Anti-static vs. conductive vs. dissipative foam?

Question

UPDATE: I have included some basic answers inline that I received after talking in depth with an application engineer at a foam company. I think it would still really help for someone to help flush out the theory behind these answers.

I am having some custom foam inserts cut to transport a fully assembled tablet/laptop inside a polypropylene hardshell case (Pelican brand). This kit will be used in a very low humidity environment, which is why I thought it might make sense to use either a conductive, dissipative or anti-static foam.

I am not an electrical engineer, so please excuse any ignorance that follows, but these are my thoughts (I would greatly appreciate clarification!):

Conductive Foam: Conductive foam is a polyethelyne foam filled with carbon (as are all black ESD foams), giving it its conductive properties and color. Conductive foam is designed for repeat use, and will act like a Faraday cage when an item is fully enclosed within it. Because of this a silver conductive bag is not needed as an outer container when using conductive foam.

Conductive foam will drain batteries if their contacts have a path the foam, so care must be taken to prevent this (by insulating the contacts or using a static dissipative foam layer between the conductive foam and the item being packed). There are two grades of conductive foam: lead-insertion grade and component grade.

Lead-insertion grade conductive foam is designed to have components directly mounted into the foam via their leads, and will drain charge off of attached components.

Cushion grade conductive foam is designed to package something like a circuit board or hard disk drive.

Conductive foam should be used when it's properties as a faraday cage and conductor are needed (e.g. shunting a component to ground). It has a surface resistance of 10^4 Ohms or less. It is the most expensive type of ESD foam.

Anti-Static Foam: Anti-Static foam is a polyurethane foam that is chemically doped with an anti-static agent (surfactant) and colored with a pink dye for identification. Anti-static foam will not generate static charge when it rubs against itself, but will happily pass a charge through itself to anything contained within (thanks to EEVblog for explaining that one).

Because anti-static foam does not provide shielding it must be placed within a shielding bag. Another key property of anti-static foam is that it has a shelf life when exposed to the environment. Thus it is usually used as a one-time foam for shipping components. It is the least expensive type of ESD foam. It has a surface resistance of 10^9 to 10^10 Ohms or less (the higher this number the more slowly a charge is dissipated).

Static Dissipative Foam: Static Dissipative foam is a polyethelyne foam that is either black carbon-impregnated, or pink colored and doped with surfactants. The black carbon version is permanent, and has a lower carbon content than conductive foam. The pink version has a limited lifetime, but lasts much longer than pink anti-static foam.

Static Dissipative foam is the general goto foam for reusable applications, unless you require the specific properties of conductive foam. Like anti-static foam, static-dissipative foam must be contained within a faraday cage.

Unlike conductive foam it will not drain batteries (or at least, not quickly). It has a surface resistance of 10^5 to 10^10 Ohms.

I wasn't able to confirm this, but it may have some of the properties of both anti-static and conductive foam, in that it prevents static buildup from the foam rubbing against a surface (or itself), and also allows a charge to flow through the foam to ground (although more slowly than conductive foam).

Based on my understanding I would lean towards static dissipative foam, but would be sure to contain it within a conductive layer that works as a faraday cage. I am leaning towards using conductive fabric sealed with a conductive hook and loop (velcro) tape.

The closest I found to an answer on this board was here.

I would be very grateful if someone with a better understanding of electricity than myself could elaborate on the theory behind these foams. I put together a rough guide to the best application of the three types of foam for the five following broad usage categories:

1. Bare populated boards with no battery. Single-use: Anti-static (pink) inside of a silver conductive bag. Multi-use: Conductive (black) foam. Alternately you can use static dissipative (black) foam inside of a conductive container/faraday cage.
2. Battery cells: Single-use: Anti-static (pink) inside of a silver conductive bag. Multi-use: Static dissipative (black) foam inside of a conductive bag.
3. Bare boards with battery cells. Single-use: Anti-static (pink) inside of a silver conductive bag. Multi-use: Static dissipative (black) foam inside of a conductive container/faraday cage.
4. Assembled devices without battery cells. Single-use: Anti-static (pink) inside of a silver conductive bag. Multi-use: Static dissipative (black) foam inside of a conductive container/faraday cage.
5. Assembled devices with battery cells. Single-use: Anti-static (pink) inside of a silver conductive bag. Multi-use: Static dissipative (black) foam inside of a conductive container/faraday cage.

Answer 1

As anti-static foam isn't reusable, if the laptop has no exposed electrical connections, and the threat you're guarding against is ESD damage to the laptop, and the cost of the foam is small compared to that of the laptop, then use the most conductive foam you can find, namely cushion grade conductive foam.

(I feel faintly embarrassed posting this. You should really get credit for this answer.)

Answer 2

As far as the Faraday cage around the foam, the ESD "Shielding" bags are perfect to put around the product rather than the foam. These bags are made with multi-layers that block electric charges and provide a conductive path around the product. Putting it around the product can save a lot of cost of the much larger bag needed to get around the bulky foam.

Technically, using this type of bag, any foam will work. In practicality, since the foam is in the presence of the product, the bag may be punctured or torn, or the product is often removed from the bag near the foam, the ESD foam is still desirable.

Bottom line, with a shielding bag, I would use anti-static foam; with a static dissipative bag (not recommended for products containing microelectronic devices), I would use the dissipative foam. I have not worked with products that required the conductive foam but I would not rely on it to provide a great Faraday cage with a surface resistance of more than a few ohms.

Answer 3

Conductive foam is the best because it disallows for one pin to have more electrical advantage than another when it is not used. Anti-static only deals with damage from static electricity, but doesn't nullify (totally) the possibility of the pin getting a current through it. Plus conductive foam is just more general and broad.