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 PCB the cases of cell phones, e-book readers and similar hand-held devices PCB 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 PCB subjected to contaminants but not physical breakage in PDAs and larger devices.
How to Clean PCB
Before we get into a proven PCB cleaning practice it’s a good idea to cover how not to do the job. This can be covered quickly. Manual cleaning with brushes and isopropyl alcohol or a similar solvent is not recommended for three reasons:
1. Brushing can damage delicate connections and no matter how adroitly administered it cannot reach tightly configured components on the board.
2. IPA is flammable.
3. Inhaling IPA fumes is not good for your health.
How about using a detergent?
Detergents do eliminate the drawback of manual cleaning with IPA or a similar solvent but it does not address the main disadvantage of manual cleaning: total removal of contaminants without damaging delicate components.
Cue Ultrasonic PCB Cleaning
Ultrasonic cleaning has a long history of removing all sorts of contamination from all sorts of surfaces. Previously confined to highly regulated applications, now ultrasonic cleaning is even used in the household due to the availability of biodegradable cleaning solution concentrates and low-cost cleaning equipment.
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 PCB.
Cavitation is 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.
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.
While these numbers may sound as if they are harmful, the process is so fast that it does not damage parts being cleaned. In fact it in no way compares to the potential damage of hand scrubbing board surfaces with a toothbrush and IPA. The temperature of the cleaning solution will rise as cavitation proceeds but does not reach harmful degrees. That said it is not a good idea to reach into an operating ultrasonic bath.
Specifics on PCB Cleaning
Let’s start with the premise that PCB are pretty much the same whether for complex or simple apps. Let’s add the fact that ultrasonic cavitation reaches all surfaces of parts wetted by (immersed in) the ultrasonic bath. We’ll now look at some details involved in setting up a cleaning operation.
Ultrasonic Frequency
Measured in cycles per second, ultrasonic cleaners can produce from 25 kHz to 100+ kHz. Lower frequencies create larger cavitation bubbles compared to higher PCB 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.
The relative size of the bubbles is another topic but take our word for it. All sizes are small – really small.
PCB Ultrasonic Cleaners
There are many ultrasonic cleaners on the market. Selecting one for cleaning PCB requires a bit of intelligence on features that should be offered.
A good frequency for cleaning PCB is 37 kHz. The cleaner should have a feature called “sweep” which causes a slight variation in ultrasonic frequency to eliminate potentially damaging harmonic vibration and what are called “standing waves” of cavitation that can damage components.
A thermostat to set cleaning temperature and a timer that automatically starts cavitation when the temperature is reached are other desirable PCB features.
