Conductive Fabric Surface Resistance Testing

Conductive fabric surface resistance testing mainly conducts scientific testing and analysis on the surface resistivity testing, point-to-point resistance testing, shielding effectiveness verification testing, resistance re-testing after the environmental stability testing, resistance changes after the friction/abrasion testing, and square resistance uniformity testing, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Surface resistivity testing: This testing is the core indicator for measuring the conductive performance of the conductive plane on the surface of the conductive fabric. By measuring the resistance value between the opposite two sides of the square on the material surface under the specific conditions, it is characterized with the unit of ohm (Ω) or ohm per square (Ω/□). The lower surface resistivity means that the material has the better surface conductivity and electrostatic discharge ability.

Point-to-point resistance testing: This testing is used to evaluate the resistance between the two specific contact points on the conductive fabric. It simulates the conductive path performance between the connection points such as the grounding wire clamp connection points or the contact points of the two conductive parts in the actual applications. The test results directly affect the effectiveness of the conductive fabric at the electromagnetic shielding or electrostatic protection connections.

Shielding effectiveness verification testing: Although the surface resistance is the basic electrical performance indicator, some inspections will verify it in correlation with the shielding effectiveness (SE). By measuring the shielding effectiveness at the specific frequencies (such as 30 MHz-1.5 GHz) and comparing it with the surface resistance data, the correlation of the actual performance of the conductive fabric in the electromagnetic interference (EMI) shielding applications can be evaluated.

Resistance re-testing after the environmental stability testing: This testing aims to evaluate the stability of the conductive performance of the conductive fabric after the environmental stresses. Usually, the sample is placed in the constant temperature and humidity, high temperature and high humidity, temperature and humidity cycling or salt spray environments for a certain time, and then its surface resistance is re-measured, examining the coating adhesion and anti-corrosion oxidation ability.

Resistance changes after the friction/abrasion testing: Simulates the performance attenuation of the conductive fabric caused by the friction in the actual use. After rubbing the sample surface for multiple times with a standard abrasion tester, the surface resistance change rate of the abraded area is measured, thereby evaluating the durability and wear resistance of the conductive coatings (such as the nickel-plated and copper-plated coatings).

Square resistance uniformity testing: The samples are taken at the different positions of the conductive fabric width (such as the centre, four corners and edges) for the surface resistivity testing, evaluating the uniformity of its conductive performance. The poor uniformity may lead to the poor local shielding or grounding effects of the products, and is the key for measuring the stability of the production process.

Testing Scope

Metal-plated fibre fabrics: Mainly for the conductive fabrics made by depositing the metal layers such as the copper, nickel and silver on the base fabrics such as the polyester and polyamide through the electroless plating or electroplating processes. This is the most common testing object, and its performance depends on the metal type, thickness, density of the coating layer and the adhesion with the base fabric.

Carbon fibre/metal wire hybrid woven fabrics: The testing scope includes the fabrics made by blending or interweaving the conductive fibres such as the carbon fibres and stainless steel wires with the other insulating fibres. The surface resistance testing needs to focus on the distribution uniformity of the conductive fibres and the influence of the weaving structure on the continuity of the conductive network.

Conductive foam composite gaskets: The testing objects are the conductive foam electromagnetic sealing gaskets with the surface wrapped with the conductive fabric. During the testing, the resistance testing is carried out on the exposed outer surface of the conductive fabric, evaluating its ability to form the low-impedance connection with the chassis shell as the contact interface.

Conductive tapes and conductive hook-and-loop tapes: Covering the tapes made of the conductive fabric as the base material, coated with the conductive pressure-sensitive adhesive on one or both sides, as well as the conductive hook-and-loop tapes in the form of the Velcro. The testing includes the surface resistance of the conductive surface of the tape and whether the backing adhesive affects the overall conductivity.

Products with the differentiated coating structures: The testing scope extends to the products with the special coating structures, such as the Cu/Ni, Cu/Ni/Au and Ni/Cu/Ni multi-layer coated conductive fabrics. The contributions of the different coating combinations to the surface resistance, oxidation resistance and contact resistance need to be evaluated separately.

Customized cutting and formed parts: Including the conductive fabric products cut into the specific shapes according to the customer requirements (such as the finger-shaped spring contacts, grounding gaskets and shielding cover liners). The testing needs to be carried out at the key contact parts after forming, ensuring that the processing process does not damage the conductive layer.

Testing Methods

Four-probe method (four-point probe method): This is the classic and most commonly used method for measuring the surface resistivity of the thin film or sheet materials. Four probes arranged at equal intervals linearly contact the sample surface, a constant current is passed through the outer two probes, the generated voltage is measured by the inner two probes, and the resistivity is calculated through the formula. This method can effectively eliminate the influence of the contact resistance, with the accurate results.

Two-point method (parallel electrode method): Two parallel strip electrodes contact the conductive fabric surface with a certain pressure, and the resistance between the two electrodes is measured. This method is simple to operate and is commonly used for the rapid inspection and production line quality control. However, its results include the contact resistance, and it is usually used for the relative comparison of the samples of the same batch.

Concentric ring electrode method: In accordance with the standards ASTM D257 or GB/T 1410, a concentric ring electrode system is used. The sample is placed between the two concentric ring electrodes, and the surface resistance is calculated by measuring the resistance between the electrodes. This method is suitable for the standardized precise laboratory measurement, and the electrode system needs to meet the specific dimension requirements.

Non-contact eddy current method: The high-frequency alternating magnetic field is used to induce the eddy currents on the conductive fabric surface, and the surface resistance is indirectly calculated by measuring the eddy current losses. This method does not require the physical contact and will not scratch the sensitive coating layers. It is particularly suitable for the testing of the ultra-thin or high-value conductive fabrics whose surfaces are easy to be damaged.

Shielded room method correlation testing: In the electromagnetic shielding anechoic chamber, the conductive fabric is used as a part of the shielding body, and while measuring its shielding effectiveness, the surface resistance data is synchronously recorded. Through the accumulation of a large amount of data, the empirical relationship model between the specific material structure, surface resistance and shielding effectiveness is established.

Environmental test chamber built-in testing method: In order to monitor the resistance changes under the environmental stresses in real time, the test electrode leads can be connected into the high and low temperature damp-heat test chamber or salt spray chamber, and the online or interval measurement of the sample can be carried out. This method can obtain the continuous performance attenuation curves, with the more continuous data.

Testing Standards

Domestic Standards:

1. GB/T 1410, Methods of test for volume resistivity and surface resistivity of solid electrical insulating materials

2. GB/T 2439, Rubber, vulcanized or thermoplastic — Determination of resistivity of conductive and antistatic products

International Standards:

1. ASTM D257, Standard test methods for DC resistance or conductance of insulating materials

2. ASTM D991, Standard test method for rubber property — Measurement of resistive and conductive materials

3. ASTM D4935, Standard test method for measuring the electromagnetic shielding effectiveness of planar materials

4. IEC 60093, Methods of test for volume resistivity and surface resistivity of solid electrical insulating materials

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

High-precision digital resistance meter/megohmmeter: The core measurement equipment, requiring the high input impedance, high precision and wide range (usually from 10⁻³ Ω to 10¹² Ω). It is used to connect the various test electrodes and directly read the resistance values. The high-end models are equipped with the temperature compensation, data storage and statistical functions, meeting the requirements of the standards such as ASTM D257 and IEC 60093.

Four-probe tester: An integrated equipment specially used for the four-probe method, including a constant current source, high-sensitivity voltmeter and four-probe probe. The probe spacing of the probe is fixed (such as 1 mm), and the material is tungsten carbide or gold-plated, ensuring the hardness and conductivity. The instrument usually automatically calculates and directly displays the resistivity (Ω·cm) or square resistance (Ω/□).

Standard dead-weight electrodes: Manufactured in accordance with the standards such as ASTM D991 or GB/T 2439, including the parallel strip electrodes, concentric ring electrodes or cylindrical electrodes. The electrode material is the brass or stainless steel with the good conductivity, with the flat bottom, and is equipped with the dead weights of the specified mass (such as 2 kg or 5 kg), to ensure the pressure applied on the sample is constant and repeatable.

Shielding effectiveness testing system: The equipment used for the correlation analysis, usually including a vector network analyser, coaxial flange fixture (such as ASTM D4935), or a system composed of a shielded room, antennas, signal source and receiver. It is used to measure the shielding effectiveness of the materials in the range of 30 MHz to 18 GHz or higher frequencies, and is compared and analysed with the resistance data.

Environmental reliability testing equipment: Including the constant temperature and humidity test chambers, high and low temperature shock test chambers and salt spray corrosion test chambers. They are used to carry out the accelerated aging or environmental stress treatment on the conductive fabric samples before the testing, to evaluate the long-term stability and reliability of their conductive performance.

Abrasion tester: Such as the Martindale abrasion tester or linear friction tester, equipped with the standard friction heads (such as the wool felt) and the specified loads. It is used to simulate the friction situation in the actual use, and after the testing, combined with the resistance meter, the wear life and performance retention rate of the conductive fabric coating are evaluated.