about antistatic bags

Hello,
The antistatic bags that come with PC motherboards, for example, are used to carry static electricity devices.. but, what do these bags really do?
If you put circuits on top of one of these bags, it is safe to work on?

Of course, I suppose that grounded antistatic mat is better.
If you don't ground a mat, does it still work? (it might sound stupid, but many people recoment you just connect your wrist band to ANY metal object in the room, or even, not using any wrist band at all, just touching some metal agains the floor. These solution just don't convince me.. )

Static electricity is disipated by ANY condutive medium, it doesn't have to be grounded. The main reason for grounding is have a stable reference voltage/medium that can replicated . If you shuffle your feet on carpet (in a dry environ) then touch a tin can, your charge is diipated by the tin can whether it is earthed or not. Me, I always try to have the conductive surface earthed.

"The main reason for grounding is have a stable reference voltage/medium that can replicated "

Why could this be usefull? Any situation where it is important??

if you have two surfaces you're working on it's nice if they at the same potential.

Paul Obrien said:

Static electricity is disipated by ANY condutive medium, it doesn't have to be grounded. The main reason for grounding is have a stable reference voltage/medium that can replicated . If you shuffle your feet on carpet (in a dry environ) then touch a tin can, your charge is diipated by the tin can whether it is earthed or not. Me, I always try to have the conductive surface earthed.

Click to expand...

Sorry, not true.
If the can is not earthed, both you and the can will be charged.
You will be at a lower potential than before the contact, and the can will be at a higher potential.

In reply to the original question, there are different types of antistatic bags.
Some are made from a (slightly) conductive material, others are made from a plastic film which has a thin coating of metal on the inside, the idea being to create faraday screen when the bad is sealed.

Wrist straps and antistatic mats should be connected to earth.

It is not a good idea to power up a circuit board while it is sitting on an antistatic bag or mat.

JimB

The key to eliminating ESD damage is to get you, you tools and the device at the same potential so that there can be no current flow among the various elements, grounded or not. Having you and the mat grounded usually simplifies everything, but carries absolutely no guarantee of ESD safety if you don't do things right.

Dean