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June 19, 2006

Ultrasonic cleaning for PCB assemblies

We are thinking to use ultrasonic cleaning for PCB assemblies, SMD 2 sides with some through hole.

1. Is Ultrasonic cleaning recommended for PCB assemblies with built-in oscillator ICs & crystal, electro magnetic relays?

2. Does the Ultrasonic cleaning affect on the component & IC chip bonding?

3. Is there a mil standard or any other standard that recommends ultrasonic cleaning?



Sunil S. Bhurake

Experts Comments

There was an article published in the October, 2004 edition of SMT Magazine titled "Why Not Ultrasonic Cleaning?" that should help answer your questions.

It discusses IPC test methods and reliablity testing for electronic assemblies.

Ram Wissel
Engineering Services Manager
Kyzen Corporation
Mr Wissel works with Kyzen's Engineering Services Team specializing in developing unique process control solutions that are aimed at optimizing chemistry usage.
NOTE: Mr. Wissel is no longer working at Kyzen.

Safe ultrasonic cleaning of PCB assemblies is a very common question. For that reason, we maintain several published articles on the Smart Sonic web site: http://www.smartsonic.com/article.html. Most of the studies regarding ultrasonic cleaning of electronic assemblies have been performed by the EMPF Laboratory and GEC Marconi (Dr. B.P. Richards' Group).

In a nutshell, the history of ultrasonic cleaning of PCBs got off to a bad start in the 1950s when Westinghouse developed ultrasonic cleaning technology for the U.S. Military. The original technology used very low frequencies and high power densities (the amount of electronic energy used to cavitate the ultrasonic wash bath measured in watts / liter). Low frequencies (20 35 kHz) = aggressive ultrasonic cavitation and high power densities = strong scrubbing action. The very strong and uncontrollable ultrasonic technology of the 1950s was not compatible with the delicate transistors and wire bonds of the times. However, today's ultrasonic cleaning technology is the most reliable and controllable precision cleaning technology available and has shown to be very compatible with today's robust SMT assemblies.

An analogy can be made by comparing it to manual scrubbing with a brush. Low frequency would be like using a wire brush and high frequencies would be like using a tooth brush. Low power density (10 15 watts / liter) would be similar to gripping the brush with only two fingers and exerting mild pressure. High power densities (>15 watts / liter) would be similar to gripping the brush with two hands and exerting heavier pressure.

Ideally, high frequencies (>35 kHz) and low power density (<15 watts liter/) is the combination of choice for cleaning electronic assemblies.

With all of that said, remember, ultrasonic cleaning technology is just a
mechanical scrubbing action. As with any cleaning application, the
appropriate cleaning chemistry must be determined before the mechanical
scrubbing action will prove effective.

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Bill Schreiber
President
Smart Sonic Corporation
Mr. Schreiber developed the original ultrasonic stencil cleaning process in 1989. Obtained the only EPA Verification for specific parameters of Environmental Safety, User Safety and Cleaning Efficiency for a stencil cleaning process.

Regarding the question about cleaning circuit boards with ultrasonics, this is a question people have been asking for 50 years. In the old days, most ultrasonic-equipped vapor degreasers operated at a single-frequency, usually in the 40 KHz range for solvents. It was found that certain components, usually diodes and other large devices, could suffer damage to their leads from the harmonic vibrations constantly pounding of the ultrasonic frequencies.

Remember the collapse of the Tacoma Narrow's bridge, "Galloping Gertie"? It is the same effect only with wind instead of ultrasonics providing the energy.... See http://www.enm.bris.ac.uk/research/nonlinear/tacoma/tacoma.html#mpeg

However, in today's world almost all ultrasonic systems using frequency "sweeping" to enhance the cleaning process. Thus, the transducer will start at 25 khz and race up to, say, 90 khz, and then drop back down in random patterns. Most companies using modern, digitially controlled sweep ultrasonics find they can do so, either in aqueous system or in solvents, without damage to their boards. (Of course, a lot depends on the amount of energy the transducers are blasting into the liquid... so get some expert assistance before specifying a cleaning machine.)

More importantly, how do YOU determine if the cleaning cycle is damaging any componnets? There are standardized tests for this specific issue. Almost always, the components are cleaned in a cycle ten-times longer than your worst-case cleaning cycle. Then, simply mechanical "pull strength" tests and electronic functional testing determines if the cleaning has harmed the components. A ten-times margin of safety is considered sufficient for most applications.

For more info see:  Electronic Ultrasonic Cleaning by a fellow named Kenyon, pretty well explains the process.

Best regards,

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Mike Jones
Vice President
Micro Care
Mr. Jones is an electronics cleaning and stencil printing specialist. Averaging over one hundred days a year on the road, Mike visits SMT production sites and circuit board repair facilities in every corner of the globe, helping engineers and technicians work through the complex trade-offs today's demanding electronics require.
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