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Electrical resistance: For electrically conductive tapes, test methods matter
Electrical resistance is one of the main performance characteristics engineers quickly scan for when evaluating an electrically conductive tape for shielding and grounding performance. More then ever, they rely on manufacturer test results to help ensure devices function properly. Yet individual manufacturer test methods seem to vary. Many don’t supply information on test methods at all.
Measuring for electrical resistance
Many electrically conductive tapes are tested and measured according to conductivity. On the surface, this makes sense – shielding and grounding depends on the conductivity of the material. However, resistance can be a stronger and a more accurate performance characteristic for product performance in real-world end-use applications.
Conductivity is a fundamental property of a material, regardless of its size or shape. The resistance of a material depends on the real-world dimensions such as length, width and cross-sectional area – leading to a more specific and accurate performance measurement. Resistance is also very useful for measuring performance on specific substrates. Accounting for contact resistance, which can affect measurements based on a substrate’s resistivity, roughness and hardness, can lead to more accurate performance calculations for specific components in practical applications.
While it is possible to measure the conductivity of a material according to its precise physical dimensions and geometry, it may require much more complex testing and may not be easily reproducible. Further, it takes time: calculating conductivity is typically an extra step which involves accounting for contact resistance and depends on accurate resistance measurements.
Testing for replication – the right tools
It’s always best if subsequent material tests can easily replicate the performance indicated on manufacturer data sheets. Without the right tools, measuring electrically conductive tapes can present challenges. A technician may use a multimeter with a sharp probe. A conductive tape will typically feature an adhesive with electrically conductive fillers spread throughout. Narrow or sharp probes may not contact enough filler particles to achieve accurate or consistent resistance measurements.
Also, electrically conductive tapes from 3M typically deliver very low resistance – so low that accurately measuring it is beyond the capability of some handheld multimeters. For best results, technicians should use feature-rich bench multimeters with flat head probes.
The value of standardized manufacturer electrical performance testing
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Many manufacturers of EMI control products such as electrically conductive tapes rely on standard industry test methods specs such as MIL to measure their products’ performance. For evaluators, the results can be difficult to replicate.
3M tests its electrically conductive tapes for resistance using its own highly standardized test methods – 3M ETM-7 and ETM-12. Proprietary manufacturer test methods like these show specific results in environments close to real-world applications. Results are published in product technical data sheets. So evaluators can more easily replicate these results conducting their own testing, making the application more reliable early in the design stage.
Our testing methods, step by step
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The 3M ETM-7 test method measures XY-axis electrical resistance through the adhesive. The ETM-7 testing board contains Au plated Cu electrodes, and a strip of conductive tape is placed adhesive side down between them to simulate a typical manufacturing process. The tape is hand laminated and rolled with a 2kg rubber roller for a smooth 10 mm x 10 mm contact area between the tape and electrodes. After dwell time, DC resistance between the electrodes is measured with a micro-ohm meter. The resistance results are recorded after 5 ~ 30 seconds for resistance.
For measuring Z-axis electrical resistance through the adhesive, two strips of tape are placed on the electrodes of the ETM-7 board and an ETM-12 test board is applied on top of them, the Au plated Cu electrode side down.This method creates a complete circuit for measuring the resistance of double-sided tapes on specific substrates – the ETM-12 board can be made of any number of metals, such as aluminum or stainless steel. The assembly is hand laminated for a 10 mm x 10 mm contact area between the tapes and electrodes and rolled with a 2kg rubber roller to simulate typical manufacturing processes. After dwell time, DC resistance between the electrodes is measured with a micro-ohm meter. The resistance results are recorded after 5 ~ 30 seconds for initial resistance.
Testing for the real world
3M conducts this resistance testing on our wide range of 3M™ Electrically Conductive Tapes. The results are published on our technical data sheets – a reliable and reproducible basis for evaluating shielding and grounding performance in specific applications. Additional testing on test boards with different substrates can also be done. To learn more, speak to a 3M technical expert for help optimizing EMI solutions for real-world applications.
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