Industrial operations rely on computerized systems to perform various operations. Manufacturers develop printed circuit boards (PCBs) to perform these simple and complex functions. An important issue experienced by manufacturers involves keeping circuit boards clean during production. Any types of contaminants introduced into the process may cause printed circuit boards to experience damage or malfunctions.
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While clean rooms assist in preventing contaminants from entering workspaces and shop floors, particulates may inadvertently come from other places. Such contaminants may include fingerprints, resins such as adhesives, salts, flux residues, ions, board particles, or solder balls. Circuit board cleaning tackles the dirt, contaminants, and other particles.
Circuit board cleaning go through different cleaning stages depending on the production step. The cleaning steps may occur before and after wafer sawing, drilling, soldering, or plating processes. It may also occur before and after adding fluxes as well as during the times when workers have been handling the circuit board constantly.
The circuit board cleaning process is based on the types of equipment and cleaning chemicals used. They may also depend on the types of circuit boards, such as lead-free or lead-based. In addition to removing the common contaminants that have to be removed to ensure the printed circuit board works properly, cleaning processes also focus on removing residues introduced during production.
These residues may simply be perspiration from hands, salts, acids, flux activators, or plating chemicals. While the residues may be there in small enough quantities that will not impact the functions of the circuit board, removing these residues may improve the PCBs operations and longevity in the application.
The cleaning process typically begins by pre-washing the PCBs to remove any types of large contaminants on the surface. This process helps to get rid of most contaminants and residues.
Next, workers and equipment perform a more specialized cleaning to get rid of any leftover contaminants of the circuit boards. During the washing process is an anti-drag out phase, where the liquid of the water or chemicals is prevented from being dragged out with the circuit board
After the washing, the circuit boards become rinsed off as methods are used to get all surface areas and crevices to ensure no remaining cleaning solution is left on the board. Just like during the washing stage, an anti-drag out have is performed during rinsing.
Lastly, the PCBs go through a drying phase. The drying phase utilizes blowers that propel high pressurized and high temperature gasses, such as nitrogen, or air onto the circuit board. This prying stage helps to remove all moisture.
PCB manufacturers rely on a wide range of cleaning equipment. The equipment may be as simple as soft-bristle brush, microfiber cloth, and cleaning agent such distilled water or isopropyl alcohol to perform fast cleanings on individual circuit boards. Other equipment may consist of large standalone machines made to perform batch circuit board cleanings in larger quantities. Here are some clearing equipment options:
Alkaline texturing equipment uses modules connected in a series to direct cleaning chemicals onto the circuit boards using flat fan nozzles, which are available from Lechler. This method is used during wafer sawing processes to remove contaminants and damage. The machine may employ ready-to-use chemicals that are sprinkled onto the wafers, then blowers dry the wafers.
Wafer moistening equipment helps prevent impurities dry onto to the printed circuit board after the sawing process. This equipment uses may use Lechler hollow cone nozzles to direct a very fine spray mist onto the wafer's surface. The spray softens the wafers and moistens the contaminants and residues so they can be removed.
Other types of equipment and components are employed during the etching, stripping, and development phases of the PCB. The Lechler 646 series nozzles are used that have a bayonet quick-locking system used in an inline process.
Lastly, some manufacturers still have ultrasonic cleaners to remove circuit board contaminants. Many industries, including NASA, have moved away from this method due to end cap separation of components and bond wire/pad damage.
Manufacturers may determine specific circuit board cleaning methods based on the number of circuit boards going through the production process and the types of common contaminants found on the boards. Three types of cleaning methods include manual, batch/inline, and gas phase.
Manual: The manual method simply relies on the worker performing all the stages of the cleaning process using various tools. While this procedure provides the most attention to detail, it also involves increased handling of the circuit boards and may lead to human errors.
Batch/inline: Batch/inline methods are automated processes that are used to clean no-flux residues. The method can clean small and large batches of circuit boards at once and in a uniform clearing production line.
Gas phase: For this method, a chemical solvent is boiled until it becomes steam or a gaseous state. The vapor is directed onto the circuit board to separate the contaminants from the surface. This method is also called vapor cleaning.
Cleaning agent types vary based on the equipment and methods used during the washing and rinsing methods. Selecting a cleaning agent may depend on the contaminant and its location.
Water-based methods use a water-based PCB cleaning agent and deionized water for rinsing.
Semi-aqueous methods may employ solvents and deionized water in dip tanks during spray-under-immersion applications. Many of the solvents are organic or halogen-free.
Water-free methods rely on modern solvent-based cleaners applied as vapor rinses or in dip tanks.
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.
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
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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.
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!
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.
White flux residue with visible copper corrosion (photo courtesy of Foresite)
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:
Flux-Off® Rosin with a brush attachment
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
Automated or Semi-Automated Flux Removal Methods
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.
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:
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 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.