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(ultra)acoustic in circuit component level crack detector?
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rhb:
The ideal glue would have a density of  2.65 grams/cc and be water thin.  I'm going to look for a source of sodium silicate.

A single transducer as both source and receiver (e.g. an old style medical ultrasound)  is the only way I can see to make this work.  Otherwise you have to solve for the response of the tweezers and correct for it.  That's a major PhD dissertation project over the course of several years.

But it *might* be possible to get distinct reflections from the chip to board interface and the back of the board.  A strong reflection before the back of board reflection would indicate cracking.  But there is the problem of the reflection from the backside of the transducer support.

Last night I hit on the idea of a piece of thin sheet ( e.g. 0.040 aluminum or plastic) supported by a spring.

The requirement is that we get the reflections from the front and back of the xtal to arrive at different times.  If we can't do that, then it turns into another PhD project.

This whole thread is completely insane.  But I'm having a lot of fun playing so I'll continue until we succeed or it's no longer fun.

We are fighting very basic physics, so I expect we will lose.  But I never expected the clean pulse response I got from the bare xtal. 

Another experiment that I just thought of is to place two xtals as close together as possible, but not touching and examine that response.  The xtal may be operating in shear mode in which case there will be no response from the receiver. That changes the physics a good bit.
coppercone2:
with the tweezers and reflection, can you make lopsided tweezers where one portion is really massive compared to the other one, so it acts like a anvil? Maybe dissimilar metals (use a dense steel for the anvil and a aluminum for the transducer or w/e? Just so you can do the measurement through the component in the direction where cracks are likely to form.

Sodium Silicate is cheap, I use it for making sand molds in lead casting that are hardened with carbon dioxide.
rhb:
If the transducer is attached to a large mass it won't vibrate.  That's the first order problem that needs to be solved.
coppercone2:
no I mean just do a reflection test but put a backer on the part so it mimics the PCB board.
chris_leyson:
For crack detection you're going want to use pusled ultrasound to detect the distance to a crack or boundary and in some ways it's not too dissimilar to time domain reflectometry. You transmit a short burst of ultrasound into to device under test and then use the ultrasound transducer as a receiver and measure the time taken for the signal to return, if you know the propagation velocity in the material you can work out the distance to the fault or boundary. Given that the DUT is going to be small and the propagation velocity is quite high you will need to operate at some tens of MHz. I've seen 20MHz transducers used for medical Doppler at very short distances, just under the skin and 20MHz is probably the upper limit for medical ultrasound although I have seen 40MHz transducers.

There are a few companies that sell bare unmounted PZT, lead zinc titanate, transducers with metalized surfaces. Mounting them is a problem as you have to make electrical contact with both surfaces and for larger low frequency transducers used in medical applications you can get away with soldering small, say 38AWG, wire onto both sides. At 20MHz or 40MHz the disc transducers are much smaller and become difficult to mount and attach electrodes to. Small high frequency transducers are rather speciallized and expensive and not something I would contemplate building at home. These guys have a good range of PZT transducers, mostly low frequency though. https://www.steminc.com/ Maybe send them an email, you never know they might have something suitable.
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