Hi,
I've bought myself one of these ultrasonic cleaners.
http://cgi.ebay.co.uk/NEW-6-5-LITER-INDUSTRIAL-ULTRASONIC-CLEANER-WITH-BONUS_W0QQitemZ390172147214QQcmdZViewItemQQptZUK_Jewellery_Watches_JewelleryBoxes_Supplies_CA?hash=item5ad8127e0e
Mainly bought to clean the carburretors in my motorcycles (and my dentures when my teeth fall out 
)
I had the pleasure of seeing an ultrasonic cleaner rip forty years of crud off of some old Greyhound bus parts once, and it was near magical. They're absolutely awesome for automotive parts, but you do have to be a bit careful about overdoing it when working with parts that have a fine tolerance. I went a bit overboard once and pitted the heck out of a machined surface, lesson learned.
I'm assuming that the output is a single frequency at 50W (misdescribed as 150W). I've been reading that single frequency units can cause damage to a PCB due to resonance. Articles suggest the use of a swept frequency instead.
My question is it possible to modify the oscillator in these units to produce a frequency sweep of 40kHz +/- 3%? I was thinking some kind of varactor diode voltage controlled by a ramp waveform.
Does anyone have any experience of using ultrasonics to clean boards?
Thanks.
David.
Ultrasonic cleaners are fairly simple to manufacture and their use is usually quite forgiving of tolerance, so I wouldn't trust the specs to be very precise unless you can get info from the manufacturer (and trust it). A cleaner's power output can be subject to a similar level of ad-wizardry and jackassery as a hi-fi stereo. If you have the capability, I'd recommend taking your own measurements to get a baseline.
The problem with frequency sweeping is the transducer. The electronics can be adjusted to output whatever power and frequency you like, but the transducer itself is only capable of a certain frequency response. It's similar to an audio speaker, but the physics is a bit weirder for having to deal with metal and fluid instead of just air. A typical transducer is made up of bonded layers of various materials in addition to the crystal and designed to have a resonant frequency at which it is most efficient, and departing from it's resonant frequency at significant power levels will cause attenuation and (worst case) possible damage to the transducer. Broadband ultrasonic transducers with wide flat frequency response do exist, and are likely available at high enough power to clean with, but my experience with them is low power non-destructive testing. It's very unlikely that the manufacturer used a high power (multiple watts plus) broadband capable transducer and left out the ability to sweep the frequency over a large range, as I imagine such a unit would be relatively expensive (and heavily marketed as such). The frequency response of a tuned transducer will have one good peak that it's designed for and usually at least one other peak (far) above and below that it can operate at with reduced efficiency, but will operate poorly in between. The cleaner I used for bus parts had the ability (highly touted) to sweep frequency a few KHz to keep standing waves from developing within the parts, and this is definitely a good thing.
I imagine that the problem with cleaning a stuffed PCB ultrasonically is the parts themselves responding unpredictably. Sure enough, the
IPC has a test method for it (love those guys, they've got something for everyone). You might have to upgrade to military spec parts, but if you can find parts that are IPC-TM(Test Method)-650 certified then you'll know what they're guaranteed to endure. My guess is that frequency sweeping is designed to cut down a good deal on the parts, leads, and joints resonating as opposed to the PCB itself. I think a bare PCB is much less likely to be damaged by a single frequency cleaner, but I suppose track adhesion and plated through holes would also be potential vulnerabilities. Then again, I suppose ultrasonically cleaning a naked PCB is a bit of overkill anyhow, and nowhere near as useful as cleaning a stuffed board.
All things considered, were I in your shoes I'd definitely experiment with the cleaner as manufactured, you never know! Modifying it without replacing the transducers may work to some extent, and the reduced average power from broadband sweeping might work in your favor, but you'll still be delivering a huge spike of energy at the transducer's resonant frequency (plural, if you sweep that far), just less often. Using different transducers is an option, but they tend to be the bulk of the cost of a cleaner, so at that point you might want to consider building from scratch. It can be a real pain to get a transducer to behave efficiently, but it could be a neat project.
I hope I got the details right, it's been a few years since I worked with ultrasonics. I trust peer review to straighten me out if I drifted to far.
Hope that helps some.
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