“Is this batch of bags antistatic?” — most people put a surface resistance meter on the bag, read a number and call it done. But the same bag can read several orders of magnitude apart with different humidity, a different setup, or one control step skipped. A number by itself means nothing; only a reproducible number does.
Using measurement records from JIAHON’s own LDPE antistatic bags, this article explains which steps a valid surface resistance measurement should include, and what happens when each one is missing.
1. The measurement record
| Item measured | Reading | Relative humidity | Temperature |
|---|---|---|---|
| Control: insulator | Greater than 2 × 10¹² Ω (above the range limit) | 74.4% | 23.0 °C |
| LDPE bag, inner layer | 4.55 × 10¹⁰ Ω/sq | 74.4% | 23.0 °C |
| LDPE bag, outer layer | 3.10 × 10¹⁰ Ω/sq | 72.5% | 23.3 °C |
The instrument was a surface resistance meter with a weighted concentric-ring electrode, reading in Ω/sq (ohms per square). It has a calibration certificate issued by an external laboratory, dated December 26, 2024.
2. Four things this record does right
- First measure a known insulator as a control: a reading of “greater than 2 × 10¹²” shows the electrode, leads and instrument have no short or leakage and the range is set correctly. If the control reads a middle value, nothing measured afterwards can be trusted — the problem is the equipment, not the sample. This step is often skipped, yet it is the foundation of the record’s credibility.
- Record temperature and humidity with every reading: surface resistance is extremely sensitive to humidity; the same bag measured on a dry winter morning and on a damp day can differ by two or three orders of magnitude. A resistance value without temperature and humidity is data without measurement conditions, and an auditor can’t judge it. This instrument shows temperature, humidity and resistance on one screen precisely to tie the reading to its conditions.
- Measure inner and outer layers separately: antistatic bags are often multilayer, with different treatments inside and out, so values naturally differ (here 4.55 × 10¹⁰ inside, 3.10 × 10¹⁰ outside). If only one side is measured and reported as “this bag’s surface resistance,” which side is meant?
- Use an instrument with a calibration certificate: readings from an instrument without traceable calibration records aren’t accepted in audits. Keep the certificate and schedule periodic calibration under your metrology management procedure.
3. Conditioning before measurement is the step most often overlooked
Formal surface resistance methods (for example ANSI/ESD STM11.11 for planar materials) require specimens to sit at specified temperature and humidity for a set time before measurement, so the material’s moisture content reaches equilibrium with the environment. The reason is simple: most antistatic treatments conduct through moisture adsorbed on the surface, so a material just taken from the warehouse, not yet in equilibrium, reads “the history of its transport,” not the material’s own property.
- How far to go depends on the purpose: quick incoming screening and a formal report for a customer have different requirements. For the former, measure on site and record temperature and humidity; for the latter, follow the conditioning and specimen preparation in the applicable standard.
- The record on this page is the former: it was measured under site conditions and is not a formal test report under a standard procedure. If your plant needs a formal report, send samples to a qualified laboratory, or run the full standard procedure and keep conditioning records.
4. What to compare the reading against
A reading of 3 × 10¹⁰ doesn’t by itself mean pass or fail — it depends on which specification your plant or customer follows and what range it requires.
- First confirm the applicable document: requirements for ESD packaging materials usually cite ANSI/ESD S541; the surface resistance method for planar materials is ANSI/ESD STM11.11. Another system is the IEC 61340 series. Judge by whichever your plant’s ESD control program specifies.
- Then check the customer’s audit checklist: many customers specify limits and measurement conditions directly. If so, the customer’s requirement comes first; keep records measured under those conditions.
- Don’t apply someone else’s thresholds to your material: shielding, conductive and antistatic bags are three different things with different requirements. For the differences, see the ESD bag selection article in Further reading below.
5. Five common mistakes in on-site measurement
- No insulator control: equipment problems go completely unnoticed.
- No temperature and humidity: the data can’t be reproduced, and measurement conditions can’t be explained in an audit.
- Electrode not seated properly: a weighted electrode uses its own weight for steady contact pressure; pressing by hand or propping it up changes the contact.
- Measuring on creases, printing or seals: these areas differ from the bag body; measure flat areas of the body.
- Concluding from a single point: different spots on the same material vary; measure several points and record all readings rather than picking the nicest one.
On SMT/EMS lines: measure packaging, worktops and totes together
- One meter, different measurements. This article measures packaging surface resistance; lines also measure worktops and floors to ground. Before measuring, confirm whether you need surface resistance or resistance to ground — the electrodes differ too.
- Sample packaging at incoming inspection. When ESD bags, trays and totes arrive, measure a few by the steps above and record temperature and humidity; returning an out-of-spec lot costs less than finding out on the line.
- Standards. For packaging classification and testing, see ANSI/ESD S541; for the international IEC 61340 series, see the IEC website; for the plant-wide program, see ANSI/ESD S20.20.
FAQ
What is the difference between Ω/sq and Ω? Surface resistivity is expressed in Ω/sq and is independent of the measured area — the point of the concentric-ring electrode design, so specimens of different sizes can be compared. People often say “surface resistance in ohms”; strictly it is “surface resistivity in Ω/sq.” With a concentric-ring electrode the two have the same value but different meanings.
Why do the inner and outer layers read differently? Antistatic bags are mostly multilayer, and the inner and outer materials or treatments may differ, so differences are expected. Here 4.55 × 10¹⁰ inside and 3.10 × 10¹⁰ outside is a normal difference within the same order of magnitude. What matters is stating which side was measured.
How much does humidity matter? Most antistatic treatments conduct through adsorbed surface moisture, so resistance rises clearly at low humidity. That is why a lot can “fail” a site check in the dry season yet pass in a controlled-climate laboratory — not because anyone cheated, but because the conditions differ. So every reading needs its temperature and humidity.
What does a control reading of “greater than 2 × 10¹²” mean? That is the instrument’s range limit. The insulator’s resistance exceeds what the instrument can resolve, so it shows as over range. That is exactly what a control should show — it proves the setup responds correctly to a real insulator rather than producing a middle value from nowhere.
Can this record be given to customers as a test report? Not recommended. It is a record measured under site conditions, not a formal report from a qualified laboratory under a standard procedure (including specimen conditioning). It is fine for internal acceptance or incoming screening; if a customer audit requires a formal report, send samples for testing.
Standards cited (please use each publisher’s current version):
- Overview of ESD standards (including ANSI/ESD STM11.11 for planar materials and ANSI/ESD S541 for packaging) — EOS/ESD Association, Inc.
- ANSI/ESD S20.20, ESD control program standard — EOS/ESD Association, Inc.
- IEC 61340-5-1:2024, Electrostatics – Part 5-1, Ed. 3.0 — International Electrotechnical Commission (IEC)
Further reading:
- How to choose an ESD bag: antistatic, conductive and shielding
- How often should wrist straps be tested? Daily checks, continuous monitoring and pass values
- How to choose an ESD mat: material, resistance, grounding and size
- SL-031A personal ESD equipment tester manual
- JH-038A grounding monitor alarm specification
Related products:
- Weighted surface resistance meter (the instrument used in this record)
- Antistatic foil bags (shielding)
- ESD cleanroom tool bags
- ESD mats
- ESD shielding bags (open-top and zip-lock, 142 sizes; one of the items measured)
For questions, please contact JIAHON or email [email protected].
From JIAHON’s engineering department. The surface resistance meter is named by function; JIAHON is not the authorized distributor of ACL in Taiwan.