A Guide to No Clean Flux

Small, complex PCBs (printed circuit boards) used in modern electronics are often fabricated using no-clean solder and pastes. The original intention of no-clean products was that the no-clean flux residue could stay on the PCBs indefinitely and not negatively impact the lifespan or performance of the PCBs.  No-clean solders and pastes were supposed to reduce both production time and labor by eliminating the need for cleaning circuit boards after reflow. Ironically, this time and cost saving idea is now proving to be more of a hindrance than a help. 

Millions of electronic assemblies are made using no-clean solder pastes. For non-critical applications like inexpensive toys or single-use, disposable items, this works fine. However, for higher-reliability electronics used in the medical, aerospace or defense industries, the cleaning becomes critical. In other words, if a PCB were to fail due to flux residue contamination, it might result in some form of human catastrophe.  Cleaning these PCBs is not optional. It is essential to ensure their long-term performance, functionality and reliability.

Many of these high-reliability PCBs are also growing in complexity while shrinking in size. They are multi-layered and tightly packed with bottom termination components like BGAs, CSPs, MLFs, QFNs, and D-Paks. Low standoff or zero clearance components are now the norm.  This trend in circuit board miniaturization, complexity and high density may cause a greater likelihood for problems if the PCBs, especially those produced using no-clean solder pastes, are not properly cleaned. 

Contaminated PCBs are susceptible to many types of trouble including parasitic leakage, electrochemical migration, shorting and dendrite growth. Today, the need to clean no-clean flux residue is no longer optional, it is essential to ensure long-term PCB performance, functionality and reliability.

This comprehensive guide delves into the realm of no-clean flux in soldering, encompassing its origins, progression, and methodologies associated with no-clean solder fluxes. It discusses the attributes of no-clean flux and provides insights into effectively cleaning it using specialized products such as VeriClean No Clean Flux Remover, along with guidance on sourcing these items from local retailers or online platforms

Origins of No-Clean Flux

To comprehend the necessity for cleaning no-clean flux, we must rewind to the late 1980s. Prior to this era, electronics manufacturers heavily relied on cleaning flux and solder paste residues using aerosol spray solvents containing chlorofluorocarbons (CFCs). However, the adverse environmental impact of CFCs, particularly their role in ozone layer depletion, spurred global action. In 1987, the Montreal Protocol prohibited the use of CFCs, prompting a transformation in PCB manufacturing practices.

Rather than seeking alternative cleaning solvents, the electronics industry pursued a different avenue. They developed solder fluxes that obviated the need for cleaning altogether. Thus, no-clean solder flux was conceived.

The Challenge with No-Clean Flux

The success of these new no-clean fluxes hinges on proper heating. No-clean flux relies on precise heating processes to oxidize activators into benign, non-ionic, or low-ionic residues, encapsulating them to prevent corrosion of electronic components or connectivity issues. Adhering to the solder paste manufacturer's recommended heating profile is crucial. Inadequate heat exposure can leave behind detrimental residues, while excessive heat may cause flux residue to darken and solidify, rendering it more challenging to clean.

In automated surface mount technology (SMT) environments, where heating profiles can be meticulously controlled, no-clean fluxes perform admirably, leaving benign residues on PCBs without necessitating cleaning. However, challenges arise in hand soldering or touch-up operations, where achieving precise heating for residue encapsulation proves more challenging. Additionally, as electronics progress towards high-density surface mount boards featuring components like BGAs, Micro-BGAs, Flip Chip, FPGA, and QFNs, densely packed boards offer limited space for adequate flux heating, prompting the industry to reconsider the necessity of cleaning no-clean flux white residues.

White Residues are the Enemy

There are many different types of contaminants that can impact PCB performance. Solder balls may get trapped between tightly-spaced components. Ink and fingerprints may mar the surfaces. But white residue is really the enemy.  When the salt activators in the no-clean solders and pastes come in contact with heat or other chemicals, they often leave behind a white residue. The residue may corrode fragile circuits and enable dendrite growth. This can interfere with signal transmission or create noise on the board, especially on high-voltage systems.  White residue is one of the most stubborn and difficult PCB cleaning challenges.

Four Reasons to Clean No-Clean

1. Preservation of Signal Integrity: No-clean flux residues have the potential to disrupt signal transmission, leading to diminished performance, especially in high-frequency or high-voltage assemblies.
2. Conformal Coating Adhesion: Even minute residues of no-clean flux can impede adhesion in conformal coating applications, underscoring the importance of thorough cleaning for optimal coating performance. Flux residue absorbs and traps moisture on the board which may then be released during curing operations, causing the conformal coating to separate from the PCB. This leaves the PCB vulnerable to damage, especially if used outdoors in wet, harsh environments. Corrosive materials, dust, or water could penetrate the PCB assembly causing corrosion, signal transmission problems, or even complete component failure.
3. Improved Aesthetics: Visible residues detract from the overall quality of the assembly, highlighting the importance of cleaning for enhanced aesthetics and a favorable end-user perception. Leftover flux residue on PCBs looks messy and may be misconstrued as lax or below-average work. Cleaning the flux residue from circuits simply produces a higher quality, better-looking board that is more acceptable to customer.
4. Improved Quality Control: No-clean flux residue can pose challenges during inspections, complicating quality control assessments and field repairs. Dirty PCBs are more difficult to inspect during quality control operations because residue or other contaminants could interfere with visual inspection equipment. Thoroughly cleaning no-clean flux remnants can simplify and enhance the accuracy of PCB inspections.

Cleaning No-Clean Flux – Selecting the Right Fluid

Cleaning no-clean flux residues involves selecting the appropriate cleaning fluid and method tailored to the specific flux used. Given variations in flux formulations across manufacturers, accurately matching the cleaning solution to the flux chemistry is essential. Cleaning fluid suppliers offer a range of flux removers tailored to different flux chemistries, facilitating efficient removal of stubborn residues.

PCB manufacturers are encouraged to consult with their cleaning fluid supplier to choose a fluid that not only effectively removes flux residue but also remains compatible with the PCB substrate and other materials on the board. The selected flux remover should be potent enough to remove residues effectively without causing damage to metal or plastic components or coatings.

VeriClean Flux Remover

Discover the unparalleled advantages of VeriClean No Clean Flux Remover, your ultimate solution for effectively cleaning no-clean fluxes and pastes. This versatile product excels in a myriad of applications, effortlessly removing residue from both through-hole and SMT designs, accessing hard-to-reach areas such as under BGA chips, and thoroughly rinsing residue from connectors. Its efficacy extends beyond fluxes and pastes, tackling oils, grease, adhesives, and even fully-cured silicone-based conformal coatings.

Featuring a non-corrosive and ESD-safe formula, VeriClean ensures a safe work environment while boasting fast drying for swift cleaning. Its mild, low-odor composition, based on methyl siloxanes, not only enhances user comfort but also aligns with environmental standards, being ozone-safe with a minimal global warming potential. Plus, with its economical jumbo-sized aerosol can delivering 30% more cleaner at only 15% extra cost, and the convenience of a pocket-sized cleaning pen, VeriClean stands as the epitome of efficiency and efficacy in flux removal

Three Ways to Clean No Clean Flux

PCB manufacturers have a plethora of options for PCB cleaning, with manual benchtop cleaning, aqueous cleaning and automated vapor degreasing being popular choices.

Benchtop Cleaning

Benchtop manual cleaning involves a meticulous four-step approach of wetting, scrubbing, rinsing, and drying the PCB. This method effectively addresses concerns of damage when working with delicate or moisture-sensitive components. It is also popular for cleaning PCBs that have been selectively hand-soldered and require secondary spot cleaning of just a few components.

Aerosol no-clean flux removers are available in a variety of strengths. Fabricators should choose one by carefully matching the flux remover to the contaminant. In addition, the cleaning fluid should be compatible with the PCB substrate and other materials on the board. It should be strong enough to effectively remove the flux residue, yet not cause damage to soft plastics or coatings.  When possible, PCB fabricators should opt for a nonflammable, flux remover for facility safety. Also, using a complementary cleaning fluid dispensing tool minimizes the amount of cleaning fluid used and the fumes released into the air.

Aqueous Cleaning

Aqueous Cleaning typically uses heated deionized water and a detergent in a series of washing and rinsing cycles which takes place in large machines. A second operation, using heat or air, dries the PCBs. Aqueous systems are considered by many to be environmentally friendly, however, they consume relatively large amounts of electricity, require continuous water monitoring and stringent wastewater management. As a result, operating costs can be high.

Vapor Degreasing

Vapor degreasing is an efficient and effective cleaning processes available to clean PCBs. Vapor degreasing offers excellent performance when cleaning miniature components and uses an environmentally sustainable cleaning fluid, instead of water.

Vapor degreasers are a closed-loop system that require two elements: a specially designed cleaning machine, and a specific low-boiling, non-flammable fluid as the cleaning agent. Vapor degreasers contain two chambers: the boil sump and the rinse sump. In the boil sump, the cleaning fluid heats, and the PCBs are immersed and cleaned in the fluid. Once cleaned, the PCBSs mechanically transfer to the rinse sump for final rinse in a pure, uncontaminated fluid. The PCBs come out clean, dry, and cool enough for immediate coating or packaging. Because vapor degreasers recycle and reuse their cleaning fluid for hundreds of hours before the fluid needs to be refreshed, it makes it a cost-effective and environmentally sound cleaning method.

Finding No Clean Flux Products Locally

In the past, PCB fabricators had to rely on local electronics suppply stores, hobby shops or hardware stores to purchase no-clean flux removers. Today there are a number of suppliers that offer no-clean flux products and related soldering materials. Check out our Where to Buy page to see the full list of the MicroCare no-clean flux remover distributors in your area. Use the search filter to locate products based on your location. all have online stores and some have retail locations where MicroCare flux removers are available to purchase. 

Conclusion

While the introduction of no-clean solder flux has transformed electronics manufacturing by eliminating the need for traditional PCB cleaning processes, there are instances where cleaning becomes imperative for optimal PCB performance, reliability, and aesthetics. By understanding the nuances of different flux chemistries and employing proper cleaning fluids and methods, PCB manufacturers can uphold the integrity and quality of their electronic assemblies, even in the era of "no-clean" fluxes.