How does aluminium foil or esd foam protect ICs from esd damage when stored

Question

I see people storing ICs in plastic containers with aluminium foil or ESD foam on the bottom and then put their ICs on top of it to prevent ESD damage.

I dont understand this concept tbh and im confused also, how does it prevent ESD damage ?

My understanding so far:

Plastic is an insulator and insulators store/accumulate charge when rubbed against other insulators. So if the ICs inside the plastic container rattles around, it accumulates charge, this charge cant flow anywhere, then if the potential difference between the IC and the plastic box is high enough it discharges, this could cause damage.

Right so far ?

Ok now putting something conductive between the IC and the storage box solves this problem, but how. What i dont understand is how does it prevent ESD, since aluminium foil is conductive, doesnt it pass the charge to the IC when rattling around ? Makes no sense to me.. The foil is touching the plastic bin and the IC touches the foil, so still everything is connected together or not ?(the IC is still in "direct" contact with the plastic since the foil is conductive )

For me, it seems, its the same like without foil. So how does it work.

EDIT: That foil/foam only prevents ESD damage inside the storage container but not for when i grab the IC, right ?

Answer 1

The problem solved by the conducting foam is that if there is a "discharge" from the foam to earth, then the voltage between the pins of the IC remain pretty low with respect to each other. It's a differential voltage across chip pins that causes damage.

When you grab the IC, grab the foam first so that you become equalized in potential to all the pins on the IC then, when you want to place the IC onto a PCB (or breadboard) you touch the PCB to equalize you and the chip (in your other hand) to the PCB.

Answer 2

ESD protection requires a system. You are part of that system, as is your workstation, the way components are packed in storage and transit, and the way you move components between them. The purpose of the whole system is to avoid, at any point, putting a differential voltage across two pins of the sensitive IC.

A conductive liner to the storage/transit container keeps the potentials of any pins that touch it the same.

When you move a component from one conductive environment (say a transit box) to another conductive environment (say placed on the conductive bench top), you first of all bring those two conductors to the same potential. We usually use ground for convenience, but it's not essential. So touch the metal liner with your grounded finger (you are wearing a wrist strap, aren't you?) before removing the IC and placing it on the grounded bench top.

Answer 3

The foil keeps all of the outside of the device at the same potential. As others have said it is difference in potential and the currents associated with those voltages that kills devices.

If you zap the foil it acts as a shield and a Faraday cage. The voltage on all the pins rise almost in unison so no damage is done.

However, when using through hole parts, do not use foil.

The legs will perforate the foil and they can become "disconnected" from it. This leaves them exposed to ESD. All too often I have seen this done and then seen the thing plugged into a piece of non conductive foam.... (Styrofoam)..sigh.

BTW: Chips should not be bouncing around in "plastic boxes". ESD plastic boxes have a filler or coating in them that make them conductive. The foil trick is used as a stop-gap when you do not have access to proper containers and bags.