Let’s Talk About Flexible Absorber Sheets

We covered absorbing materials used in the construction of anechoic chambers for electromagnetic compatibility (EMC) testing in a September 2018 article¹.  In this article, we address a different type of absorber material, the flexible absorbing materials used for radio frequency (RF) noise suppression and EMC shielding in electronic products such as cell phones, MP3 players, and rugged handheld devices.

Demand

In recent years, the technical need for RF suppression components has grown. Several factors, including use of faster rise-time and hence noisier components, integration of wireless (IoT) into almost anything and everything, more densely populated PCBs, and the use of plastic housings over metal enclosures, have contributed to expanded demand for RF suppression.

Applications

In specific applications, flexible absorber sheets act as low-pass filters, absorbing or attenuating high frequency conducted noise.

Flexible absorber sheets are also used to suppress cavity resonances, an issue arising when reflected energy from a noisy electrical/electronic circuit in a metal enclosure destructively combines to create a highly intense field.  In this application, the absorber attenuates the resonance by reducing internal reflections.

A third application for flexible absorber sheets is for use in high-density layouts and circuits that require some form of good magnetic field decoupling.  In this scenario, placing the sheet either on the noise source or on the part requiring protection from the noise source allows attenuation to work.

Characteristics to look for when choosing flexible absorber material:

• Frequency Range: What is the frequency or range of frequencies used within your product that require suppression?  Select a material with a wide enough frequency range to suppress all offending signals of concern.  One manufacturer specifies their absorber material suppresses RF emissions from 1 MHz to 6 GHz. When working to suppress an electrostatic discharge (ESD) event, such as when testing to IEC 61000-4-2, note that its bandwidth is approximately 300 MHz (see reference 2 for a discussion on design for ESD immunity).
• Operating Temperature Range: Don’t forget to factor the expected operating temperature range of the device you intend to use the flexible absorber sheet in into your selection criteria.  Carefully consider the environment felt by the absorber material internal to the device.  Interior temperatures can be 5, 10, 15 or more degrees hotter than the ambient temperature of the end-product.  Also consider the lowest temperature expected and select absorber material that will accommodate these lower temperature extremes.
• Humidity: Humidity goes hand in hand with the temperature specification.  What kind of humidity levels do you expect your product to experience and will the adhesive backing used on the flexible absorber keep its adhesiveness under these conditions?  You may have to contact the supplier directly for more information concerning the material’s ability to survive in a highly humid environment.  
• Permeability: Is it high enough for your needs?  Higher permeability materials usually mean better shielding at lower frequencies.  Permeability is frequency dependent upon the flexible absorber sheet material. Consideration of the material used to construct a flexible absorber sheet is a must to achieve effective noise level suppression of the required frequency range.
• Flexibility: Is the flexible absorber sheet truly flexible and easy to apply as the manufacturer states? It’s best to request samples to try before specifying for purchase.
• Material Safety Certification: An example is the UL flammability standard, UL94 V-0.  Is it required for your design and does the flexible absorber material sheet have it?
• Thickness: Is the material too thick (or too thin) for your application?  Can you get different thicknesses based on your specific needs?
• Customization: Does your application require a custom sheet size, and can the manufacturer provide it?

Performance

In addition to absorber permeability and other characteristics, there are many variables which play a part in determining the performance of a flexible absorber sheet.  These include sheet thickness, size, geometry, and the distance between the noise source and absorber material.  These factors make it nearly impossible to estimate the real attenuation capabilities of the suppression material. The good news is that it’s relatively easy to set up and run experiments that more closely characterize the actual performance of the material.  References 5 and 6 describe these methods in greater detail.  These methods make it possible to test several sheets with different compositions or thicknesses to obtain the maximum transmission attenuation power ratio for any specific application.

Closing

The EMC landscape is changing, and it is getting more difficult to design products to meet EMC emissions, immunity and functionality requirements.  Today’s design constraints are so burdensome that we’re no longer able to employ EMC solutions that were more than adequate in the past.  Flexible absorber sheets can help save the day. We can use flexible absorber sheets in situations where design constraints prevent us from employing older, previously adequate EMC suppression techniques or where time constraints prevent us from completely redesigning our product to comply using more traditional methods.

References and Further Reading

1. In Compliance Magazine. (2018, September 28). What Every Electronics Engineer Needs to Know About: Absorbing Materials. 
2. In Compliance Magazine. (2019, February 5). Let’s Talk About Design for ESD Immunity.
3. Würth Electronik, WE-FAS EMI Flexible Absorber Sheet.
4. Würth Electronik, How Flexible Absorber Sheets Solve EMI Problems: Properties + Materials [Part 2 of 2].
5. Würth Electronik, Characterization Methods for Flexible Absorber Sheets WE-FAS, Application Note ANP059.
6. Wyatt, K., Insertion-loss measurements of ferrite absorber sheets. July 25, 2019.