Dissipative or conductive? Choosing an ESD plastic grade

ESD dissipative conductive plastic covers three separate jobs, and a grade chosen for one of them will often make another worse. The resistivity band, not the colour or the brand name, decides whether a fixture protects a board, drains a charge away from it, or quietly does neither.

At Top Plastic Co., Ltd. we machine dissipative and conductive stock shapes in Dongguan for electronics, semiconductor and industrial customers, and the most expensive mistake we see is a drawing that asks for ESD material without naming a resistivity band or a test method. This article sets out how we help buyers close that gap.

The resistivity bands and what each protects

Resistivity is quoted in ohms per square for surfaces and ohm-centimetres for volume, and those two numbers are the pair most often confused with each other. The bands below are the ones we work to, quoted as surface resistivity at low humidity unless a specification says otherwise.

The tighter dissipative window, roughly 10^6 to 10^9 ohms per square, is where most sensitive-device programmes sit. The wider band up to 10^11 still counts as dissipative in the standards, but it discharges more slowly and will fail a specification that was written around the tighter window.

Read the two numbers together before you select an ESD dissipative conductive plastic, because a grade that drains a carrier quickly can be too conductive to protect a sensitive device. We hold both ends of that range as standard stock, and the grades are listed on our ESD material pages.

Surface versus volume conductivity

Surface resistivity is measured across a square of the outer skin, and volume resistivity is measured through the thickness. A carbon-loaded compound usually behaves as a three-dimensional network, so it is reasonably consistent in both directions, but the skin of an extruded or moulded shape is never identical to its core.

That difference becomes practical the moment you machine a part. Cutting removes the original skin, and the freshly exposed face is what the customer eventually measures. On a well-dispersed compound the reading barely moves. On a poorly mixed one it can shift by decades, which is why we test the finished surface rather than relying on the number printed on the delivery note.

Carbon black, carbon fibre and inherently dissipative grades

Carbon black is the cheapest route to a dissipative grade. The loading level sets the band, and a small change in loading moves the reading a long way, so lot-to-lot variation is real and worth testing for. Black grades also tend to lose some impact strength and can shed marginally more particles than the unfilled polymer.

Carbon fibre gives conductivity and stiffness in the same grade, which suits structural parts such as end effectors and load frames; carbon-filled PEEK is the usual answer there. A third family reaches the band through an additive system rather than a fixed filler loading, and it is chosen when the band is narrow and has to stay narrow after cleaning, wiping and repeated handling.

Where the target band comes from in each industry

Electronic assembly generally specifies 10^6 to 10^9 ohms per square, and that figure usually arrives from the customer’s own ESD control plan rather than from a published standard. Semiconductor front-end handling is stricter again, and adds outgassing and particle limits on top of the resistivity requirement, which we cover in our industry notes.

Explosive atmospheres and some powder-handling lines go the other way and ask for genuinely conductive material, in the 10^3 to 10^5 range, because the aim is to drain charge as fast as it is generated. Conductive acetal is often the least expensive part in that assembly, provided the temperature stays within its normal working range.

Machining, cleaning and how the surface ages

Machining ESD grades is mostly a matter of not undoing the network. Sharp tooling and moderate feed rates keep the cut clean, and heat is the main enemy, because local melting can redistribute the filler and leave a skin that reads several decades away from the rest of the part. Fixtures and handling jigs for this sector are usually machined from stock in the same grade for that reason, as described in our semiconductor fixture notes.

Cleaning matters as much as cutting. Alcohols and aggressive solvents can either pull additive to the surface or strip it away entirely, and abrasive blasting will remove a conductive skin outright. Wipe with the mildest agent that does the job, dry the part, and let it settle at ambient conditions before anyone takes a reading.

Where a part is machined, cleaned and then measured, an ESD dissipative conductive plastic that passed on the sheet can fail on the finished face for reasons that have nothing to do with the material itself. Keeping the process and the measurement in the same conversation is the simplest way to avoid that.

Questions buyers ask about specifying an ESD grade

Should I specify surface or volume resistivity on the drawing?

Name both, and name the test method and the humidity alongside them. A surface figure without a humidity condition is only half a specification, because readings move noticeably as the environment dries out. If only one figure will fit on the drawing, use surface resistivity, since that is the number most incoming inspectors will measure.

Does machining change the resistivity of a grade that was tested in sheet form?

It can, and it does more often than most drawings allow for. The safe approach is to qualify the finished surface on a sample part rather than the delivered sheet, then set the drawing limit from what the process actually delivers with margin to spare.

Which grade is safe for semiconductor handling?

The resistivity band is only the first gate. Wafer-contact parts also have to pass particle and outgassing limits, which rules out many carbon-black compounds, so the usual answers are a clean dissipative PEEK or a filled PEI. If your line handles wafers, tell us the band and the cleaning agent together and we will advise on the grade that satisfies both.

How often should the band be verified?

On every new lot, after any process change, and whenever the surface is cleaned with a different agent. For a stable production part, a periodic sample check is normally enough, typically once a quarter, but the band should be re-confirmed each time the supplier, the grade or the machining route changes.