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As I understand it, metallic static shielding bags act as a Faraday Cage once sealed which protects the contents against external electrostatic discharges. They also prevent triboelectric charge buildup, protecting the contents inside the bag.

However, does this stay true if for example a PCB is wrapped in plastic or paper before being inserted into a metallic static shielding bag which is then sealed? Would this material be able to damage the PCB even though it is contained inside the faraday cage?

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However, does this stay true if for example a PCB is wrapped in plastic or paper before being inserted into a metallic static shielding bag which is then sealed?

No

Would this material be able to damage the PCB even though it is contained inside the faraday cage?

Yes

They primarily prevent triboelectric charge buildup on the body of a person handling the contents from being transferred by conduction. They also prevent such transfer from other materials that the might come into contact with such as mechanically-protective packaging materials used for shipping and storage.

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[Is it safe when] a PCB is wrapped in plastic or paper before being inserted into a metallic static shielding bag which is then sealed?

I wouldn't think so.

Would this material be able to damage the PCB even though it is contained inside the faraday cage?

Seems possible.

The triboelectric effect describes how electric charge is generated from material contact and movement.

Here is a thought exercise. Imagine a circuit board is completely wrapped in a packing product then is placed into a grounded metal box. Then the box is shaken. The triboelectric effect says electricity could be generated. Since the box is grounded, that charge will dissipate. But what about the circuit board? Since it is completely covered and most packing materials are good insulators, there is no way for this charge to dissipate. Someone comes along, opens the box, unwraps the board, and touches it. ZAP! Dead board.

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As I understand it, metallic static shielding bags act as a Faraday Cage

No. They are neither conductive nor thick enough to effectively block electrical fields.

which protects the contents against external electrostatic discharges

They do, but mostly because they're OK insulators between in- and outside.

A normal plastic bag works better than the antistatic bags at this: there's never a path through the contained electronics that is of lower or comparable resistance as along the outer surface of the bag for a good insulator.

However, does this stay true if for example a PCB is wrapped in plastic or paper before being inserted into a metallic static shielding bag which is then sealed?

That sounds more like a ESD endurance test than antistatic packaging:

Static electricity will form on the insulating packaging material due to friction surface effects, and one unlucky shift of weight suffices to make the board be the electrically easiest way for built up charges to travel - either to oppositely charged surfaces, or through the board to the metal box.

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