in improving soldering quality.

However, flux components remaining after soldering can affect product reliability under certain conditions. Flux cleaning is therefore an effective way to prevent or address problems associated with flux residue.

This page explains why flux is cleaned, representative PCB and flux cleaning systems, how to improve cleaning performance, and how to evaluate cleanliness after cleaning.

Why Is PCB Cleaning (Flux Cleaning) Necessary?

What problems can occur if flux residue is not removed?
The primary purpose of flux cleaning is to help prevent electrical and physical problems such as the following.

	Reduced Insulation Reliability Ionic components in flux residue can promote ionic migration under high-humidity conditions, potentially causing metal deposition between conductors and resulting in short circuits.
	Corrosion Under high-temperature and high-humidity conditions, ionic substances such as halide ions in flux residue can contribute to corrosion. Galvanic corrosion may also occur where dissimilar metals are in contact.
	Connection Failures Dry flux residue can have high electrical resistance. If it adheres to electrical contact areas such as connector contacts, it may interfere with electrical continuity.
	Leakage or Short Circuits Caused by Residual Solder Balls Solder balls generated during solder paste printing or preheating can remain on PCB surfaces together with flux residue and may contribute to leakage or short circuits.
Other potential problems include: Reduced wire-bonding strength Poor adhesion or curing interference with molding resins and conformal coatings Poor underfill penetration, filling, or curing Outgassing Interference with visual inspection or poor contact with in-circuit test equipment Flux cleaning can help reduce the risk of these problems.

Applications Where PCB Cleaning (Flux Cleaning) May Be Required

No-clean fluxes are widely used today, which can lead to the question: “Is PCB cleaning still necessary?”

However, cleaning may still be required in high-reliability applications such as automotive electronics, medical devices, communication equipment, and industrial electronics. Even small amounts of flux residue or ionic contamination can affect insulation performance, corrosion resistance, or migration behavior depending on the design and operating environment.

Cleaning may also be required when downstream processes such as wire bonding, conformal coating, or underfill application depend on clean surfaces for reliable bonding and adhesion.

The need for cleaning should therefore be determined not only by the type of flux but also by product application, downstream processes, operating environment, and required reliability.

Application	Typical Requirements
Automotive Electronics	Long-term reliability and resistance to demanding temperature environments
Medical Devices	High reliability and prevention of functional failures
Communication Equipment	Stable signal performance and insulation reliability
Industrial Equipment	Long service life and maintainability

For these applications, PCB cleaning is not simply intended to improve appearance. It can be an important process for supporting the quality and reliability of electronic assemblies.

Key Points When Selecting a PCB Cleaning (Flux Cleaning) System

Evaluating a PCB cleaning system solely by cleaning power can create operational problems later. Cleaning-agent cost, drying time, equipment safety measures, waste-liquid treatment, material compatibility, and running costs should also be considered.

As assembly density increases and environmental requirements evolve, cleaning processes are increasingly evaluated as complete systems, including cleaning, rinsing, and drying, rather than by cleaning chemistry alone. Important considerations include cleaning performance, environmental impact, stable drying, process control, and component compatibility.

KAKEN TECH designs and develops both cleaning agents and cleaning equipment, enabling us to propose complete cleaning processes instead of cleaning chemicals alone. Through cleaning tests and evaluation, we can propose cleaning conditions suited to your PCBs and manufacturing requirements.

Types and Characteristics of Flux Cleaning Systems

When selecting a flux cleaning method, it is important to consider not only flux removability but also environmental impact, safety, operator workload such as bath replacement and waste treatment, and running costs.
The table below compares representative flux cleaning systems.

Water-Containing Alcohol Flux Cleaning System

Cleaning	High-boiling glycol ether-based cleaner
Rinsing	Water-containing alcohol rinse
Drying	Drying performance comparable to IPA

Water-containing alcohol flux cleaning systems combine high cleaning performance with relatively fast drying and are used for high-density PCB assemblies and applications requiring high cleanliness. A glycol ether-based cleaner removes rosin and other flux components, while the water-containing alcohol rinse helps remove ionic contamination and residual cleaning-agent components.

Compared with systems that use a pure-water rinse, shorter drying times can be achieved more easily. In KAKEN TECH's closed-process design, the rinse agent can be regenerated and circulated within the equipment, helping reduce wastewater generation. Appropriate process design combining the cleaner, equipment, and rinse agent supports stable PCB cleanliness and product quality.

Semi-Aqueous Flux Cleaning System

Example System

Cleaning	Cleaner containing solvents such as glycol ethers, surfactants, and water
Rinsing	Purified water
Drying	Hot-air drying; generally slower than IPA-based drying

Semi-aqueous systems can offer good handling characteristics depending on the formulation. When the cleaning agent contains water, concentration and water content may require process control. As contamination accumulates and bath composition changes, redeposition can occur. Water-rinse processes can also generate wastewater and may require appropriate wastewater treatment.

Hydrocarbon Flux Cleaning System

Example System

Cleaning	Hydrocarbon cleaner
Rinsing	Clean solvent or solvent vapor
Drying	Vacuum or reduced-pressure drying may be used

Hydrocarbon cleaners generally dissolve resinous flux components such as rosin effectively, but their ability to remove ionic contamination can be limited. Depending on the formulation and operating conditions, flammability controls and equipment safeguards may be required. Compatibility with plastics, elastomers, adhesives, and mounted components should also be evaluated before use.

Brominated / Fluorinated Solvent Flux Cleaning System

Example System

Cleaning	Brominated or fluorinated solvent cleaner
Rinsing	Clean solvent vapor
Drying	Natural evaporation / vapor-process drying

These solvent systems have been used for degreasing and certain flux-cleaning applications and can offer excellent drying characteristics. Their cleaning performance for modern lead-free flux residues varies with chemistry and process conditions. Because some solvent chemistries have environmental, health, or regulatory considerations, applicable local requirements and the latest SDS should be reviewed when evaluating these systems.

Main Contaminants Removed by PCB Cleaning

PCB cleaning targets more than flux residue. Various contaminants can be introduced during manufacturing and storage, so cleaning chemistry and process selection should reflect the type of contamination present.

Contaminant	Typical Source	Potential Problem
Flux Residue	Soldering Process	Insulation or adhesion problems
Activator Components / Ionic Contamination	Flux Components / Manufacturing Environment	Migration or corrosion
Solder Balls	Printing / Reflow Process	Leakage or short circuits
Unreacted Flux Components	Heating Conditions / Flux Type	Coating or bonding problems
Oils / Fingerprints	Operator Handling	Coating adhesion problems
Dust / Foreign Matter	Manufacturing / Storage Environment	Contact problems

The optimum cleaning system depends on the contamination involved. It is important to characterize the contamination and select the cleaning chemistry and equipment as an integrated process.

The optimum method also depends on PCB structure, mounted components, subsequent processes, and required reliability.

Because KAKEN TECH designs and develops both cleaning agents and cleaning equipment, we can propose complete cleaning processes based on the residues to be removed and the required cleanliness level.

Through cleaning tests and evaluation, we support the selection of cleaning agents, equipment, and process conditions suited to each PCB and manufacturing process.

How Should PCB Cleanliness Be Evaluated After Flux Cleaning?

PCB cleaning does not end when the cleaning cycle is complete. It is important to confirm that the required cleanliness level has actually been achieved.

Evaluation Method	What It Evaluates
Visual Inspection	Presence of visible flux residue or foreign matter
Ionic Contamination Measurement	Amount of residual ionic contamination
Insulation Resistance Testing	Electrical reliability
Surface Analysis	Composition of residual material

For high-reliability applications, quantitative evaluation may be appropriate even when contamination is not visible.

For more information, see Methods for Evaluating Cleaning Performance After Flux Cleaning.

Cleaning Methods Used to Improve PCB Cleaning Performance

Cleaning efficiency can be improved by combining the dissolving power of the cleaning agent with appropriate physical action. Many flux cleaning systems use one or more of the following methods.

Cleaning Method	Immersion	Agitation / Rotation
		Liquid Circulation / Jet Flow / Boiling
		Ultrasonic
		Bubbling
	Spray Cleaning
	Vapor Cleaning
	Brushing
	Other Methods

Immersion Cleaning

The workpiece is immersed in cleaning solution so that flux residue can be dissolved by the cleaning agent. Because simply soaking the workpiece does not always promote sufficient diffusion of dissolved contamination, immersion cleaning is often combined with physical action such as the following.

Jet Flow
Cleaning solution circulated by a pump is directed toward workpieces while they are immersed. This can provide relatively uniform cleaning throughout the tank, and turbulent flow can promote removal and dispersion of dissolved contamination.

Ultrasonic Cleaning
Ultrasonic energy is transmitted through the cleaning liquid, and cavitation and acceleration effects help remove contamination. It can be particularly effective for removing solder balls and contamination from fine features. Some components may be sensitive to ultrasonic energy, so frequency, power, exposure time, and component compatibility should be evaluated.

Spray Cleaning

Cleaning solution is sprayed onto the workpiece in a shower-like pattern.

Advantages include applicability to many cleaning chemistries and the ability to increase mechanical cleaning action by adjusting spray pressure. However, highly volatile or flammable cleaning agents may require enclosed equipment, ventilation, vapor control, and other safety measures. Excessive spray pressure can damage sensitive assemblies, atomized cleaner can increase solvent loss, and shadowed areas that are not reached effectively by the spray may be more difficult to clean.
Always confirm the cleaner's SDS, equipment compatibility, and local requirements before use.

For small quantities or localized cleaning after repair, handheld flux-cleaning sprays may also be used. These products are commonly marketed as “flux cleaning spray,” “flux remover,” or “flux cleaner.” The terminology differs, but the basic purpose is to remove flux contamination.

Vapor Cleaning

Cleaning-agent vapor condenses on the workpiece, and the condensed liquid dissolves contamination as it flows from the surface. Vapor cleaning can be used with compatible solvent systems and equipment designed for the selected chemistry. Depending on the solvent, vacuum or closed-system operation may also be used.

Brushing

The surface of the workpiece is physically scrubbed with a brush, pad, or similar tool to remove contamination.
Brushing provides strong mechanical action but can damage the substrate and may create secondary contamination from shed fibers or abrasion debris. It also cannot effectively reach areas where the tool cannot physically contact the surface, such as fine gaps and spaces under components.

References

Standard Microsoldering Technology, 3rd Edition
Edited by the Microsoldering Education Committee, Japan Welding Engineering Society
Nikkan Kogyo Shimbun, Ltd.

Frequently Asked Questions About PCB Cleaning (Flux Cleaning)

Related Technical Information & Products

	Technical Solutions Methods for Evaluating Cleaning Performance After Flux Cleaning Introduces representative analysis and evaluation methods for confirming whether flux residue and ionic contamination have been sufficiently removed after cleaning.
	Technical Solutions What Is Flux Residue? Explains the electrical and physical problems that can be associated with flux residue after soldering and measures for managing or removing residue appropriately.
	Product Information Flux Cleaner WS-923 WS-923 powerfully removes flux residue during manual PCB cleaning and hardened flux contamination from equipment such as reflow ovens.
	Product Information Flux Cleaning Chemicals MICRO CLEAN & MARKLESS® Introduces KAKEN TECH's flux cleaning process combining MICRO CLEAN glycol-based cleaner with MARKLESS® water-containing alcohol rinse agent for high cleaning performance and stable finishing quality.
	Product Information High-Performance, Low-VOC Flux Cleaning Chemicals MICRO CLEAN ECO & MARKLESS® ECO Introduces MICRO CLEAN ECO low-VOC flux cleaner and MARKLESS® ECO rinse agent for precision cleaning of high-density PCB assemblies while reducing VOC emissions.
	Product Information Flux Cleaning Equipment MICRO CLEANER® MICRO CLEANER® is cleaning equipment designed to provide stable removal of flux contamination from printed circuit boards and electronic components, supporting quality and reliability in high-density assembly.