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I took apart a Huawei E3372 LTE Modem, and this is what I saw. A jelly like substance on the electromagnetic shield.

Teardown Huawei E3372 LTE Modem

It is very soft and a bit sticky. I guess it is there to dissipate the heat from the modem, since it couple well to the back enclosure of the modem.

Enclosure Back

What is this substance and what is the function?

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    \$\begingroup\$ These things can get really hot after several hours of operation, so this extra dissipation is a pretty good idea to make it last longer and also not burn your fingers. \$\endgroup\$ Nov 20 '16 at 15:29
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It's a thermal pad. It performs the same function as thermal grease except that it's easier to apply but is slightly less effective.

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  • \$\begingroup\$ Look up "thermal gap filler" or even just "gap filler" and that will tell you more about it. \$\endgroup\$ Nov 20 '16 at 21:56
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I do second Ignacio answer, but that thing might also be something else.

It does look like a thermal pad, but RF shields usually do not contact the chips underneath, plus it is pressing against plastic, which is far from optimal to dissipate power.

The design do not include any screw to fasten the PCB to the casing, this leads me to believe that the pad was thrown in also for mechanical stability. It is probably more cost effective than glue or something similar, possibly they even had it lying around in the assembly house.

It does improve thermal performance to a certain extent, of course, but I do not believe it's the entire story.

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    \$\begingroup\$ @pstan Can the RF shield be popped off? Or is it soldered down? (photo is not convincing). As Vladimir says, it may be a pressure pad to ensure that RF shield contacts the surrounding "ground" on the printed circuit board, if it is not soldered down. \$\endgroup\$
    – glen_geek
    Nov 20 '16 at 17:43

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