Defluxing and PCB Cleaning - ITW EAE

As high reliability printed circuit boards become more dense and complex, PCB assemblers are seeking ways to enhance reliability. Cleaning is viewed as a process solution. Defluxing is the cleaning process designed to remove solder flux and by-products from the PCB assembly. PCB cleaning typically includes de-fluxing, but it may also include removing impurities from the board assembly such as solder balls, dirt, dust, organic materials, and other contaminants.

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In many electronics manufacturing facilities, throughput volume, as well as cleanliness and process control requirements justify the capital expense of a batch and/or in-line type cleaning system. Electrovert’s batch and in-line cleaners are designed for defluxing, PCB cleaning, surface preparation, and specialty cleaning applications. The Electrovert Aquastorm™ series (batch and in-line) cleaners are built to deliver ‘dynamic’ energy at the board level to maximize cleaning effectiveness and performance. The MicroCel™ batch cleaner uses centrifugal energy in an immersion wash environment.

Aquastorm series cleaners (batch and in-line) and MicroCel batch cleaners are used in many cleaning applications:

• PCB assembly
• De-fluxing
• Smartphone and tablet mobile devices
• Microelectronics
• Semiconductor
• Hybrid electronics
• Misprint cleaning
• Industrial parts cleaning
• Wet cleaning prior to wire bonding
• Specialty applications

What Is Ionic Contamination?

What is Polar Contamination?

Ionic contaminants are remains of flux that are left behind during the assembly process. Ionic compounds are held together by electrostatic forces and the compound itself has a zero net charge. These materials will disassociate when exposed to water.  These are composed of positively charged cations and negatively charged anions.  A simple example is table salt (sodium chloride), composed of a single positive sodium cation which lacks one electron, and a negatively charged chloride anion (Cl), which has an extra electron.  Polar compounds, on the other hand, can have a positive charge on one side of the molecule and a negative charge on the other side of the molecule; these molecules never split apart. Water and Isopropanol (or IPA) are examples of polar molecules.

When populating the board with components, the components themselves can carry various ionic/conductive contaminants to your assembly including cutting oils/fluids, biocides, and corrosion preventatives. Be aware of common nonionic materials that can also affect the assembly steps – process oils, mold releases, etc. can be detrimental down the line.

What Are The Most Common Ionic Contaminants From The Bare Board Fabrication?

Common Moisture Trapped in the “Layering” Process

Water is a polar contaminant.  It is conducive for dissociating other ionic materials which then lays the foundation for conductive mishaps (dendritic growth, ECM, etc).  It is common practice to “bake” the boards to remove extraneous moisture.

Corrosion from PCB fabrication contamination (photo courtesy of Foresite)

Etching Chemicals

Etching chemicals are highly conductive and can be corrosive as well. They must be chemically neutralized and removed/rinsed and are well-known as sources for current leakage. 

Flux Residues from Soldering

Heavy no-clean flux residue with visible copper corrosion (photo courtesy of Foresite)

Everyone is familiar with flux residues. Fluxes, whether in liquid, cored wire, or compounded as a paste, can leave residues that can cause serious reliability defects if not removed. Common conductive flux residues from the soldering process can include various unreacted activators, binders, rheology components, and saponifiers. Among these are numerous iterations of acids (abietic, adipic, succinic among others), highly basic ingredients (amino compounds), and even constituents found in “soaps” such as phosphate and sulfate ions. All of these must be cleaned from the substrate, whether by strict solvent cleaning such as vapor degreasing or by aqueous chemistries in the common batch or inline cleaners seen on the manufacturing floor.

Inter-Layer Residues from Drilling and Via Plating Processes

Dendritic grown between solder pads, caused by ionic contamination (photo courtesy of Foresite)

In addition, residues from the cleaning process chemistry itself must also be removed.  This is noticed more in the aqueous cleaning systems.  Many use saponifiers to neutralize and emulsify the flux residues and make them easier to rinse and remove from the substrate.  These components themselves are highly polar and ionic and can also enhance the dendrite and/or ECM mechanism if not removed.  In addition, corrosion preventatives and surfactants are commonly employed in these products.  This is not a bad thing in itself, but care must be taken to ensure they are removed along with the soils during the cleaning process.

How Do You Perform Ionic Contamination Testing?

Poor quality control in fabrication, poor soldering or component population, and even final cleaning stages are all potential sources of contamination. Many of these can be found by ionic contamination testing and analysis such as ROSE testing, ion extraction, and chromatography. Initial high humidity validation testing at the beginning of the project can also identify potential issues. 

Strict quality control and standard operating procedures during the PCB assembly, manufacturing stages, and validation testing can go a long way in preventing a reliability nightmare. Just think – simple mishandling of a part by an operator not using gloves can transfer salts and oils from skin to the substrate that could potentially be catastrophic for your item!

What is White Residue on an Electronic Circuit Board?

White residue is generally a symptom of ineffective PCB cleaning. Common conductive flux residues from the soldering process can include various unreacted activators, binders, rheology components, and saponifiers. Among these are numerous iterations of acids (abietic, adipic, and succinic among others), highly basic ingredients (amino compounds), and even constituents found in “soaps” such as phosphate and sulfate ions. When a cleaner does not fully dissolve all the constituents, or the cleaner is not allowed to flow off the PCB, the remaining solvent can evaporate off and leave behind residue that is either white or like water spots.

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White flux residue with visible copper corrosion (photo courtesy of Foresite)

How Do You Remove White Residue From an Electronic Circuit Board?

White residues can generally be cleaned by a flux remover. If the residues are the result of insufficient solvency of the original cleaning process, a stronger solvent cleaner may be required. Often agitation is required to remove the residues, which may include a wipe, swab, brush, or an aerosol with a brush attachment. Follow these steps to remove white residue:

1. Spray the residue with a strong solvent.
2. While the area is still wet, scrub with a clean tool like a wipe, swab, or brush.
3. Spray the cleaned area and surrounding areas with the same solvent, angling the board so the rinse is able to run off.

Flux-Off® Rosin with a brush attachment

What are the Different Methods of Cleaning Electronic Circuit Boards?

PCB flux removal can either happen at the benchtop, which generally requires a manual cleaning method, or in automatic or semi-automatic processes. This is common for low volume electronic PCB assembly, rework, and repair. Manual cleaning methods are generally more laborious and less repeatable, so results may vary from operator to operator. For higher volume assembly or reduced variability, more automated cleaning methods are used.

Manual Flux Removal Methods

• Aerosol– Aerosol flux removers have the advantage of a sealed system (ensuring fresh solvent) and agitation (provided by the spray pressure and pattern). A straw attachment is generally included to spray into areas with greater precision.
• Aerosol w/brush attachment– A brush can be added to the aerosol nozzle, so the solvent sprays through the brush as you scrub.
• Trigger spray– Trigger spray bottles are more common for water-based cleaners and isopropyl alcohol (IPA), but not for aggressive solvent cleaners.
• Liquid immersion– The PCB can be immersed into a tray or bucket of solvent cleaner, with cleaning tools like swabs and brushes used as needed for tenacious soils. Cleaning performance can be further improved by heating the solvent but this should only be done with nonflammable flux removers.
• Spot cleaning with a swab– A cotton or foam swab can be saturated with a mild solvent like isopropyl alcohol, often from a pump dispenser or “dauber”.
• Presaturated wipes and swabs– For added convenience, wipes and swabs are available presaturated with a mild solvent like isopropyl alcohol.

Automated or Semi-Automated Flux Removal Methods

• Ultrasonic– Ultrasonic cleaning equipment use sound waves to create implosions within the flux residue, breaking it apart and lifting it off the PCB. Most equipment have the option of heating the solvent to increase cleaning performance. Only use this option with a nonflammable flux remover. Cross-contamination can be a concern so change the solvent regularly. Ultrasonic cleaning might be too rough on sensitive components like ceramic-based resisters.
• Vapor degreaser– Vapor degreasing is the go-to process for the highest precision cleaning, as in aerospace and medical electronics. PCBs can be submerged in a sump of boiling solvent, in a rinse sump with ultrasonics, or rinsed in solvent vapors. Special solvents need to be used that are azeotropes or near-azeotropes, so will not change as the solvents are boiled off and reconstituted in a continuous cycle.
• Batch flux remover– This is basically a dishwasher for electronic circuit boards. PCBs are stationary in a rack, and the flux remover (usually water-based) is sprayed over the assembly. The PCB stays in place as the machine goes through the wash, rinse and dry cycles.
• Inline flux remover– An inline washer is more like a carwash for electronic circuit boards. PCBs travel on a conveyor through wash, rinse and dry zones. Water-based flux removers are used.

Does the Flux Type Affect the Effectiveness of the PCB Cleaning Process?

The type of flux can have a big impact on the cleaning process. R, RA and RMA fluxes are generally easier to remove with standard flux removers and isopropyl alcohol. No-clean fluxes are intended to stay on the PCB, so can be more difficult to remove. They may require a more aggressive solvent flux remover, additional agitation like brushing, or a heated solvent. Aqueous fluxes are generally designed to be removed in a batch or inline cleaning system with straight deionized water or water with a saponifier. Alcohol-based or specially formulated solvents can also be used to clean aqueous fluxes, but the same cleaners may have mixed results on other types of fluxes.

The short answer is to match the flux remover with the flux type. However, this can be challenging for an EMS supplier that may have to use a variety of fluxes as required by their various customers. Flux removers are available that can break down a large variety of fluxes while changing the variables, like cleaning time, agitation, or additional heat, can treat distinctive needs adroitly.

For water-based cleaners in batch or inline cleaning systems, cleaner concentration can be adjusted, cycle time increased, and temperature increased to improve performance against various flux types.

What Factors Cause Difficulty Cleaning Flux Residues from PCBs?

Any process engineer will tell you that the key to designing a repeatable process is to control the variables. When removing flux from electronic circuit boards, there are a number of variables that can drastically change the cleaning performance of a cleaner and process:

• Flux type – The type of flux can have a big impact on the cleaning process. R, RA and RMA fluxes are generally easier to remove with standard flux removers and isopropyl alcohol. No-clean fluxes are intended to stay on the PCB, so can be more difficult to remove. They may require a more aggressive solvent flux remover, additional agitation like brushing, or a heated solvent. Aqueous fluxes are generally designed to be removed in a batch or inline cleaning system with straight deionized water or water with a saponifier. Alcohol-based or specially formulated solvents can also be used to clean aqueous fluxes, but the same cleaners may have mixed results on other types of fluxes.
• Higher solids flux – Cleaning a PCB made with a mix of soldering technologies can be a particular challenge. Tacky fluxes or other types with a high level of solids can be more challenging to clean, require more cleaning time, soak time, or additional agitation.
• Amount of flux – A thicker layer of flux residue is more soil to remove and can create flux dams under low stand-off components. This prevents flux remover from fully penetrating under the component.
• Soldering temperature – Higher temperatures have a greater tendency to bake-on flux residues, making them more difficult to remove. High temperature soldering may require more cleaning time, soak time, or additional agitation.
• Lead-free solder> – Lead-free soldering generally requires higher soldering temperature and more highly activated fluxes. Flux residues left from a lead-free soldering process may require more cleaning time, soak time, or additional agitation, and you may actually have to consider a more aggressive flux remover that is engineered for lead-free processes.
• Time between soldering and cleaning process – It is not unusual to finish the assembly on Friday, come back on Monday to clean and be surprised with white flux residues. As flux residues sit on the PCB, volatiles continue to flash off and it becomes more difficult to remove.

If you are suddenly surprised by white residues or some other clear evidence of a cleaning problem that didn’t exist before, step back and look at your process before calling for help. Has anything changed? That will be the first question a technician will ask, and necessary to know before you can identify and solve the problem.

How do you clean a PCB after soldering?

How do you remove solder flux?

The most common way to clean flux residues from a repair area is to saturate a cotton or foam swab with isopropyl alcohol or another cleaning solvent, and rub it around the repair area. While this may be adequate for no-clean flux, where the goal is a visually clean PCB, this may not be clean enough when more heavily activated fluxes are involved, like RA or aqueous. The dirty little secret is that flux residues will not evaporate along with the solvent. You may dissolve the flux, and some of the residues will soak into the swab, but most of the residues will settle back onto the board surface. Many times these white residues are more difficult to remove than the original flux.

Flux residues don't evaporate along with the solvent.

One quick and easy improvement to this process is to rinse the board after swabbing around the repair area. While the solvent is still wet, spray over the entire board with an aerosol flux cleaner. Hold the PCB at an angle to allow the solvent to flow over the board and run off, along with any residues that are picked up.

The straw attachment that comes with aerosol flux removers is a good way to increase the spray force and penetrate under the components.

Aerosol with straw good for cleaning under components

Chemtronics offers the BrushClean™ system with many of their flux removers. The cleaning solvent sprays through the brush, so agitation can be increased by scrubbing while spraying. To absorb the flux residues, a lint-free poly-cellulose wiper can be placed over the repair area, and the spraying and scrubbing can occur over the material. Then remove the wipe and brush attachment, and spray over the board for the final rinse.

Aerosol brush attachment over a wiper dissolves and absorbs flux residues at the same time.

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