Resistance Testing Methods for Semiconductor Cleanroom Gloves: A Practical Guide

1. Why Is Glove Resistance Testing Necessary?

In a semiconductor workshop, operators wearing gloves can separate charges from workpieces. When charges accumulate on the human body and are then conducted to the chip, electrostatic discharge (ESD) may occur. The glove is the final link in the entire anti-static chain: wrist strap → skin → glove → workpiece. If the resistance of any link is abnormal, the protection fails.

Many people assume that a glove labeled as ESD-rated is sufficient, but the standard points out two key facts:

1. Resistance decreases after the glove is worn: once the inner surface of the glove fits against the skin it becomes hydrated, and the measured in-use resistance is often lower than the material resistance measured at the factory. Therefore, the factory report alone is not enough; the actual wearing condition must be tested.
2. A glove can only be tested once: oil and sweat from the fingers contaminate the inner surface of the glove, and a second measurement will no longer be accurate.

2. Two Types of Resistance: Do Not Confuse Them

Intrinsic material resistance: measures the glove fabric itself, using STM11.11 surface resistance, STM11.12 volume resistance, and STM11.13 point-to-point resistance.

In-use resistance: measures the glove and the operator together as one system, using the constant area and force electrode (CAFE), tested while worn on the hand.

For on-site acceptance and incoming inspection, the focus is on the second type: worn on the hand and measured as a connected system.

3. Equipment to Prepare

1. Anti-static wrist strap with a ground cord (some steps require a built-in 1 megohm resistor, while others do not, as explained below).
2. Low-resistance multimeter: range 0.1 Ω to 10 MΩ, open-circuit voltage 1.5 to 10 V.
3. High-resistance meter: capable of outputting 100 V (for measurements above 10^9 Ω) and 10 V (for measurements below 10^9 Ω), with a range of 10^3 to 10^11 Ω.
4. Constant area and force electrode (CAFE): a small weighted metal electrode connected to the resistance meter through a banana plug and pressed onto the fingertip, ensuring a consistent contact area and pressure each time.
5. Isopropyl alcohol: used to clean the electrode.

4. Before Testing: Environment and Samples

Environment: 23 ± 3 °C, with comparative testing under two humidity conditions: a dry environment of 12% ± 3% RH and a normal environment of 50% ± 5% RH.

Sample conditioning: gloves are left in the standard environment for at least 48 hours before testing.

Sample size: six gloves of each type are taken as one group.

5. Operating Steps

Step 1: Bare-hand baseline test (without gloves). First confirm that the test system itself works properly:

1. The operator does not wear a glove, selecting the hand that will later wear the glove in actual use.
2. A wrist strap is worn on this wrist, and the ground cord without the 1 megohm resistor is connected to one terminal of the resistance meter.
3. The CAFE electrode is connected to the other terminal of the resistance meter and placed flat on the fingerprint area of the bare thumb or index finger.
4. After 15 seconds of charging, take the reading and record the bare-finger-to-wrist-strap resistance.
5. Repeat the test on other fingers selected for testing.

This value represents the minimum resistance baseline of the person plus the wrist strap.

Step 2: Test the system resistance with the glove worn (three voltage levels, selected according to the resistance). After putting on the glove, wait at least 15 seconds for the inner surface to fit against the skin before starting the measurement. The CAFE electrode is pressed onto the fingerprint area of the gloved thumb or index finger.

Level A · low voltage (expected below 1 × 10^6 Ω): use the low-resistance meter (range 0.1 Ω to 10 MΩ), with the wrist strap not containing the 1 megohm resistor. Read after 15 seconds. If the measured value is below 10^6 Ω, record it; if higher, switch to Level B.

Level B · 10 V (expected 10^6 to 10^7 Ω): the high-resistance meter outputs 10 V, with the wrist strap still not containing the 1 megohm resistor. Read after 15 seconds. If below 10^7 Ω, record it; if higher, switch to Level C.

Level C · 100 V (expected above 1 × 10^7 Ω): the high-resistance meter outputs 100 V, and at this level the wrist strap must include the 1 megohm resistor (simulating the real working circuit). Read after 15 seconds.

Replace with a new glove after each glove is tested; repeat all six gloves under both humidity conditions.

6. How to Interpret the Results

SP15.1 is itself a test method standard and does not specify acceptance limits; the limits are determined by the company's ESD control program (usually based on ANSI/ESD S20.20).

Common reference ranges in engineering:

• A glove system resistance between 10^6 and 10^9 Ω falls within the anti-static dissipative range, which is the target range for ESD gloves.
• A value below 10^6 Ω warrants caution about excessive conductivity, which instead poses an electric shock risk near high-voltage equipment.
• A value above 10^9 Ω indicates that charge dissipation is too slow and cannot provide ESD protection.

Record data using the standard attached form: record the resistance of each finger under both humidity conditions for each glove, then calculate the average, standard deviation, minimum, and maximum.

7. Five Common Pitfalls

1. Reading before waiting 15 seconds: the glove has not yet hydrated against the skin, so the data is biased high.
2. Repeatedly testing one glove: after oil contamination the data becomes distorted, so discard it after testing.
3. Not cleaning the CAFE electrode: wipe it with isopropyl alcohol before each test, otherwise residue affects the contact resistance.
4. Using the wrong wrist strap level: the 100 V high-voltage level must include the 1 megohm resistor in series, while the low-voltage levels must not.
5. Testing only one humidity: resistance rises significantly under low humidity (12% RH), which is the extreme condition that best exposes differences in glove performance and cannot be omitted.