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PCB flux plays an important role in soldering, as well as electrical contact, including removal of oxides and other particles that take part in flux removal, enhancing the flow and bonding capacity of solder. However, flux that remains in PCBs could bring corrosion, electrical leak, and ultimately circuit breakdown when not managed appropriately.
Learn the different flux types, effective cleaning methods, and solutions to prevent flux residue issues with ELEPCB!
What is Flux in PCB? Why is it Important?
Can You Solder without Flux?
Flux plays a critical role in soldering, as it removes oxides from the metal surfaces, ensuring the surface is clean for the solder to adhere well. Soldering without flux is possible, but connections turn out to be very fragile and break easily.
That is a very big problem with fragile PCB assemblies where even slight mistakes can have serious consequences. Flux improves both the quality of solder joints and eases the soldering work; hence, flux is needed for manual as well as automated soldering work.
Is Flux Conductive?
Flux is normally insulating or non-conductive electrically after activation. However, if one uses some fluxes, for instance water soluble or rosin, some will retain moisture from the air, which leads to partial conductivity and may thus give way to short circuits or other reduced performances.
The cleaning may thus be necessary when flux is applied in a given environment to remove this risk or not depending on the nature of flux and the type of environment. Flux residues should not interfere with function or life if proper cleaning techniques are performed.
Does Flux Go Bad?
Flux is not eternal. Usually, manufacturers give the expiration date and storage instructions that should be followed to the letter.
- Time will eventually decrease its cleaning and sticking abilities. This can be due to various reasons such as improper storage in hot places or in humid conditions.
- Old flux will result in weaker solder joints, more residues left, and less reliable circuits. Therefore, it is very important to renew flux timely.
Solder Paste Vs Flux
Solder paste and flux achieve somewhat related but different functions.
- Solder paste is a mixture of powdered solder alloy and flux, mainly used in surface-mount technology to assist in soldering and holding the components in place until reflow.
- Flux, when in the form of a distinct material, be it SMT or through-hole soldering process, serves to enhance the flow of solder and reduce its surface tension.
| Aspect | Solder Paste | Flux |
| Definition | A mixture of powdered solder (metal) and flux used to create solder joints. | A chemical agent used to remove oxidation and improve the flow of solder during the soldering process. |
| Composition | Contains metal particles (e.g., tin, lead, or silver) suspended in flux. | Contains cleaning agents, activators, and sometimes resin to remove oxides and improve wetting. |
| Purpose | Primarily used to deposit solder material on PCBs for reflow soldering. | Primarily used to clean and prepare the surfaces for soldering by removing oxidation. |
| Form | A thick, paste-like substance. | Available in liquid, gel, or solid (rosin) forms. |
| Application | Applied using stencils or syringes to specific areas on a PCB. | Applied with brushes, pens, or directly on joints before soldering. |
| Function During Heating | Melts and provides solder to join components and PCB pads. | Activates to clean the surface and help solder flow and adhere better. |
| Role in Soldering | Provides both the solder and flux required for the soldering process. | Works as an auxiliary material to support the soldering process by cleaning and reducing surface tension. |
| Types | Lead-based, lead-free, and no-clean solder pastes. | Rosin-based, water-soluble, and no-clean fluxes. |
| Usage Scenarios | Primarily for automated processes like SMT reflow soldering. | Used in hand soldering, wave soldering, or reflow soldering processes. |
| Residue | Leaves residue that may require cleaning (unless using no-clean solder paste). | Leaves residue that may or may not require cleaning depending on the flux type. |
| Key Advantage | Combines solder and flux in one product, making it efficient for mass production. | Flexible and versatile, suitable for various soldering techniques. |
When to Use Clean or No-Clean Flux?
Clean Flux
Clean flux is used in many sensitive applications such as aerospace, car electronics, and medical appliances. The remaining clean flux after soldering needs to be cleaned to prevent rusting or electrical problems. Cleaning is usually done by solvents or specific cleaning machines.
No-Clean Flux
No-clean flux, by name, is made so that it leaves minimal residue that does not react and won’t cause damage. Still, no-clean flux might have to be cleaned when high reliability or appealing looks become important. It’s mostly used in consumer electronics, where cost-cutting and speed of production are watched keenly.
Different Flux and Cleaning Solution Types
Cleaning solutions can include IPA, deionized water, and special chemical cleaners created to successfully dissolve certain flux residues. The type of flux and the method of cleaning will determine which cleaning solution should be used. Fluxes are categorized into three main types:
Rosin-Based Flux
Natural one is also present in flux and is derived from natural resins. It works but is highly residue-creating. It produces residue that can lead to potential issues if they are not cleaned properly. Residues require cleaning, too, by solvents such as isopropyl alcohol and IPA, and the PCB should be ensured free from any other contaminations.
Water-Soluble Flux
This type of flux is easily cleaned using water and mild soaps. It is one of the environmentally friendly solution as the flux dissolves in water. Besides, the water-soluble properties make it easier to remove any remaining flux residues. But make sure you have a thorough cleaning to prevent any corrosion risk.
No-Clean Flux
Leaves very minimal residues, hence less cleaning is required, though at the expense of considering specific application requirements. This type of flux is used in an application that is hard to clean. But still, the sensitive electronics could experience some failures because of impurities and thus cleaning is recommend with no-clean flux as well.
How to Clean Flux Off a Circuit Board?
Manual Cleaning
It is done using a brush and IPA to remove specific residues. It is useful for small tasks or prototype assemblies.
Ultrasonic Cleaning
Ultrasonic cleaning involves a mixture of sound waves with cleaning solutions; it is most effective in intricate or close boards.
Automated Cleaning Systems
Automated cleaning systems, which use immersion, sprays, or vapor-phase techniques for the uniform cleaning of large batches for mass production.
Vapor Degreasing
Vapor Degreasing is mainly used for densely packed board. It is the automated process where it utilized the solvent to eliminate the flux residue and other contaminations fir OCB. It uses tools like vapor degreasing machines, suitable solvents, and protective gear.
Aqueous Cleaning
It is semi-automated process that is eco-friendly and is popular method for flux cleaning. For effective cleaning, it uses water-based solutions with specially designed machines. It uses aqueous cleaning tool, proper cleaning agent and deionized water for rising.
Flux Residue on PCB
Flux residue can appear on a PCB as a tacky or powdery residue. This might lead to corrosion, electrical leakage, or visual imperfections. The type of residue must be identified and the suitable cleaning method used to make sure that the PCB works well. For critical applications, testing methods such as ion chromatography can check residue levels and confirm how well cleaning worked.
Organic Solvent Cleaning
Specialized Rosin Cleaner
- It can be soaked in an organic solvent, such as alcohol (ethanol or isopropanol), acetone, and other organic solvents. After a period of time, use a soft brush to gently scrub the parts with residue and blow dry with clean compressed air. This method can effectively dissolve rosin residues, but organic solvents are volatile and somewhat dangerous. The operation requires attention to ventilation and safety.
- Now there are cleaners on the market specially for cleaning rosin flux residue; those cleaners usually have strong solubility for rosin and are comparatively more friendly to the environment, harmless to people.
- According to the instructions, soak or spray the PCB with such a cleaner; afterward, it’s ready after being washed and dried.
No-clean Flux
Residue Removal
Water-soluble Flux
Residue Removal
- No-clean fluxes can be wiped with a mild organic solvent like isopropyl alcohol. Dip a clean non-woven cloth or cotton wool into a small amount of isopropyl alcohol and lightly rub the place with the residue, avoiding over-rubbing as this may damage components on the surface of the PCB.
- For thicker, no-rinse type residues, plasma cleaning technology can be used. The active particles in the plasma can be chemically reacted with the residue, which will be decomposed into small molecules and evaporated, this method of PCB surface damage is smaller and better cleaning effect.
- This is the most direct method.Use deionized or distilled water; dip the PCB in water and, by using a soft-bristled brush, gently brush the residue out. If some of the residues are not easy to take out, then add a small amount of any mild detergent to water.
- Addition of detergent should be carefully done as it should not cause corrosion of PCB. Rinse with deionized clean water after cleaning and then bake dry in a low temperature to prevent damage to some high-temperature components.
Do you need to know more about PCB solder flux?
Check out this blog:
An Essential Guide to Solder Flux in PCB Manufacturing, written by ELEPCB.
Conclusion
Flux plays a very important role in PCB assembly. It aids in the making of good and reliable solder joints. However, the associated risks with flux residue cannot be compromised. It is paramount to know the properties of clean or no-clean flux and how they should be cleaned for better performance and the life span of PCBs. Following best practices, manufacturers can make high-reliability assemblies by reducing the associated risks of flux residue.
FAQs
A1: Possible, however, the joint soldered without flux exhibits poor surface preparation, so the joint will be poor and unreliable.
A2: Usually, flux is non-conductive; however, some residues become conductive if it is exposed to moisture.
A3: Solder paste is a mix of solder metal plus flux while flux itself is used as a cleaning/wetting agent.
A4: Absolutely, flux degrades over time and must be replaced at end of life.
A5: Clean no-clean flux in high-reliability scenarios or when appearance matters.
References
- Zhan, Y., & Azarian, M. H. (2008). “Reliability of Printed Circuit Boards Processed Using No-Clean Flux Technology in Temperature-Humidity-Bias Conditions.” IEEE Transactions on Device and Materials Reliability, 8(2), 445-451.
- Kester. (2016). “Why Clean a No-Clean Flux.” International Conference on Soldering and Reliability.
- Orapi Applied (S) Pte Ltd. (n.d.). “Flux Removal: Key to Enhanced PCB Performance and Longevity.”
- NW Engineering Solutions. (n.d.). “Guide to No-Clean Flux in PCB Assembly.”
- Global SMT & Packaging. (n.d.). “Cleanliness of Electronic PCB Assemblies Leads to Device Reliability.”
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