How to clean PCBs during daily production?
Printed circuit boards, especially those used in cell phones such as PDAs (personal digital assistants), are susceptible to abuse. In addition to collecting dust that can penetrate the casing of cell phones, e-book readers and similar handheld devices, PCBs are known to suffer from liquid immersion and splashing during daily use. As a result, a service industry has emerged that provides cleaning and repair services for PCBs affected by contaminants, but not physical damage to PDAs and large devices.
Cleaning printed circuit boards (PCBs) to provide high-use products is as sophisticated a process as manufacturing them. If the wrong cleaning method is used, connections can be damaged, components can be loosened and materials can be damaged. To avoid these pitfalls, the same care needs to be taken when choosing the correct cleaning method as when designing, specifying and manufacturing the board.
What are these pitfalls? How can they be avoided?
Below we will discuss proven PCB cleaning solutions and some of the methods you may need to be wary of.
Different types of contaminants
A variety of contaminants can accumulate on PCBs. Using the correct and appropriate method to deal with violations will be more effective, efficient, and less of a headache.
Dry Contaminants (Dust, Dirt)
One of the most common scenarios is the accumulation of dust in or around the PCB. Gently use a fine, delicate brush (such as a horsehair brush) to remove dirt and dust without affecting the components. Even the smallest brush can reach the limit, for example under the elements.
Compressed air can reach many places but may damage important connections, so use it with care.
Vacuum cleaners specially designed for electronic components are also available, but they are ubiquitous.
Moist contaminants (dirt, wax oil, flux, soda)
High temperature operation can cause some wax coated components to become dust and dirt magnets, resulting in sticky dirt that cannot be removed with a brush or vacuum cleaner. Or soak the product in sticky soda water, which makes the board sticky. In either case, these substances should be addressed before they accumulate and affect performance.
Cleaners such as isopropyl alcohol (IPA) and Q-tip, a small brush, or a clean cotton cloth can be used to remove most dirt. Solvents such as IPA should only be used to clean PCBs in a well-ventilated environment, preferably in a fume hood.
Deionized water can be used instead. Make sure to remove excess moisture and dry the board properly (a few hours in a low-temperature oven can effectively remove any residual moisture).
In addition to IPA, there are many commercially available PCB cleaners, from acetone to chemicals used to clean electronic equipment. Different cleaners can address specific types of contaminants, such as flux or wax. Keep in mind that harsh cleaners may remove traces from components or damage plastic or electrolytic capacitor jackets or other foreign objects (such as humidity sensors), so make sure the cleaner you use is not too strong. If possible, test the cleaner on old components or connectors that do not need it to ensure no more damage is caused.
Ultrasonic PCB Cleaning
Ultrasonic cleaners use high frequencies to cause cavitation, i.e., billions of tiny bubbles explode violently in the cleaning solution in the tank of the ultrasonic cleaner. These bubbles are generated by sensors that stick to the bottom of the tank and are excited to ultrasonic frequencies by the generator. The bursting of these bubbles will be blown away by contaminants on the surface of the cleaned parts.
Ultrasound can be defined as sound waves with a frequency above the upper limit of the normal human hearing range, which is about 20 kHz (20 kHz or 20,000 times per second). Although this is true, ultrasonic cleaners can be heard during operation because of what we call ultrasonic cavitation.
Because the technique can damage components or loose connections as well as dirt and dust, it is no longer suitable for use as a cleaning method. In fact, the National Aeronautics and Space Administration (NASA) has ordered that ultrasonic cleaning not be used because it could inadvertently cause component end caps to separate and conduct ultrasonic energy through the lead frames of the integrated circuits. Damage to bonding wires and bonding wire pads.
Having said that, there are still ultrasonic cleaning applications. The ultrasonic cleaning process can reach the most difficult and hard-to-reach locations under the high-density components of most components on the board. However, this is not the case for SMD devices with small gaps smaller than the surface tension coefficient of the cleaning fluid. However, the process is fast, and there are many high-capacity machines available to address large-scale cleaning needs.
PCB ultrasonic cleaning machine
Cavitation is not a gentle process. It is calculated that temperatures in excess of 10,000°F and pressures in excess of 10,000 PSI will be generated at the bursting point of the cavitation bubbles.
In terms of cycles per second, ultrasonic cleaners can generate frequencies from 25 kHz to over 100 kHz. Low frequencies produce larger cavitation bubbles compared to high frequencies. Larger bubbles will explode more violently, for example, to remove serious contaminants from manufactured metal parts. Higher frequencies produce smaller bubbles that clean more gently, but penetrate cracks, crevices and blind holes better. Higher frequencies are used to clean highly polished or delicate surfaces.
There are companies that specialize in PCB cleaning. Please note that we (San Francisco Circuits) do not offer PCB cleaning services. We are a high-end supplier for PCB manufacturing and assembly.
Depending on your needs - such as a large number of boards, what needs to be cleaned, and the level of refinement of the board - may leave you looking for an outside source to meet your cleaning needs.
If you have frequent problems with boards that need cleaning, there may be more important things to check during the design or manufacturing process. Our consulting engineers can help you identify problems and develop long-term solutions to better ensure that your printed circuit boards remain clean and within operating tolerances.
Cleaning PCBs does not have to be a daunting task. Keeping the above tips and suggestions in mind will help ensure that cleaning is done correctly.