What surface resistivity should an ESD case have?

For most electronics handling the target is the dissipative band. Under ANSI/ESD S20.20 that runs from 1x10^4 ohms to below 1x10^11 ohms; under IEC 61340-5-1, from 1x10^5 to 1x10^11 ohms. In semiconductor and IC tray practice a narrower working range of 10^6 to 10^9 ohms per square is commonly specified, because it drains charge slowly enough to avoid a discharge spike while being fast enough not to accumulate.

Why not just use the most conductive material available?

Because very conductive packaging drains charge fast, and a fast drain is a discharge event. A component dropped into a highly conductive container can see a rapid equalisation current that damages it. The dissipative band exists precisely so that charge moves away slowly and controllably. Conductive materials are specified where shielding from external fields matters more than drain rate - for the most field-sensitive devices, or where an explosive atmosphere makes any spark unacceptable.

Does anti-static protection wear off?

It depends entirely on which technology was used. Migratory anti-static agents work by blooming to the surface and depend on ambient humidity; they are consumed by wiping and washing, and their performance drifts over months. Permanent anti-static compounds build a conductive network inside the polymer itself and broadly hold their resistivity through cleaning and long storage. Carbon-filled materials are permanent but opaque. If a case must stay anti-static after repeated alcohol wiping, specify the permanent type and say so.

Do I need an ESD case, or is anti-static foam inside a standard case enough?

Usually the foam. The charge risk is at the interface between the device and what touches it, so a dissipative foam cavity inside an ordinary case addresses the dominant mechanism at a fraction of the cost. A fully ESD shell becomes relevant when the case itself is handled in an ESD-controlled area, when the contents are extremely field-sensitive, or when the operating environment makes triboelectric charging of the shell itself a hazard.

How do I verify an anti-static claim?

Measure it. A surface resistivity meter with the appropriate probe, used on a sample conditioned at the standard temperature and humidity, gives a number you can hold the supplier to. Ask which standard the measurement was made under, what the conditioning was, and whether the figure is surface or volume resistivity - they are different quantities and the numbers are not interchangeable. A datasheet that says anti-static without a number and a standard is not a specification.

ESD Anti-Static Protective Cases: Surface Resistivity Guide

Anti-static is a measurable property with defined bands, published standards and test methods. Treating it as a marketing adjective is how buyers end up with a case that measured correctly on the day it was made and does not measure correctly six months later.

The three bands

Band	ANSI/ESD S20.20	IEC 61340-5-1	Typical use
Conductive	Below 1x10^4 ohms	1x10^2 to 1x10^5 ohms	Field-sensitive devices, shielding, explosive atmospheres
Dissipative	1x10^4 to below 1x10^11 ohms	1x10^5 to 1x10^11 ohms	General electronics handling - the usual target
Insulative	Above 1x10^11 ohms	Above 1x10^11 ohms	Not static-protective; ordinary polymers sit here
Shielding	Below 50 nJ energy, or below 1x10^3 ohms	Below 50 nJ energy	Where an external discharge must not reach the contents

Note that the two standards do not draw the conductive-dissipative boundary in the same place. A figure quoted without its standard is ambiguous near the boundary, which is one more reason to require both.

Compact conductive case suitable for ESD-sensitive electronics — Small size, low mass, frequent handling — exactly where static builds up.

Why dissipative is usually the target

Charge has to go somewhere. The purpose is to prevent accumulation, not to create a path to ground through the device. Dissipative materials remove charge at a controlled rate.
Speed is a hazard. A very fast drain is a discharge event. The dissipative band is slow enough to protect the component and fast enough to prevent accumulation.
A narrow working range is common. In semiconductor tray and carrier practice, 10^6 to 10^9 ohms per square is the range typically specified - narrower than either standard's full band, because it is the region where both hazard mechanisms are comfortably avoided.

Permanent versus migratory: the question that decides longevity

Two technologies produce a low-resistivity polymer, and they behave very differently over the life of the case.

	Migratory anti-static agent	Permanent anti-static compound	Carbon filled
Mechanism	Agent blooms to the surface and attracts moisture	Conductive network formed within the polymer	Conductive particles in contact throughout
Humidity dependence	Significant	Low	None
Effect of wiping or washing	Degrades; consumed over time	Broadly retained	Retained
Transparency	Can be transparent	Can be near-transparent	Opaque
Cost	Lowest	Higher	Varies
Right for	Short-life, low-cost, single-campaign packaging	Reusable cases that get cleaned	Where opacity is acceptable and resistivity must be lowest

If your cleaning regime includes alcohol wiping - common in electronics and medical environments - a migratory agent will not survive it. This is the single most common reason an anti-static case stops measuring correctly, and it is entirely predictable at specification stage.

Case, foam, or both

Start with the interface. Charge transfer happens where the device touches something. That surface is the foam cavity, so a dissipative foam in a standard case addresses the dominant mechanism.
Add the shell when handling matters. If the case is carried, stacked and slid within an ESD-controlled area, the shell itself is a charging surface and should be dissipative too.
Add shielding for field-sensitive contents. Some devices are damaged by an external discharge rather than by their own accumulated charge. That is a shielding requirement, and it calls for a different construction again.
Consider the atmosphere. Where a spark is unacceptable for reasons beyond the device - flammable vapour, dust, some ordnance and propellant contexts - resistivity is a safety specification, and it should be set with that consequence in mind rather than by copying a datasheet.

How to write the specification

State the band you require and the standard it is measured under - for example dissipative per ANSI/ESD S20.20.
Give a numeric range, not just the band name, if your process depends on it.
State whether the requirement is surface or volume resistivity.
State the conditioning: temperature and relative humidity, and how long the sample was conditioned before measurement.
State the cleaning regime the case must survive, and specify permanent anti-static if wiping is involved.
Require a measurement on production parts, not only on a laboratory sample, and at a defined interval if the case is reused.

None of this is burdensome, and it converts a claim into a specification you can test on delivery. For JUNZHIJIA cases, anti-static options are matched to the application you describe - tell us the devices being carried, the cleaning regime, and which standard your ESD programme is run to, and the material and target resistivity are selected against that rather than picked from a list.

ESD and Anti-Static Protective Cases: Surface Resistivity and When It Matters

The three bands

Why dissipative is usually the target

Permanent versus migratory: the question that decides longevity

Case, foam, or both

How to write the specification

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