How to Clean PCBs

Printed circuit boards, especially those used in PDAs (personal digital assistants) like cell phones, get a lot of abuse. In addition to collecting dust and dirt that penetrates the cases of cell phones, e-book readers and similar hand-held devices PCBs have been known to suffer from immersion in and splashing from liquids in day to day use. As a result, a service industry has emerged to provide cleaning and repair services for PCBs subjected to contaminants but not physical breakage in PDAs and larger devices.

Cleaning a Printed Circuit Board (PCB) to service a high-use product is as delicate a process as is making the board. Connections can be damaged, components loosened and materials compromised if the wrong cleaning method is employed. To avoid these pitfalls, you need to take the same care in selecting the right cleaning method as you did when designing, specifying and producing the board in the first place.

Different Types of Contaminants

There are a variety of contaminants that can accumulate on a PCB. Attacking the offending problem with the correct corresponding method will be more effective and efficient, and cause fewer headaches.

Dry Contaminants (Dust, Dirt)

One of the most common occurrences is the accumulation of dirt or dust in or around the PCB. Gently using a small, delicate brush, such as a horsehair paint brush, can remove dirt and dust without affecting the components. There are limitations on Where even the smallest brush can reach, such as beneath a component.

Compressed air can reach many areas but may damage vital connections, so it should be used with extreme care.

Specially designed vacuum cleaners for electronic components are also an option but cannot reach everywhere.

Wet Contaminants (Grime, Waxy Oil, Flux, Soda)

High temperature operations can cause some components that are coated in wax to become magnets for dust and dirt, causing tacky grime that cannot be removed with a brush or vacuum. Or a product gets a bath of sticky soda, making the board a gooey mess. Either way, these substances should be tackled before it builds up and affects performance.

Most grime can be removed with a cleaning agent, such as isopropyl alcohol (IPA) and a Q-tip, small brush or clean cotton cloth. Cleaning a PCB with a solvent like IPA should only be done in a well vented environment, ideally under a fume hood.

Demineralized water can be used as an alternative. Be sure to remove any excess moisture and properly dry the board (a few hours in a low-temperature oven can effectively remove any residual moisture.)

There are a number of commercially available PCB cleaning agents aside from IPA, ranging from acetone to chemicals made for cleaning electronics. Different cleaning agents can address specific types of contaminants, such as solder flux or wax. Keep in mind that harsh cleaning agents can remove component markings or cause damage to plastics or electrolytic capacitor jackets, or other exotic components like humidity sensors, so make sure you aren’t using a cleaning agent that is too strong. If you can, test the cleaner on an old component or connector you don’t need to be sure you don’t do more harm than good.
 
Ultrasonic PCB Cleaning

Ultrasonic cleaning machines use high-frequencies to cause cavitation; the violent implosion of billions of minute bubbles in the cleaning solution contained in an ultrasonic cleaner tank. The bubbles are created by transducers bonded to the bottom of the tank and excited to ultrasonic frequencies by generators. The implosion of these bubbles blasts away contaminants on the surface of parts being cleaned.

Ultrasonic can be defined as sound waves that have frequencies above the upper limit of the normal range of human hearing, which is about 20 kilohertz (20 kHz or 20,000 cycles per second). While that is true, ultrasonic cleaners when operating can be heard due to the action created by what we call ultrasonic cavitation.

This technique has lost some favor as a method for cleaning as it can cause damage to components or loosen connections, along with the dirt and grime. In fact, NASA has issued a directive that it no longer employs ultrasonic cleaning as it can inadvertently cause separation of component end caps and actually cause damage to bond wires and bond wire pads internal to the IC via ultrasonic conduction of the ultrasonic energy via the integrated circuit lead-frame.

That being said, there is still a place for ultrasonic cleaning applications. The ultrasonic cleaning process can reach the most difficult, hard to reach places underneath high density components on most any parts of a circuit board. This is not the case for SMD devices that have small gaps that are smaller than the surface tension coefficient of the cleaning fluid. However, the process is fast and there are numerous high-capacity machines available to address high-volume cleaning needs.

PCB Ultrasonic Cleaners

Cavitation is not a gentle process. It has been calculated that temperatures in excess of 10,000°F and pressures in excess of 10,000 PSI are generated at the implosion sites of cavitation bubbles.

Measured in cycles per second, ultrasonic cleaners can produce from 25 kHz to 100+ kHz. Lower frequencies create larger cavitation bubbles compared to higher frequencies. Larger bubbles implode more vigorously and are used, for example, to remove gross contaminants from fabricated metal components. Higher frequencies create smaller bubbles that clean more gently but are better able to penetrate cracks, crevices and blind holes. The higher frequencies are used for cleaning highly polished or delicate surfaces.

 Conclusion

If you are constantly having issues with boards needing to be cleaned, there may be something more significant in the design or manufacturing process that needs to be examined.

Cleaning a PCB doesn’t have to be a daunting task. Keeping the above tips and advice in mind will help in ensuring get the cleaning job done right.