Ask for an ESD box and you will get a surface resistivity figure. It is a useful number and it is routinely interpreted as “lower is better”, which is not how static control works. Choosing the wrong end of the range can be as unhelpful as having no protection at all.
What the number measures
Surface resistivity is the resistance to current flowing across the surface of a material, expressed in ohms per square. It tells you how easily charge moves across the box surface, which determines whether a charge drains away slowly, quickly, or not at all.
The three ranges and what they are for
Conductive – low resistivity. Charge moves quickly. Used where you need fast dissipation and where a discharge path is controlled. Generally not what you want for carrying sensitive components, because a rapid discharge through a component is exactly the event you are trying to avoid.
Static dissipative – the middle range, and the one most component handling needs. Charge drains at a controlled rate rather than suddenly. This is the usual specification for trays, boxes and workstation surfaces handling electronics.
Insulative – high resistivity. Charge does not move. Standard polypropylene is insulative, which is precisely why it generates and holds static in the first place.
Why lower is not better
The failure mode that damages components is a fast discharge. A highly conductive box in contact with a charged device can produce exactly that. The point of static control is to remove charge at a controlled rate, and that means the middle range rather than the bottom of it.
If a supplier quotes a single very low figure as a selling point, it is worth asking whether that is actually appropriate for your application.
What else matters as much
- How the box is grounded. A dissipative box only drains charge if it has somewhere to drain to. In practice that means the person, the bench, or a grounding point – and it often is not connected at all.
- Tribocharging. Any material generates charge through contact and separation. Antistatic additives reduce it; they do not eliminate it, and they can be worn off or washed off.
- Humidity. Many antistatic mechanisms depend on surface moisture, so performance degrades in a very dry environment. This is the most common reason ESD packaging that worked in one season stops working in another.
Two different additive types
Migrating antistatic additives bloom to the surface and work by attracting moisture. Effective, cheap, and they deplete – performance fades over time and is lost by washing.
Permanent antistatic or conductive fillers – carbon black, carbon fibre, or inherently dissipative polymers – give a stable resistivity that does not depend on humidity and survives cleaning. More expensive, and carbon-filled materials are black, which limits colour coding.
For a reusable box that will be cleaned, the permanent type is usually the right answer despite the cost.
How to check what you have
A surface resistivity meter is the proper tool, and readings should be taken at several points and at the humidity you actually run at. A reading taken in a humid laboratory tells you little about a dry production floor in winter. If you are qualifying a supplier, ask for readings at controlled humidity.
What we supply
We supply ESD corrugated PP boxes in the static dissipative range, using permanent antistatic construction for boxes that get cleaned. Tell us what you are carrying, how dry your environment runs, and whether the boxes are washed – those three answers determine the right specification.