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.
Unfortunately most of the manufacturers and resellers of these foams don't bother explaining in detail why one might use one vs. another. 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: My understanding is that the conductiveConductive foam is a polyethelyne foam filled with graphitecarbon (as are all black ESD foams), giving it its conductive properties and color. I am imagining that it would essentiallyConductive foam is designed for repeat use, and will act like a floating ground inside of the polypropylene case, which isFaraday cage when an insulatoritem is fully enclosed within it. What I am unsureBecause of this a silver conductive bag is whether Inot 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 worry aboutprevent this (by insulating the possibility ofcontacts or using a static dissipative foam layer between the charged battery insideconductive foam and the device potentially shortingitem being packed). There are two grades of conductive foam: lead-insertion grade and component grade.
Lead-insertion grade conductive foam is designed to the ground throughhave components directly mounted into the exposedfoam via their leads, and will drain charge connectoroff 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/or USB ports on conductor are needed (e.g. shunting a component to ground). It has a surface resistance of 10^4 Ohms or less. It is the devicemost expensive type of ESD foam.
Anti-Static Foam: From what I understand the pinkAnti-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). This doesn't seem
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 offer muchthe environment. Thus it is usually used as a one-time foam for my usage caseshipping 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: From whatStatic 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 can gatherwasn't able to confirm this offers, but it may have some of the benefitproperties 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).
So basedBased 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.
I would be very grateful if someone with a better understanding of electricity than myself could elaborate and help recommend an appropriate foam for my applicationon the theory behind these foams. It might also be helpful to othersI put together a rough guide to classify the best application of the three types of foam for the five following broad usage categories:
- 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.
- Battery cells: Single-use: Anti-static (pink) inside of a silver conductive bag. Multi-use: Static dissipative (black) foam inside of a conductive bag.
- 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.
- 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.
- 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.
