no-clean flux

Guide to No-Clean Flux in PCB Assembly

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Solder formulation is most commonly selected based on the need for RoHS compliance, and the PCB assembler is left to handle any remaining issues with fluxes and cleaning. In most cases, cleaning is either assumed by the design team or it is an afterthought, yet in many assemblies it is a very important part of ensuring reliability. This is where no-clean flux enters the picture for many assemblies as it eliminates the need for a cleaning step at the end of the PCB assembly process.

There are different kinds of no-clean fluxes, each tailored for specific soldering needs and applications—from those that leave almost no visible residue to thicker types used for touch-ups. The type you choose depends on the assembly process, the components used, and where the PCB will be used. Flux-cored solder wire and solder pastes are available with no-clean and water-soluble fluxes as part of common solder alloy formulations, so designers and assemblers have some freedom to select solder formulations and specify cleaning needs.

Understanding No-Clean Flux

According to Bastow in iConnect007, "an estimated 80% of all SMT assembly in the world is performed with a no-clean soldering process, largely due to the predominance of consumer electronics." Clearly, no-clean flux is widely prevalent and is not the main cause of all device failures. However, as we will see below, certain instances demand removal of flux residues due to their interaction with the surrounding environment during temperature cycling.

No-clean flux is a specific type of flux used in the soldering process that is engineered to leave behind non-conductive, non-reactive residues on the PCB after soldering. This residue is quite non-conductive and non-corrosive, which theoretically eliminates the need for any post-soldering cleaning operations. The development of no-clean flux was driven by the electronics industry's need for faster production times and reduced cleaning costs, all while maintaining high standards of solder joint integrity and reliability.

Chemistry of No-Clean Flux Residues

No-clean flux primarily serves to improve the wetting characteristics of the solder by removing oxidation from the metal surfaces involved in the soldering process. It facilitates the flow of solder to form a good bond between the components and the PCB. The unique formulation of no-clean flux is such that it decomposes during the soldering process, leaving behind very little residue. This minimal residue is designed to be left on the board without causing harm in typical operating environments.

No-clean flux residue contains base resin/rosin and solder flux activators, and it is known to spread around or get trapped beneath low-standoff SMT components during soldering. The types of flux activators used in reflow solder paste are:

  • Organic acids
  • Organic amines
  • Organic halogen compounds
  • Organic halide salts

During reflow soldering, the activator removes any oxides on the metal contacts so that the melted solder alloy can wet onto component pads and leads. After soldering, the corrosive activator residue can be encapsulated by the film of base resin/rosin part and hence this makes the flux residue "no-clean". Due to a glass transition temperature of the remnant film being near 60 Celsius in a humid environment, the encapsulated flux residue may degrade and lead to the release of flux activators due to humidity interaction. This would lead to corrosion of nearby solder joints, reduction of SIR, and release in an active component in electrochemical reactions with other contaminants.

Why Cleaning Might Still Be Necessary

The term "no-clean flux" suggests that once applied and reflowed, the residue left behind does not require any further cleaning. This is because the residues are designed to be non-conductive and non-corrosive, posing no immediate threat to the functioning of the PCB under normal conditions. However, this assumption can be misleading and does not universally apply to all scenarios or end-use environments. The prior paragraph outlines the chemistry that primarily motivates cleaning in certain cases.

Failures have been reported via the standard corrosion or electrochemical mechanisms in boards where no-clean flux was used during PCB soldering. It is thought that remnant activator and binder/resin in the remnant flux residue is a contributor to hygroscopicity and later electrochemical activity in these residues. The effects on surface insulation resistance (SIR) measurements and optical inspection show degradation of the film to a less resistive state, which is related to uptake of moisture and interaction with remnant metal salts in the no-clean flux residue film.

 

No-clean flux study

No-clean flux study

 

Image source:

  • Li, Feng, et al. "Effect of flux activator in reflow process related flux residue on the climatic reliability of surface-mount electronic devices." Journal of Materials Science: Materials in Electronics 34, no. 16 (2023): 1315.

Despite the convenience of no-clean fluxes, there are compelling reasons to consider cleaning the residues in certain cases:

Long-term reliability: In critical applications, such as aerospace or medical devices, any potential risk of failure needs to be minimized. Residues, even if non-conductive and non-corrosive, can attract moisture or dust over time, leading to issues like ECM failure as discussed above.

High-density assemblies: As PCBs become more complex and component-packed, the tiny spaces left between components can trap flux residues. These trapped residues can become problematic, especially if they absorb moisture or interact with other materials.

Thermal cycling: Devices that undergo significant temperature changes can experience changes in the physical properties of the flux residues. This can lead to cracking of the flux residue film and the SIR values as shown above. However, thermal cycling shows apparent healing of the films despite environmental exposure, as shown below.

The thermal cycling aspect may be surprising for some readers, however this has been studied from the perspective of evolution of SIR measurements. In a long-term reliability study by Bastow, it was found that the leftover no-clean flux residues can crack after exposure to a temperature-cycled environment, and that some regions in the cracked residues can heal themselves during a subsequent round of thermal cycling. While the physical changes were observable, no major changes in the SIR were observed in the study.

 

No-clean flux study

 

The full text of this study can be found here:

One conclusion that can be drawn from this is that the potential reliability challenges due to the SIR of the flux residue does not change simply due to temperature cycling: it will remain a potential problem in some assemblies, cracking of the flux residues notwithstanding.

Understanding that "no-clean" does not always mean "no consequences" is crucial for manufacturers and designers. Evaluating the specific needs of the PCB and its operating environment is essential to decide whether to clean the residues or leave them as is. In conservative environments, such as mil-aero or automotive, why bother taking the risk? The additional effort to clean these residues is minimal compared to the reduced liability for equipment failure.

Cleaning Methods for No-Clean Flux

Even though no-clean flux is designed to minimize residue, there are instances where cleaning these residues becomes necessary to ensure the reliability and functionality of PCBs. Understanding the various methods available for cleaning no-clean flux is crucial for manufacturers who need to incorporate this step into their production process.

Solvent Cleaning

Solvent cleaning is one of the most common methods used to remove no-clean flux residues. This method involves using chemicals that can dissolve the residue without damaging the PCB or its components. Solvents can vary from mild, environmentally friendly options to more aggressive chemicals, depending on the nature of the residue and the components involved. It's important to select the right solvent that effectively removes the residue while being compatible with the PCB materials.

Ultrasonic Cleaning

Ultrasonic cleaning involves using high-frequency sound waves in a cleaning solution to agitate and remove residues from the PCB. This method is highly effective for cleaning densely packed PCBs where residues hide in hard-to-reach areas. Ultrasonic cleaning must be carefully controlled to avoid damaging sensitive components through excessive vibration.

Manual Cleaning

In some cases, manual cleaning with brushes or swabs might be necessary, especially for prototypes or low-volume production where setting up automated cleaning systems is not feasible. Manual cleaning allows for targeted cleaning but requires skilled personnel to ensure thoroughness and to avoid damage to the PCB.

 

Whether you're designing high-speed PCBs for mil-aero embedded systems or a complex RF product, you should work with a design and development firm that can ensure your product will be reliable and manufacturable at scale. NWES helps aerospace OEMs, defense primes, and private companies in multiple industries design modern PCBs and create cutting-edge embedded technology, including power systems for high reliability applications and precision control systems. We've also partnered directly with EDA companies and advanced ITAR-compliant PCB manufacturers, and we'll make sure your design is fully manufacturable at scale. Contact NWES for a consultation.

 



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