Conductive, dissipative or insulative: what the rack under your boards actually is

Three words get used as if they were grades of the same thing. They aren't. Conductive, dissipative and insulative are three different materials that behave three different ways, and for a rack that holds boards between operations, the difference matters more than the label suggests.

What the three words mean

They're sorted by surface resistance, which is how easily charge moves across the material. A conductive material drains charge fastest. A dissipative material drains it more slowly. An insulative material barely lets it move, so charge stays where it lands.

The number matters more than the label, because labels get applied loosely across the industry. A rack described as antistatic might be any of the three. Ours are conductive, 10⁴ Ω. How surface resistance is measured is in ESD surface resistance explained.

Why conductive, for a rack

A rack's job is to hold boards at ground while they wait. A conductive rack does that directly. Any charge on a board edge has a short path through the rack to ground, and every board in it sits at the same potential as the rack.

The usual worry is speed. A charged part that touches bare metal discharges at once, because there's next to nothing in the path to limit the current. A conductive plastic at 10⁴ Ω isn't metal. The resistance in the path limits the current, so the charge leaves quickly without the hard dump of metal contact. And a rack touches a board at its edge, usually bare laminate or a breakaway rail, not a component pin.

Dissipative materials drain charge more slowly. They're a common choice for surfaces people work on all day, like bench mats. For a rack that keeps boards at ground between steps, conductive does the job with margin.

Where insulative belongs

Almost nowhere near boards, and yet it's common. Untreated plastics are insulative. So is ordinary cardboard once it dries out, and so is most packaging that wasn't bought as ESD packaging.

An insulative surface doesn't just fail to drain charge. It holds it, and it can hold a charge next to a board the whole time the board sits there. An insulator near an assembly is an active hazard rather than a neutral one, and that's the part that gets missed.

The practical decision

For bare boards, conductive. For populated boards waiting between operations, conductive as well, in a rack that's grounded and loaded by people who are grounded too.

For the structure around the racks, carts, shelves and frames, conductive or grounded metal, because that's the path to ground that makes the rest of it work. Keep insulative materials away from boards altogether.

The part that undoes all of it

A conductive rack that isn't connected to anything is conductive in name only. Charge has to have somewhere to go. A rack sitting on an insulative bench, handled by an ungrounded operator, is doing a fraction of the job it was bought for.

The material is the first half of the decision. The path to ground is the second half, and it's the half that gets built once and then changed over the years as benches are replaced and racks move around.

Our universal racks and magazine racks are conductive, 10⁴ Ω, and every product page states the material, so you have the number before you buy. The wider standards picture is in the IPC and ESD standards hub.