Two suppliers quote the same part number: anti-static XPE foam, 5 mm, black. One test report says 1×10⁶ Ω; the other says 4×10⁹ Ω. Both numbers sit inside the range people call “anti-static”, and the two differ by roughly two orders of magnitude. Which one is right?
Quite possibly both. Surface resistance is not a material constant; it is a measurement result. It moves with four variables, and those four are frequently left off the report. This page is about what has to accompany a number that ends in Ω before it can be accepted. The difference between conductive and anti-static as product categories is on ESD foam; this page is about the number itself.
The first variable: surface, or through the thickness
“Resistance” here covers two different quantities.
- Surface resistance. Both electrodes sit on the same face, and the current travels along the surface. It answers: can charge leave the surface?
- Volume resistance. One electrode on the top face, one underneath, and the current passes through the thickness. It answers: can charge pass through the material?
Anti-static packaging needs the first. Static charge accumulates on the surface, the packaging contacts the component by its surface, and the acceptance probe lands on the surface too. So the quantity of record on this page is surface resistance.
That also means a specification reading only “resistance 10⁶ Ω” has not said which one is meant, and on acceptance day both sides can be measuring different things. The two can be far apart: a material that dissipates on its surface can still be an insulator through its body, and the reverse also happens.
The second variable: by which method
Surface resistance measured by different methods gives different readings. The difference comes from three things: electrode shape and spacing, applied voltage, and how long after energising the reading is taken.
Methods in common use include IEC 61340-2-3, ANSI/ESD STM11.11 and GB/T 1410, and they do not share the same electrodes or the same applied conditions. They are listed here only to make the point that there is more than one — we do not state that Bofeng Foam reports against any of them. The practical use of this section is: if your specification names a method, accept against that method, and do not switch midway.
The third variable: under what conditions
Surface resistance is humidity sensitive, which is standard in this industry. Most polymer surfaces adsorb a very thin film of water; at high humidity that film makes the surface more conductive and the reading falls, while in dry air the reading rises. The same piece of foam measuring a different band in the rainy season and in a dry winter is not surprising.
Two further conditions are usually left out: specimen state and reading time. Which batch the specimen came from, and how long it was stabilised in the test environment, both affect the result; and some materials drift slowly after voltage is applied, so “how long after energising” has to be agreed.
Together these three say one thing: a value on a report only means something under its own conditions. A clause reading only “≤10⁹ Ω”, with no temperature, humidity or reading time, gives no basis for a reproducible acceptance.
The fourth variable: which face
Foam has two faces of different character: the moulded face from foaming, and the cut face produced later by slicing.
Conductivity can come from two routes. One is bulk blending: a conductive filler such as carbon black is mixed into the polyethylene before foaming, forming a conductive network through the body of the material. Grades made this way remain conductive on a newly exposed cut face after slicing, die-cutting or CNC routing. The other is surface treatment: conductivity sits on the surface, and the cut face may not carry the same surface resistance.
Our conductive grades use bulk blending and stay conductive on the cut face; the anti-static grades depend on the specific grade. For procurement the conclusion is the same either way: on a die-cut part, the face the probe will land on has to be written into the clause. We recommend specifying the cut face, because that is the face actually exposed during assembly.
The four together: a clause that can be accepted against
Write these four, together with the range and the form of the report, and the clause holds up.
| What to write | How to write it | What happens if it is left out |
|---|---|---|
| Quantity | Surface resistance — not “resistance”, not volume resistance | The two sides measure different quantities |
| Method | Name the test method; electrodes and applied voltage follow from it | Readings are not comparable |
| Conditions | Agree temperature, humidity, specimen state and reading time | One material, two readings |
| Face | Moulded face or cut face, stated | Disputes over die-cut parts |
| Range | A closed interval such as 10⁶–10⁹ Ω | The words “anti-static” cannot be accepted against |
| Report | Who tests it, and in what form it is issued | Verbal data is not a clause |
None of these six requires new test capability. They are all a matter of writing clearly. What they save is not material cost; it is the argument on the day the goods arrive.
Two bands: 10³–10⁶ and 10⁶–10⁹
Before writing a clause, know which band you need. Our functional grades fall into two types.
| Type | Surface resistance | Discharge rate | Typical position |
|---|---|---|---|
| Conductive | 10³–10⁶ Ω | Fast, close to a grounded conductor | Trays, dividers, circulating carriers |
| Static-dissipative (anti-static) | 10⁶–10⁹ Ω | Slow, avoiding a rapid discharge | Liners and pads in direct contact with sensitive devices |
Both types are permanent: the conductive filler goes into the polyethylene before foaming, so the resistance is a property of the material and does not depend on a surface coating that could be wiped or washed off. Which band to choose depends on how the device touches the foam, not on which one sounds more anti-static.
Conductive is not shielding
The two are often written together, but they are different physical quantities. Conductive foam here drains static charge, preventing it from accumulating and then discharging in one event that damages a device. Electromagnetic shielding effectiveness is a different question: how much attenuation a material gives in a given frequency band, expressed in dB, and quoted per band and per test method.
A conductive filler does bring some electromagnetic attenuation, but shielding effectiveness has to be measured and cannot be derived from surface resistance. Our specification documents carry no shielding figure, so this page gives none, and does not write “conductive” as though it meant “shielding”.
What we do not decide
- Which sensitivity class your device falls into. That is set by the device specification, not by the packaging.
- ESD protection grading for a finished device. We supply material and fabricate to drawing; testing and marking of the finished article belong to the device manufacturer.
- Shielding effectiveness — for the reason above.
- A service life in years. “Permanent conductivity” means the material does not lose conductivity through wiping, washing or converting. It is not a guarantee against a calendar period. Where a period matters, retain a sample and retest on your own acceptance cycle.
Common questions
Should a specification say surface resistance or volume resistance?
Surface resistance. Charge accumulates on the surface, the packaging contacts the device on its surface, and the acceptance probe lands on the surface. Volume resistance is not useless, but it answers a different question.
Why do two suppliers differ by two orders of magnitude?
Compare four things first: whether it is the same quantity, whether it is the same method, whether temperature, humidity and reading time are the same, and whether it is the same face. If any one differs, the two numbers are not comparable.
Readings run high in the dry season — has the material gone bad?
No. Surface resistance varies with humidity throughout this industry. Agree the measurement conditions, then compare batches against each other.
Does the cut face stay anti-static after die-cutting?
For conductive filler grades it does: the network runs through the body, so the newly exposed face is still in it. Surface-treated material may not carry the same surface resistance on a cut face, which is exactly why the acceptance clause has to name the face.
Can you issue ESD certification?
Not the kind that certifies a finished device as ESD-protected. We supply material against the quantity, method and range you specify, and provide material-side data in the agreed form.
Boundaries
This page is about writing surface resistance as an acceptable quantity. It is not ESD protection design and does not replace your control programme. The two bands above are the material-side supply range; within a band, actual readings still move with method, conditions and face. If you have a specified test method and acceptance condition, send them with the enquiry and we will confirm which combination we can supply.
What Bofeng Foam supplies
Bofeng Foam is a manufacturer of cross-linked polyolefin foam, with a 10,000 m² plant, five production lines and 5,000 tonnes of annual capacity in Zhuhai, Guangdong. Conductive and anti-static grades carry surface resistances of 10³–10⁶ Ω and 10⁶–10⁹ Ω respectively, both permanent; the base material can be XPE, IXPE or IXPP. XPE single layer 3–15 mm, heat laminated to 200 mm; IXPE single layer from 0.1 mm; width 500–2000 mm. Finished stock can be die-cut, punched, CNC routed, laminated to non-woven or film, and adhesive coated with a release liner. Flame-retardant grades meet UL94 V2 and GB 8410 A0.
Telling us the device, the contact method and the target band places the specification more reliably than the words “anti-static”.
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See also closed-cell vs open-cell foam, medical grade foam, XPE foam, IXPE foam, certifications and compliance and all foam materials.