Standard EVA is an insulator, and a good one. Surface resistivity on our untreated shell material sits above 10^13 ohms per square, which means a case made from it holds charge rather than bleeding it off. That matters when the contents are a bare PCB, a sensor module or a drive with exposed contacts. It matters much less when the contents are a finished consumer product already sitting in its own housing.
What changes when you need ESD control
The fix is in the material, not the shape. Adding carbon black or a permanent antistatic masterbatch to the EVA compound brings surface resistivity into the dissipative band, roughly 10^5 to 10^9 ohms per square, which is the range IEC 61340-5-1 uses for protective packaging. A topical antistatic coating is the cheaper route, but coatings wear off with handling and humidity swings, so for a case opened daily we recommend the loaded compound.
Two things have to be checked on the finished case, not only on the raw sheet. Surface-to-surface resistance across the shell, and the resistance path from the case to whatever it is standing on, since a dissipative shell resting on an insulating bench still collects charge from the person carrying it. Our QA bench measures both at 100 V and 500 V, at 12% and 50% relative humidity, and the readings go on the inspection sheet with the lot number.
Where the limits are
A dissipative EVA case slows charge decay. It is not a Faraday cage and it will not stop a direct discharge into a connector. If the contents are sensitive at that level, the case needs a conductive inner layer or a metallized shielding bag inside the foam cavity, and the usual answer is both: an antistatic shell plus a shielding bag around the board itself. Colour is the other constraint. Carbon-loaded EVA comes in black or dark grey, so a buyer who needs a white or colour-branded shell has to accept a topical coating, and that coating is measured in months rather than years.
Humidity moves the numbers more than buyers expect. A compound that reads 10^7 ohms at 50% RH can read two orders of magnitude higher at 12% RH, which is a real condition inside a heated European warehouse in January. We test at both ends and ask buyers which end their product will actually see before quoting a compound.
How it gets specified
Tell us what goes in the case and which standard the receiving end audits to. Most ESD programs are written to IEC 61340-5-1 or to ANSI/ESD S20.20, and the two ask for slightly different documentation. We supply material test reports with the resistivity figures and the humidity they were measured at, and we build the electronic accessory case with a die-cut antistatic foam insert so the whole interior sits in the dissipative band rather than just the shell. Drawings and target values can go through the contact page before quotation.
MOQ on antistatic compounds runs higher than standard, usually 1,000 pieces, because the compound has to be run as a dedicated lot rather than pulled from standard sheet stock. Sampling takes about a week longer for the same reason. The job next on the bench is a sensor module case with a 10^6 ohm target; the first molded shells are due off the press on Friday and go straight to the resistance bench.