Modify ultrasonic cleaner for swept frequency

[aandrew]

Hey everyone,

I think that the next piece of equipment I should get in my home lab is an ultrasonic cleaner for circuit boards. I know that there are cheap versions which are fixed-frequency but have heard that these are not as good as the (significantly more expensive) swept frequency units.

Now being the tinkerer and never being able to leave well enough alone, I got to thinking... What's stopping me from buying the fixed-frequency (40kHz) one and hacking the oscillator to sweep the frequency a dozen Hertz in either direction?

I understand that the actual driver might have less output when driven off its center frequency, but I don't see a real technical challenge in this. I'm sure I'm not thinking of something, which is why I've asked you all about it here... What might I be missing which would kill this idea?

And while we're here... what sort of cleaning fluid should be used? Just distilled water? Should I worry about something that would dissolve the oldschool organic fluxes?

[ebclr]

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[Kleinstein]

With the cheap US cleaners the oscillator and transducer are still on life potential. So measuring and adding things can be tricky and dangerous They might also use a self resonant circuit, so there is no independent oscillator to sweep. Of resonance there will be likely to little power.

One disadvantage of the fixed frequency can be that cleaning is not efficient at some places in the bath. So my fix to this would be a mechanical one, by moving the parts in the liquid or maybe modulating the water level.

[MagicSmoker]

Ultrasonic resonators tend to have very high Q, so might not resonate nearly so efficiently if driven even a "few dozen Hz" off of resonance. That said, I suspect the resonant frequency already drifts that much in the normal course of operation from temperature rise, shifts in loading, etc, so you might be getting the benefit - whatever there might be - of a slightly variable frequency already. Otherwise, if the goal is to eliminate uncleaned areas because of standing waves or interference or the like then mechanical agitation of the bath contents helps immensely.

[retrolefty]

Ultrasonic resonators tend to have very high Q, so might not resonate nearly so efficiently if driven even a "few dozen Hz" off of resonance. That said, I suspect the resonant frequency already drifts that much in the normal course of operation from temperature rise, shifts in loading, etc, so you might be getting the benefit - whatever there might be - of a slightly variable frequency already. Otherwise, if the goal is to eliminate uncleaned areas because of standing waves or interference or the like then mechanical agitation of the bath contents helps immensely.

 I agree, the first order of research is if the 'inexpensive' cleaner's transducer will support a frequency bandwidth that 'sweeping' might benefit.

[aandrew]

Wow, such great responses. Thank you everyone for your input and suggestions!

I agree, I think I ought to buy a cheapie and see what I *can* do with it.

I'd always thought the fixed-frequency units were frowned upon for electronics cleaning because they might damage components at resonance, didn't think it might have only been a standing wave problem.

[ian.rees]

I've got one of these https://www.aliexpress.com/item/Globe-110V-220V-PS-10A-80W-Digital-heated-Ultrasonic-Cleaner-2L-C-with-free-basket-free/32240448259.html (2L heated tank, 80W ultrasonic cleaner, from aliexpress), and went through a similar thought process.  In the end though, it works great for my purposes as-is, so I haven't modified it at all.  If I were going to modify the ultrasonic cleaner, the first thing I'd do is disable the beeper.

The cleaner does use a self-oscillating circuit driven from mains potential, can't recall the topology.  The power PCB layout is a bit sketchy in mine; there's some mains that goes awfully close to the low voltage power supply for the front panel.

I use it mainly for cleaning flux off of small parts (tiny coils) - I use tap water in the main tank, and put the parts in a glass beaker full of either 99% isopropyl alcohol or DI water (from the dehumidifier) - works great.  At the start, I stop/start the ultrasonics every few seconds for a minute or two, it helps to get any air bubbles that have coalesced out of the cleaning fluid, or were trapped in the part when it was submerged.  Have had good success with using a little bit of detergent in tap water for cleaning non-electronics things.

[John Heath]

A little old school but I have a diced tomato parts washer. One tin of diced tomatoes minus the tomatoes glued to a turn table at 45 degrees. A mini cement mixer would be a close description. Filled with warm soapy water it would clean even the dirtiest knobs after a 1/2 hour cycle of rotating.

Back to piezoelectric resonators. Cheap stereos use them as tweeters however their natural resonance is around 50 KHz so they are always off resonance. If the amplifier were to go to 50 KHz I suspect the crystal would crack much like a wine glass when energized at it's natural resonate frequency. Then again it would be fun to try just to see if set frequency cleaner could be changed to a sweep type. The bottom line to ultrasonic cleaners is those little vacuum bubbles that bash the cleaning fluid against the surfaces that need to be clean it. Without the cavitating bubbles all is lost for fixed or sweep frequency. 

[lacek]

I'd always thought the fixed-frequency units were frowned upon for electronics cleaning because they might damage components at resonance, didn't think it might have only been a standing wave problem.

I have some reservations:
1. the lowest frequency of  piece of matrial is determined, in addition to sound velocity in medium, by the size of the object. It is generally given by 2v/L, where v is the sound velocity, and L is the length. Keeping in mind how small are SMD components on PCB boards, isn't the lowest resonance frequency way above the frequency range to which such ultrasonics cleaners are set (typically around 80 kHz)?
The sound velocities for common solids are in the range 1000-6000 m/s (for water approx 1500 m/s). This means that the the object which can be resonantly shaken should be larger than approximately 1.5 cm.
The only such object is the PCB itself. This means that the function describin the wave will not penetrate the components, but will be supported only in the PCB itself. This means that the whole board may be "bending", but components will oscillate together with the board. Of course even slight bending of the board potentially can cause metal fatigue to the solder joints (especially lead free), but I want to understress that we are not in the situation when a single component oscillates.

2. In microwave oven the problem of "standing waves" is solved by rotating the food on a tray.

3. The practical problem I have with fixed frequency cleaner is that resonance frequencies depend on how much liquid is in the tank, and what is its temperature. It may mean that adding a few spoons of liquid is the difference between good and bad cleaning. A good cleaner should look for maximum energy absorption by the tank and adjust the frequency to ensure the standing waves are generated.  I want to distinguish between "random frequency sweeping" and "active resonance frequency following".

[Marco]

AFAICS the difference in the amount of water less shifts the resonance (mostly determined by the crystal, then the steel tank and finally only a little by the water) than affect the matching. Electrically the impedance of the system changes even at resonance. You need more voltage.

[poorchava]

I've once analyzed the drive circuit in an ultrasonic air humidifier (I could bet my money on cleaners using the same topology) and it used a kind of self-oscillating push-pull converter to drive the transducer. In tha case sweeping the frequency would not be possible, as it is a product of many physical properties of components being used .

[David Hess]

3. The practical problem I have with fixed frequency cleaner is that resonance frequencies depend on how much liquid is in the tank, and what is its temperature. It may mean that adding a few spoons of liquid is the difference between good and bad cleaning. A good cleaner should look for maximum energy absorption by the tank and adjust the frequency to ensure the standing waves are generated.  I want to distinguish between "random frequency sweeping" and "active resonance frequency following".

All of mine drive the transducer at the resonate frequency of the system as a whole so adding water changes the frequency such that it always follows resonance.  If the transducer was driven at a fixed frequency, then it would not work very well.  The nonlinear response of the system also means that simple inexpensive designs use an unfiltered supply voltage so that the half-sine output from the rectifier sweeps the frequency slightly.

[Marco]

This patent explains the mains frequency modulation, it also shows the large swings in electrical impedance with water depth.

Old patents are the best.

[Marco]

BTW, I don't see how a Hz level shift near the resonance frequency can help much. A 30 cm tank is only 8 wavelengths across at 40 kHz, don't you need to shift the frequency by 2.5 kHz to turn a node into an antinode? Not sure how that works in 3D though.

[woody]

Not too long ago I build a driver where I could set the frequency with a pot. Based on the generic 'self oscillating' drivers you see everywhere, but then driven from the NCO in a PIC 16F150x. It worked, but to be honest I did not see a whole lot of difference with a self-oscillating driver.

I always assumed that, apart from preventing damage to small parts in a cleaning tank, another reason for using a swept frequency is that you do not want to operate the transducer exactly on its resonance frequency, because it gets a bit out of control then. For sure the more expensive driver boards (based on a KA3525A that generates a more or less stable, adjustable frequency) employ a special circuit to generate a sweep. So whatever the reason, it is probably needed.

Paul