Why test ESD plastic box resistance at two humidities?

A: Humidity changes the conduction path. A 50 percent reading describes a normal floor, 12 percent a dry season, and only a box passing both ships all year.

If surface resistance passes, is decay time needed?

A: Yes. Resistance is static; decay is dynamic. Some boxes sit mid-band yet release charge slowly because the additive is uneven.

What loop resistance counts as passing?

A: Not above 1 ohm at 1 A. Above that, expect an oxidised crimp or coating between terminal and wall.

How much batch drift is a failure?

A: A spread above one decade in a lot, or drift above half a decade after 1000 hours of ageing.

Is sampling five boxes enough?

A: Enough for the lot, not the cavity. A drifting cavity needs three boxes per cavity over three lots.

ESD Plastic Box Standards: Inspection Items and Pass Criteria

Quick answer: Acceptance of an ESD plastic box rests on five measurable items — surface, volume resistance, static decay, triboelectric voltage and grounding loop resistance — plus batch consistency. Each needs an instrument, a condition and a pass value; with all three written down, two laboratories reach the same verdict.

ESD plastic box standards: the order of acceptance
Reading surface and volume resistance
Decay time and triboelectric voltage
Grounding and loop resistance
Batch consistency, marking and sampling
FAQ

1. ESD plastic box standards: the order of acceptance

Fix the items first, the instruments second, the pass values last. Reversed, the plan becomes whatever the lab owns. Conditions matter as much as accuracy: resistance runs at 23 °C ± 3 °C, at 50 and 12 percent humidity, because the same box reads two orders higher in dry air.

Item	Instrument	Pass value
Surface resistance	High-resistance meter, 2.27 kg electrode	1×10⁴ to 1×10¹¹ ohm at 100 V
Volume resistance	Plate electrodes, read at 60 s	Not above 1×10¹¹ ohm·cm
Static decay	Field meter with discharge plate	1000 V to 100 V within 2.0 s
Triboelectric	Nylon cloth, 500 g, 10 passes	Peak not above 100 V
Grounding loop	Loop tester at 1 A	Not above 1 ohm

Sample five ESD plastic box units per lot, five points each; one point outside fails the unit.

2. Reading surface and volume resistance

Surface resistance is the most misread number on an ESD plastic box. Three bands work better than one, and the voltage must be stated: 100 V and 500 V differ by half an order.

ESD plastic box in white on a galvanised steel shelf, lid ajar, two folding side handles and three white latches along the front seam

Volume resistance describes the compound, not the skin. With carbon black unevenly dispersed, surface passes while volume drifts past 1×10¹² ohm·cm, pointing to a coating or an additive that wipes off.

Class	Surface	Volume	Verdict
Conductive	Below 1×10⁴ ohm	Below 1×10⁴ ohm·cm	Permitted in explosive areas
Dissipative	1×10⁴ to 1×10¹¹ ohm	Not above 1×10¹¹ ohm·cm	Standard for electronics assembly
Insulating	Above 1×10¹¹ ohm	Above 1×10¹² ohm·cm	Barred from sensitive areas

3. Decay time and triboelectric voltage

Passing resistance does not mean fast discharge. Decay runs from 1000 V to 100 V, and the 2.0 s limit applies at 12 percent RH, where dry air leaves no water film and charge must travel through the compound itself.

Triboelectric voltage sits closer to the floor. A nylon cloth around a 500 g load makes ten passes while a field meter reads the peak at 25 mm; above 100 V fails. The same ESD plastic box peaks at 40 V humid and 300 V dry, so a record without humidity compares with nothing.

Item	Condition	Limit and usual cause
Decay time	1000 V to 100 V, 12 percent RH	2.0 s; additive migration
Triboelectric	500 g nylon, 10 passes	100 V peak; local insulation
Decay repeat	Same box, five cycles	0.3 s spread; surface residue

4. Grounding and loop resistance

A dissipative ESD plastic box works without a terminal, but once one is fitted the loop must be measurable: conductor not below 0.5 mm², 1 ohm at 1 A. An oxidised crimp doubles that inside three months.

With no terminal, keep surface resistance between 1×10⁶ and 1×10⁹ ohm so charge bleeds away slowly, and avoid an isolated conductor that dumps everything at once. Most lines stand it on a dissipative mat and read contact resistance, at a limit of 1×10⁹ ohm.

5. Batch consistency, marking and sampling

Batch consistency is the item most often skipped. Inside one lot the spread should stay within one decade; above 20 percent regrind the filler network dilutes and every reading shifts up. After 1000 hours of ageing, drift should stay within half a decade.

Marking has to reach the cavity: batch number, cavity number, date and material grade, by laser or in-mould label, surviving 50 alcohol wipes. The cavity number decides what goes to quarantine.

Lot size	Sample	AQL
151 to 280	20	1.0 resistance, 2.5 appearance
281 to 500	32	1.0 resistance, 2.5 appearance
501 to 1200	50	1.0 resistance, 2.5 appearance

Conclusion

Acceptance turns six items into repeatable steps: resistance at a fixed voltage, decay at a fixed humidity, triboelectric voltage at a fixed load, grounding at a fixed current. Loose conditions make the numbers meaningless; fixed ones let an ESD plastic box pass without one inspector's memory.

This article covers container-side specifications and design implications only, not the handling, loading or use of the contents.