An ESD protected area (EPA) protects electronic components by making sure charge drains away in a controlled way. Worksurfaces, floors, mats, packaging, garments and seating must conduct enough to drain charge, but not so much that a charged device discharges violently. Programmes built to ANSI/ESD S20.20 or IEC 61340-5-1 therefore set resistance limits for each item and require them to be verified, both when materials are qualified and periodically afterwards.
The Three Material Ranges
| Classification | Surface resistance | Behaviour |
|---|---|---|
| Conductive | < 1 × 10⁴ Ω | Drains charge very quickly |
| Dissipative | 1 × 10⁴ to < 1 × 10¹¹ Ω | Drains charge at a controlled rate |
| Insulative | ≥ 1 × 10¹¹ Ω | Holds charge; kept away from sensitive parts |
These ranges are used for ESD packaging classification. Worksurfaces and floors have their own limits for resistance to ground; S20.20, for example, requires worksurface resistance to ground below 1 × 10⁹ Ω.
What Is Measured
- Surface resistance (point-to-point): two electrodes on the same surface, showing how well charge moves across it.
- Resistance to ground (point-to-ground): one electrode on the surface and the other on the groundable point, showing whether charge can actually reach earth.
- Volume resistance: through the thickness of the material.
Formal test methods specify the electrodes, test voltage and conditioning. ANSI/ESD STM11.11, for planar materials, uses a concentric-ring probe at 10 V for low resistances and 100 V above 1 × 10⁶ Ω, with the sample conditioned at low humidity. ANSI/ESD S4.1 covers worksurfaces and STM7.1 covers floors. Humidity matters a great deal, because many dissipative materials depend on absorbed moisture and read higher in dry conditions.
Quick Checks vs Qualification Tests
Laboratory qualification needs a resistance meter and electrodes that meet the exact test method. Day-to-day verification in the EPA usually uses simpler portable meters that place a reading within a decade, to show that a mat, bench or bag is still in its expected range.
The MEWOI-HZR100 is a pocket-sized checker of this kind. It uses a parallel-bar probe, after ASTM D257, and a row of LEDs to indicate surface resistance from 10³ to 10¹² Ω per decade, and it can also measure surface-to-ground. The supplier gives ±10% accuracy and notes that humidity above 65% RH affects accuracy. It is well suited to routine compliance checks of worksurfaces, mats, packaging and floors; for formal qualification to STM11.11 or S4.1, use the instrument and electrodes those methods specify.
Frequently Asked Questions
Is lower resistance always better for ESD control? No. Very low resistance lets a charged device discharge rapidly into the surface, which is itself an ESD event. That is why dissipative ranges are specified for most surfaces.
How often should EPA items be checked? The programme's compliance verification plan sets the frequency, often based on how heavily each item is used and how it has performed in past checks.