What is flux PCB?

Understanding the Role of Flux in PCB Manufacturing

What is Flux?

Flux is a chemical cleaning agent used in the soldering process during PCB manufacturing. Its primary purpose is to remove oxides and other contaminants from the surfaces of the PCB and component leads, promoting better wetting and adhesion of the solder.

The Importance of Flux in PCB Soldering

Proper soldering is essential for creating reliable electrical connections between components and the PCB. Flux helps achieve this by:

  1. Removing oxides and contaminants
  2. Preventing re-oxidation during the soldering process
  3. Improving the wetting and spreading of solder
  4. Enhancing the mechanical strength of solder joints

Without flux, the solder may not adhere properly to the surfaces, leading to poor connections, short circuits, or component failure.

Types of Flux Used in PCB Manufacturing

There are several types of flux used in PCB manufacturing, each with its own characteristics and applications.

Rosin Flux

Rosin flux is the most common type of flux used in PCB soldering. It is derived from pine tree sap and is available in various activities, such as mildly activated (RMA), activated (RA), and highly activated (HA).

Flux Type Activation Level Residue Cleaning Required
RMA Low Non-corrosive, non-conductive Optional
RA Medium Corrosive, conductive Mandatory
HA High Highly corrosive, conductive Mandatory

Rosin flux is preferred for its excellent wetting properties and compatibility with most solder alloys. However, the residue left behind by activated rosin fluxes must be cleaned to prevent corrosion and electrical issues.

Water-Soluble Flux

Water-soluble flux is a type of organic acid flux that can be easily cleaned using water or other polar solvents. It is known for its excellent oxide removal capabilities and is often used in high-volume PCB manufacturing.

Advantage Disadvantage
Easy to clean May cause corrosion if not cleaned properly
Excellent oxide removal Higher cost compared to rosin flux
Suitable for high-volume production Requires special handling and storage

Water-soluble flux is an ideal choice when thorough cleaning of the PCB is required after soldering.

No-Clean Flux

No-clean flux is designed to leave minimal, non-corrosive residue after soldering, eliminating the need for post-soldering cleaning. This type of flux is widely used in PCB manufacturing due to its convenience and cost-effectiveness.

Advantage Disadvantage
No post-soldering cleaning required May leave a clear residue
Cost-effective Not suitable for high-reliability applications
Faster production cycle Limited oxide removal capability

No-clean flux is perfect for applications where post-soldering cleaning is not feasible or necessary, such as in consumer electronics.

Flux Application Methods in PCB Manufacturing

Flux can be applied to PCBs using various methods, depending on the manufacturing process and the type of flux used.

Foam Fluxing

Foam fluxing involves the use of a foam fluxer machine that generates a foam head of flux. As the PCB passes through the foam, the flux is applied evenly to the surface. This method is suitable for both wave soldering and selective soldering processes.

Spray Fluxing

Spray fluxing uses a pressurized spray system to apply a fine mist of flux onto the PCB surface. This method provides precise control over the amount of flux applied and is ideal for selective soldering applications.

Brush Fluxing

Brush fluxing is a manual method where flux is applied using a brush. This method is suitable for small-scale production or rework purposes, as it allows for targeted application of flux to specific areas of the PCB.

Wave Fluxing

In wave soldering, the PCB is passed over a wave of molten solder. Just before the PCB enters the solder wave, it passes through a wave of liquid flux, ensuring even distribution of the flux on the bottom side of the board.

Best Practices for Flux PCB Manufacturing

To ensure optimal results and reliability of flux PCBs, consider the following best practices:

  1. Select the appropriate flux type based on the PCB application, soldering process, and cleaning requirements.
  2. Control the amount of flux applied to prevent excessive residue or insufficient coverage.
  3. Maintain proper storage and handling of flux to prevent contamination and ensure consistent performance.
  4. Follow the manufacturer’s guidelines for flux application, soldering temperature, and cleaning procedures.
  5. Regularly maintain and calibrate flux application equipment to ensure consistent results.
  6. Use appropriate personal protective equipment (PPE) when handling flux to minimize health risks.
  7. Properly dispose of used flux and cleaning solvents in accordance with local environmental regulations.

By adhering to these best practices, PCB manufacturers can produce high-quality, reliable flux PCBs that meet the required performance standards.

Frequently Asked Questions (FAQ)

1. What happens if I don’t use flux when soldering a PCB?

Without flux, the solder may not properly wet and adhere to the surfaces of the PCB and component leads. This can result in poor solder joints, leading to electrical issues, short circuits, or component failure.

2. Can I use the same flux for all soldering applications?

Different soldering applications may require specific types of flux. For example, high-reliability applications may demand the use of rosin flux, while high-volume production may benefit from water-soluble flux. It’s essential to select the appropriate flux type based on the specific requirements of your PCB application.

3. How do I clean my PCB after soldering with flux?

The cleaning method depends on the type of flux used. Rosin flux residue can be cleaned using isopropyl alcohol or other solvents, while water-soluble flux can be cleaned using deionized water. No-clean flux typically does not require post-soldering cleaning.

4. Can flux cause damage to my PCB?

If left uncleaned, some types of flux residue, such as those from activated rosin flux or water-soluble flux, can cause corrosion and degrade the performance of the PCB over time. It’s crucial to clean the PCB thoroughly after soldering or use a no-clean flux when appropriate.

5. How do I store flux to maintain its effectiveness?

Flux should be stored in a cool, dry place away from direct sunlight and heat sources. Ensure that the containers are tightly sealed to prevent contamination and evaporation of solvents. Always follow the manufacturer’s storage guidelines to maintain the flux’s optimal performance.


Flux PCB is an essential aspect of PCB manufacturing, as it ensures proper soldering and reliable electrical connections between components and the PCB substrate. Understanding the different types of flux, their applications, and best practices is crucial for producing high-quality, dependable PCBs.

By selecting the appropriate flux type, applying it correctly, and following proper cleaning and maintenance procedures, PCB manufacturers can optimize their soldering processes and deliver PCBs that meet the highest standards of performance and reliability.

As the electronics industry continues to evolve, the importance of flux PCB will only grow, driving innovation and advancements in PCB manufacturing techniques and materials.