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| (ultra)acoustic in circuit component level crack detector? |
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| coppercone2:
I am actually not sure in which direction cracks in SMD parts form, I thought it was perpendicular to the PCB but I am sure both types are possible. |
| rhb:
I'd expect mulitlayer capacitors to crack parallel to the layers. So it would depend upon how they were mounted. I've got an ~1/4" length of 3/8" aluminum rod with an xtal glued to it with hotmelt. I can just barely make out the reflection around 137 nS and I've got a lot of ringing in the hotmelt layer. A friend who spent his career doing this sort of stuff in a major oil company lab suggested I try honey to attach the xtal to the sample. The signal is very faint as I can only apply 5 V drive, but at the moment I'm DC coupled, so I may be able to double that to 10 V. It's still a long way from what I've done so far to having a usable tool. But just being able to check for cracked solder joints would be very useful. But I've got to figure out a way to reinforce the xtal without loading it too much. The xtal is the thickness of 2 sheets of copy paper and very brittle. The need to make electrical connections to both side of the xtal further complicates things. I think the next test will be to sweep the xtal from 1 Hz to 120 MHz and see how flat the response is. That would solve the limited power input, but if you have to sweep for an hour it won't be much use. In any case, the DSP code to make a tool rather than a lab demo will require significant effort. I'm very familiar with it, but it's very fiddly. I'm going to look for some lower frequency xtals as they will be thicker. I've always wanted to do this with rocks, so I'm having a blast even if this doesn't work out. I never had a chance to actually do anything hands on. I just got sheets of paper and did a bunch of calculations. Just not as much fun. Have a Merry Christmas. |
| coppercone2:
a crazy option is to maybe use liquid metal that is adhered to something by surface tension to make a low mechanical resistance connection to the crystal. Like two droplets of liquid metal that are upside down that touch the crystal legs. Otherwise an option might be to take incredibly fine wire (say from mutli strand silicone wire that is very fine) and make a weave (I noticed solder braid is very nimble). Not sure what weave structure would work but people make weird bracelets that are pretty nimble. you could assemble it with fine tweezers under magnification but it would be a japanese effort. I don't know what the finest gauge wire they sell on spools is but I have a hunch crazy high strand count conductor would be finer. You can split a wire with a exacto blade or a special jig to remove the insulation and access the fine strands. glue dots, silicone putty (silly putty), double sided tape, pitch, tar, fun tack (they sell it at home depot), self bonding tape, liquid electrical tape (doubtful), all come to mind as a inbetween to honey and hot glue. I am also wondering about if you made a little gasket between the crystal and the tweezers, filled it with honey or whatever, then used some kind of rubberish zip-tie to hold it in place (they sell weird stuff like this from panduit to act as 'softer' couplers. Also I have seen that gaskets are actually bonded with cyanoacrylate (if you wan to make a custom replacement gasket with a gasket kit). This might also mean you can make a custom sized rubber band by cutting an existing one precisely and using super glue to bond it. and instead of honey there are also dielectric lubricant gels like the ones used for crimps but they might actually be worse for circuits if you follow nasa guidelines about glass formation on arcing relays as a result of silicone contamination. But in all seriousness honey could result in a life-form problem of some kind ranging from microbial to rat or maybe even bear sized if it warms up and smells nice. |
| rhb:
Dried honey is a water soluble glue that will form a very thin film. The operative word is "thin". It's also easy to handle which super glue is not. Glue dots are a nightmare. Double sided tape might work, but it's still thicker than a film of honey. The requirement is that the layer be extremely thin so that the resonance in the layer is very high. If that is not the case, most of the energy is trapped in that layer. |
| coppercone2:
oh so its not really a big gap filler, you are not trying to get a wider contact area, just a better contact area (in the edge)? I imagined a big glob. yea the only other thing that comes to mind right now is cooling down the tweezers with a peltier to increase the viscosity of something inorganic |
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