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What components populated on a PCB are not safe to clean in an ultrasonic cleaner?

It is not limited, but I specifically wonder below components if they are safe to use or some requires special attention (low temperature, low power, or what kind of attention is recommended):

  • MCU/ICs
  • SMD resistors
  • MLCC
  • SMD inductors
  • Aluminium electrolytic capacitor
  • Crystals
  • u.FL, SMA,.. type RF connectors
  • power transformer (e.g, flyback)
  • SMD transistors
  • Optocouplers
  • Choke
  • Fuse
  • SMD LEDs

I read that in some cases crystal oscillators may get damaged because of the ultrasonic power. What characteristic does it depend on? I have 24 MHz, 40 MHz, and 32.768 kHz crystals with 20 ppm tolerance. Does any of them sound on a risk group?

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  • \$\begingroup\$ What do the datasheets say? \$\endgroup\$ – venny Oct 20 '15 at 21:22
  • \$\begingroup\$ Most of them don't have any note about it. It is good to know the general practice and if some components (like crystals) require special attention, then I will email the manufacturers. \$\endgroup\$ – Angs Oct 20 '15 at 21:25
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    \$\begingroup\$ I don't really know if this is a good question format for this site, but MEMS come to my mind (and possible lots of other sensors) \$\endgroup\$ – PlasmaHH Oct 20 '15 at 21:50
  • \$\begingroup\$ @PlasmaHH This is an alright question. \$\endgroup\$ – Nick Alexeev Oct 20 '15 at 22:16
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In case someone would wonder the same question. Here are some answers from the manufacturers:

Crystals (very clear and nicely explained):

XTALs are generally sensitive for ultrasonic cleaning, as their function principle is based on a electro-mechanical oscillation.

Ultrasonic cleaning on the other hand is vibrating the XTAL and thus might mechanically excite the XTAL in a resonance frequency and thus cause some mechanical damage.

This risk is esp. high on kHz XTALs as they are based on a tuning fork crystal chip with a main oscillation frequency of 32.768kHz. There are however as well other “side” resonance frequencies in the system which might cause some problems.

So there is the general recommendation to avoid esp. kHz XTAL to be exposed to Ultrasonic cleaning. MHz XTALs like the TSX-3225 are far less sensitive (as well because their nominal resonance frequency is far higher than the Ultrasonic cleaning frequency).

Transformers:

the transformer is a little sensitive to the ultrasonic cleaning process since the transformer is not hermetically sealed.

Questions arise when it comes to coils that are not hermetically sealed and run through a wash process. Approximately half of all non-hermetically sealed coils in the industry are water-washed during the PCB cleaning process and the other half are washed with an alcohol based cleaner. Of the ones washed in water it is quite rare for any of them to exhibit problems during their life cycle. However in some case transformers in the industry have exhibited shorted turns and opens later in the life cycle which can be attributed to the water wash process.

MLCC: (Yageo)

To prevent the adhesion of the terminal electrodes being degraded, ensure that the ultrasonic energy is not too high and follow the recommendations below: - cleaning time should not be greater than 3 minutes - frequency: 40 kHz

Film resistor: (vishay)

Ultrasonic cleaning should be done with power regulated equipment. Older 25 kHz, unregulated equipment can damage joints and components

One IC manufacturer refers IEC 61760-1:2003-12 (surface mounting technology - Part 1: Standardized method for the Specification of surface -mounted devices (SMD).

Washing Process with Ultrasound <= 80ºC / <=6 min, Medium (Water, Ethanol, Isopropanol, Alkoxyde, Propanole, Aminoalcohol as well mixtures of the former).

For the rest, I didn't get any clear answers. I guess nobody wants to take any responsibility in case anything goes wrong with their products. They all recommend to test it by myself and take the responsibility.

I can do tests on some samples. However, even if no components are damaged after the test, it will be a question if the process effected the component's long term stability/life time, etc.

It is my impression that I shouldn't use ultrasonic cleaner at all unless it is a hobby circuit,

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  • \$\begingroup\$ Starring and +1. Good stuff to have. \$\endgroup\$ – Sean Boddy Oct 28 '15 at 18:48
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MEMS ICs, such as accelerometers, gyroscopes, and some small microphones, must never be placed in ultrasonic cleaners. These ICs contain mechanical silicon structures, and placing them in an ultrasonic cleaner will often destroy these structures. (Some of these parts may even be damaged when a tape of ICs is dropped, or if the board is snapped apart.)

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I have submerged whole, finished cell phone mainboards in ultrasonic cleaners and had mixed results. In these cases, non functioning units were likely already destroyed before cleaning began, as a function of having been dunked in sugary carbonic acid while operating.

In my experience, ultrasonic cleaning of a finished PCB is a last ditch effort in an attempt to remove difficult substances from hidden surfaces. In many cases, a light cleansing with isopropyl alcohol and an acid brush is all that is required, followed by a deionized water rinse and an overnight air dry.

Because of the many variables involved in the placement of parts, application of solder, mechanical strain and fatigue, there isn't a good way to predict ahead of time whether or not a round in the ultrasonic will be the last straw for a particular piece; this is why technicians generally won't jump straight to ultrasonic cleaning except for obviously durable parts, mostly made of metal, plastic, ceramic and the like.

If you shake something enough times, it will break.

Edit: Connor pointed out to me a logical error. My case above is a pretty thin slice of when and how ultrasonic cleaning is used, and as such may not actually apply to the OPs use case. That being said, better answers should follow shortly - I'll leave this here, as it narrowly applies.

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    \$\begingroup\$ Ultrasonic cleaning is fairly commonly used for cleaning flux from PCBs during the production process. I think you're assuming the wrong reason for their use in your answer. \$\endgroup\$ – Connor Wolf Oct 21 '15 at 0:01
  • \$\begingroup\$ @Connor, that may very well be - where it would be so much better controlled. And inspected. I did jump the gun, but it does beg the questions, where, why and how is this being done? And what post procedure inspections are available? \$\endgroup\$ – Sean Boddy Oct 21 '15 at 0:06
  • \$\begingroup\$ I've also used ultrasonic to clean cell phone pcb's that are water damaged. About the only failures I've encountered due to the ultrasonic cleaning are microphones and gyroscope ic. Both can be sourced and replaced easily and inexpensively, so usually not a major issue. \$\endgroup\$ – Kevin Jul 9 '17 at 18:17
  • \$\begingroup\$ This page is old, and I stumbled on it for unrelated reasons, but I will certify that if your board has any cold joints, a bath in an ultrasonic cleaner WILL reveal them. It is not an adequate quality assurance measure, but it will commonly cause poor soldering to reveal it's head, which may in fact be a good thing. I have even successfully used my ultrasonic cleaner to finish breaking solder joints after desoldering components. \$\endgroup\$ – Richard Smith Dec 21 '17 at 7:16

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