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Following are two pictures of a smartphone's main board (Moto G8 Plus). The first one is for reference; the second one is the one I got to repair a phone.

Unknown components

Damaged components


Questions

  • What are the golden sheet (rectangle) and the "foam" (circle) called? What are their respective functions?

  • The board from the second picture is the one I have. The sheet is clearly damaged and the foam is absent. How safe is it to install the board as-is (i.e. without replacing either the sheet or the foam)?

  • In case it's absolutely necessary to replace one or both of these components, what is the best/easiest course of action? (Examples: How feasible is it to salvage the components from the board that is currently installed? Could I install only the foam? Could I replace just the damaged piece of the sheet instead of all of it? Can other materials be used to replace them or would I have to buy the exact same ones?)

"Bonus" question: Is there some authoritative reference where it's possible to search for component names?

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    \$\begingroup\$ My guess it is a conductive foam pad that contacts something on the cover. This is for EMC reasons as it connects the copper shield to something on the cover. \$\endgroup\$
    – Kartman
    Commented Jun 10, 2021 at 8:21
  • \$\begingroup\$ @Kartman Thank you, that makes sense! What about the golden sheet, do you have any idea? \$\endgroup\$
    – wyomo
    Commented Jun 10, 2021 at 8:35
  • \$\begingroup\$ it looks like copper shielding. \$\endgroup\$
    – Kartman
    Commented Jun 10, 2021 at 9:46
  • \$\begingroup\$ like this: adafruit.com/product/4607 \$\endgroup\$
    – Kartman
    Commented Jun 10, 2021 at 9:50
  • \$\begingroup\$ Sorry, I missed the "copper" from your first comment. I just saw the foam is for the fingerprint reader. Last questions (hopefully): Do you suppose I can stick the foam pad directly to the silvery metal below, where the copper shield was damaged? Or is it crucial to first cover it with copper? In the second case, would it make sense to put a small piece of copper foil tape overlapping the shield or would I have to replace the whole thing? adafruit.com/product/1127 Thanks again. \$\endgroup\$
    – wyomo
    Commented Jun 10, 2021 at 10:18

1 Answer 1

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What are the golden sheet (rectangle) and the "foam" (circle) called?

The "golden sheet" is a thin sheet of conductive copper foil with an adhesive layer. Kartman identified an example

The "foam" is almost certainly a conductive foam. I don't know of a more specific name. Maybe "shielding gasket" or "conductive fabric over foam gasket".

What are their respective functions?

The copper sheet has been applied either to reduce EMF emissions from the parts underneath or to protect them from external EMF.

The foam likely connects that copper shielding electrically to the casing of the product or to some other parts when assembled. The casing might be conductive or have a conductive layer applied to it.

How safe is it to install the board as-is

Not safe.

Manufacturers keep costs down ruthlessly by not including any part that can be omitted or replaced with something cheaper. There are exceptions I guess, but I think you can usually assume that these sort of components are necessary for correct working under typical circumstances.

what is the best/easiest course of action?

Best - purchase new conductive foam of an appropriate specification.
Easiest - purchase new conductive foam and hope it is the right dimensions compliance and conductivity.

If you can separate the foam without damaging it, you could probably re-use it.

I would position the foam so it overlaps the remaining copper sheet on one side and also makes good contact on the other side with whatever component is there when reassembled. I would use a multimeter to check continuity between foam and sheet.

I don't think you can repair the damage to the sheet with copper tape unless it's adhesive is sufficiently conductive. You'd have to check that.

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  • \$\begingroup\$ Great answer, thank you! You have described options for the foam but not for the copper sheet. Is the "best" option to replace the whole sheet? Is the "easiest" to buy copper tape (adafruit.com/product/1127) and apply it, overlapping the existing sheet a little bit? Or (even after taking into consideration that this wouldn't be ideal) can I just re-use the foam directly on the metal that used to be covered by the sheet? By the way, the foam seems to be related to the fingerprint reader. \$\endgroup\$
    – wyomo
    Commented Jun 10, 2021 at 12:46
  • \$\begingroup\$ Answer updated. If the existing copper foil sheet used conductive adhesive, which seems likely, then you probably don't need to repair the damage and can position the foam over the exposed underlying metal. \$\endgroup\$ Commented Jun 10, 2021 at 13:20

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