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I have another question, and in discussing the root cause of the issue there with an experienced RF engineer, he suggested placing a piece of antistatic ziplock component bag between the two most important PCBs (the GPRS PCB and the main PCB with the affected sensors). This was in attempt to attenuate the signal induced onto the main board, if that was the culprit.

We cut out a piece of a Farnell component bag, which is made of opaque silvery-reflective material (I think it's called conductive antistatic bag - like the one on the right here), and shaped it so to snugly fit the underside of the GPRS PCB, with a cutout for the only header between the PCBs. Probing the bag material with a multimeter, we found it is non-conductive, i.e. likely covered in a thin plastic layer, so we weren't worried about shorts. We placed it and tested the device. The problem practically disappeared, so we can now subdivide and cover only a part of the area below the GPRS PCB to pinpoint the offending location, or the affected trace below.

Yet, the RF engineer asked whether we grounded the "shield" we made with the antistatic bag. We didn't and I thought that would be an easy experiment to do, but I found it impossible to connect electrically to the shiny material, which I thought was some kind of metal foil. We melted the plastic with a soldering iron, and tried puncturing the bag, as to expose the metal. But again, the DMM is showing infinite resistance, and even trying with a lab PSU at 20, 35, 50V shows no current flowing when the probes touch the shiny material. We punctured the bag with the probes so the probes can touch all layers. Still no conductivity even at 50V.

So the question is - what material is this bag really made of? Why can't we connect to it electrically?

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Its usually made from mylar (plastic) that is coated with a metal (usually aluminum) which forms a thin conductive layer on the plastic ( the correct term is metallized mylar). They take plastic and then coat it with a very thin layer of metal with a chemical vapor deposition process. This layer is only nm - um thick so there isn't any conductivity that can carry current from a meter. But it is enough to dissipate static.

I couldn't find any scientific papers of metalized mylar in the MHz/GHz range, but there is could be enough metal there to conduct RF. In that case the high frequency RF could be reflected or absorbed by the metalized mylar.

You can't do a conductivity test with a meter because the resistance is more than the meter can measure, probably in the 100MΩ's to 1GΩ range. It might also be that the outside aluminum oxide layer is oxidized in which it wouldn't be conductive but the small layer of aluminum on the inside is conductive. You might be able to measure the conductivity of mylar if it has a very thick layer.

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    \$\begingroup\$ There's usually a second layer of mylar on top of the metal layer too, to protect it. \$\endgroup\$
    – Hearth
    Mar 14, 2023 at 23:55
  • \$\begingroup\$ I see, so it seems we can't properly solder the antistatic bag, or rather we may try, but we can never be sure whether we soldered it properly, as we can't test for continuity? \$\endgroup\$
    – anrieff
    Mar 15, 2023 at 0:15
  • \$\begingroup\$ You might try using a high voltage insulation resistance tester, typically 500 to 5000 VDC and resistance readings up to 100 Gigohm or higher. And perhaps you can use a multi-tooth lockwasher and a brass screw to provide connection to the conductive layer, \$\endgroup\$
    – PStechPaul
    Mar 15, 2023 at 5:28

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