Printed circuit boards (PCBs) are the heart and brain of the electronic devices we use every day—from consumer electronics and the vehicles we drive, to medical devices and much more. They’re composed of non-conductive material and are etched with conductive tracks and pads, with additional components connected through soldering. The components and connections among them on a PCB each serve a specific purpose, and without a clean connection, our electronic devices wouldn’t work the way we need them to, whether it’s making a call on a smart phone or the lifesaving mechanisms of a pacemaker.
Cleaning the PCB after manufacturing and soldering is an important part of the electronic device assembly process, and can affect the overall durability, reliability, effectiveness and lifetime of a device. During assembly and the manufacturing process, PCBs can become soiled with flux or solder, dirt from handling or from the environment, like residues, dust, moisture, colophony or resin, fingerprints or oxides. Some contaminates may even possess erosive properties, leading to the damage of circuits and connections down the road, causing shorts and failures, shortening the lifespan of the device. Cleaning away these types of debris will ensure there are no faults in functionality over time and prolong the life of the device.
When cleaning PCB assemblies the primary goal is to remove resin and flux residues from populated printed circuit boards. PCB assemblies employed in industries such as automotive, telecommunications, military, and aerospace, require assemblies that are free from any potential contaminants. Contaminant free PCBAs are mandatory for the next step in the manufacturing process, i.e. underfill, epoxy staking or conformal coating. If left on the assembly, flux residue can cause poor wetting and delamination. Plus, the printed circuit boards look better.
A clean and clear PCB can also be a benefit when it comes to the inspection process, or for any future troubleshooting. It is much easier to find defects or spot any potential issues when the board and all of its components are cleaned of any remaining contaminates or leftover solder.
Some types of flux are advertised as ‘no clean’ flux, suggesting that it doesn’t require PCB cleaning after flux is applied. However, it’s still important to clean boards that are soldered with “no clean” flux in order to avoid electrical leakage and to improve appearance. This type of solder material is designed to produce the least amount of residue, but does not eliminate the need for cleaning altogether.
There are different methods generally used when it comes to PCB cleaning, each with their own benefits and potential drawbacks: manual cleaning method, ultrasonic cleaning method and gas phase cleaning method. The cleaning method you select can depend upon the effectiveness, complexity and density of components on your PCB, the equipment you have access to, cost and affordability, number of boards to clean, environmental impact, and time.
Manual Cleaning Method
One of the most common methods of printed circuit board cleaning is the manual cleaning method. Just as it sounds, any contaminates or leftover solder material are removed through manual, or physical removal, with an alcohol solution and a small brush. Occasionally, other tools can be used in combination with a brush to physically remove any dirt from the board.
The first step in this method is to soak the PCB in an acetone solution for roughly ten minutes in order to loosen any contaminates or dirt on the board. Following the soak, a brush is used to gently remove unwanted particles, which is typically done while in an ethyl alcohol solution. Once the board is gently brushed of debris, it is rinsed in demineralized water for a few minutes, and finally, is completely dried using a nitrogen gas gun.
The manual cleaning method is popular due to its simplicity and affordability, since it does not require any equipment and the supplies necessary are easy to obtain. It is also considered to be a method with fairly low toxicity, to both people and the environment. However, the lack of automation with this method can mean it is more time consuming, and may not be the best method for large batches of boards. The manual method can also be limiting because you are only able to clean the parts of the board that can be reached with a brush or other cleaning tool. Therefore, this may not be the best method when it comes to cleaning fairly complex or dense PCBs.
Ultrasonic Cleaning Method
The ultrasonic cleaning method employs a specialized machine, created specifically for cleaning PCBs. Like the manual method, the board is first soaked in an ethyl alcohol solution to loosen dirt, contaminates or excess soldering materials and is then placed into the bath of cleaning solution inside the ultrasonic cleaner. The machine emits high-frequency sound waves into the liquid bath of cleaning solution, which creates billions of tiny bubbles that pop and physically remove any possible contaminates on the PCB, a process known as cavitation.
The ultrasonic cleaning method is a strong option to clean very dense or complex boards with many components because its tiny bubbles can reach every bit of exposed surface. The powerful frequencies can reach even the smallest, most hard to reach places, unlike the bristles on a brush.
Some people choose to avoid the ultrasonic cleaning method because of the high frequencies’ potential to damage components or connections on the board. However, it can be the best option for a dense board where a physical brush cannot reach. Measuring the risk of a damaged component versus a soiled component is an important equation when it comes to using the ultrasonic cleaning method.
Gas Phase Cleaning
Gas phase cleaning, also known as vapor phase cleaning or vapor degreasing, is a method that boils a chemical solvent in order to create steam, which then dissolves dirt and contaminates left on the printed circuit board. This method is considered to not only be effective, but to also be an environmentally-conscious option, depending on the chemical solvent used, and the ability for the solvents to be reused after impurities are filtered. The fluid is often designed based on the particular contaminantes that must be removed, the temperature at which the chemical solvents boil, whether or not they have flammable properties, and whether they are considered to be environmentally friendly. Like the ultrasonic method, the gas phase cleaning method is also effective for complex PCBs since the steam is able to reach small parts of a board where a physical brush may not be able to reach.
Deionized Water Wash
Deionized water is a safe and effective solution for removing water-soluble flux residue. ACDi utilizes water-soluble flux in both our leaded and lead-free pastes and solders, which means it can be cleaned with deionized water. The in-line wash uses deionized water jets at elevated temperature and pressure to wash and rinse the PCB assemblies. The water temperature is 144 degrees F, and the pressure is 45 psi. The assemblies are then air dried with a series of forced air jets. Resistivity and ionics testing are used to verify the cleanliness.
ACDi also employees leaded and lead free no clean flux in our solders. In this case, the assemblies are cleaned using a manual soak in a water-based and eco-friendly alcohol cleaner. And then washed in the deionized water wash. Given that deionized water is the purest form of water, quality-focused contract manufacturers use this as a standard for board washing. The cleaning method you decide to use will depend on your particular PCB, its components, density and type of soldering materials used.
For more information about PCB cleaning, or for a complimentary consultation regarding your electronic manufacturing needs, reach out to ACDi at 301-363-4182 and on our website, here.