Understanding Different EMI Shielding Effectiveness Tests

The problem arises when radiations from different devices interfere, causing device malfunctioning, data loss, and other similar issues. The risk increases as we move to manufacturing, transportation, medical, and military applications. Any malfunctioning of equipment and devices in these contexts can cause massive loss of finances and even life.

A common method for shielding electronic devices and circuits from incoming EMI is to place them inside a Faraday Cage. Named after its inventor, Michael Faraday, this cage consists of a continuous covering or ‘mesh’ of conductive material that grounds any current produced by an external or internal electromagnetic field, effectively blocking a large amount of EMI.

Traditionally, materials such as aluminum, copper, and steel have been used to make the sheet metal housings that cover electronic devices. However, thin sheets of these materials tend to deform under the pressure required for sealing and cause leakage of electromagnetic radiation, compromising the shielding’s effectiveness. Therefore, the choice of material is an essential factor in determining the effectiveness of EMI shielding.

What exactly is EMI Shielding Effectiveness?

The purpose of EMI shielding is to prevent electromagnetic radiation from entering or escaping an electronic device. So, to determine the effectiveness of an EMI shielding gasket, we measure the amount of attenuation it achieves, i.e. how immune the device is from external sources of EMI.

Quantitatively, attenuation is measured in decibels (dB) which indicates the ratio between the field strength with and without the shielding. The least effective shielding would have an attenuation of 10 to 30 dB whereas a highly effective shield would attenuate up to 90 to 120 dB.

In other words, the greater the decrease in intensity of the signal after the barrier, the more the effectiveness of the shielding.

Regulations for EMI Shielding Effectiveness

Given the ubiquity of electronic devices in modern times, governments all around the world have had to put in place strict regulations to ensure all manufactured devices meet a minimum criterion of effectiveness.

In 1982, the U.S. Federal Communications Commission (FCC) passed the public law 97-259, amending the original Communications Act of 1934 to establish standards for EMI shielding effectiveness of all electronic devices such as those used in cars, homes, planes, trains, and medical equipment.

The defence industry has also established very strict standards and regulations for the EMI shielding effectiveness of electronic systems used in mission-critical applications such as missiles, man-portable communication, avionics, radar systems, and satellites.

This is because even a small amount of EMI can lead to the failure of critical missions and endanger human life, so it is crucial to make sure devices are able to block all external EMI.

Testing Standards

To ensure that an electronic device meets a high standard of EMI shielding effectiveness, manufacturers test their devices against several testing standards developed by organizations such as ASTM, IEEE, and the U.S. Department of Defense.

• ASTM D4935: used to measure the shielding effectiveness of planar materials for a far-field EM wave.
• MIL-DTL-83528: used by the U.S. Department of Defense (U.S. DoD) to measure the effectiveness of elastomeric shielding gaskets.
• MIL-STD-461: another U.S. DoD standard targeted towards electronic enclosures smaller than a typical equipment rack.
• IEEE 299-2006: a more general standard used to measure shielding effectiveness for any enclosure that shields the device from exterior electric or magnetic fields.

How Do We Measure EMI Shielding Effectiveness?

The effectiveness of EMI shielding is evaluated primarily by measuring the electromagnetic signal attenuation a material achieves in a constrained test. A single method may not suit each application, so we have four different ways to determine how well a given barrier shields from EMI.

Open Field Test

Also called the free space test, it aims to replicate the real-use conditions for a completed electronic device.

The concept is to test the effectiveness without removing too many variables so the results can better predict how well the shielding would work in a typical usage environment.

As part of this test, the device is placed in an open area with no metal other than the testing equipment. Antennas to monitor the signals are set at varying distances, which may also capture background noise.

Coaxial Transmission Line Test

This is a comparative technique used to test the shielding effectiveness of planar materials and follows the specifications of the ASTM D4935 standard to test frequencies between 30 MHz and 1.5 GHz.

First, the reference material is placed in a diamond-like structure, and EMI is measured at various frequencies. Then, the same procedure is repeated for the test material at the same frequencies.

After the tests are complete, the effectiveness of the test material for EMI shielding is determined by comparing its results against those of the reference material. If the attenuation produced by the test material is close to that produced by the reference, the testing material can be said to have high effectiveness.

Shielded Box Test

As the name suggests, this method uses a completely sealed box with a cut-out portion to insert the shielding material.

The effectiveness is measured by comparing the signals outside and inside the box. Ideally, an effective shielding would not let any electromagnetic waves reach the interior of the sealed box.

A limitation of this method is that the size measurements of the sample shielding material must be identical across different trials since any inaccuracies in sample sizes can affect the results. Secondly, the shielded box test does not work well for frequencies exceeding 500 MHz.

Shielded Room Test

This method is particularly useful when it is almost impossible to eliminate ambient noise and is particularly helpful in determining the susceptibility of a device to that noise.

To conduct this test, two shielded rooms are used with a wall in between. The test material and test equipment are then placed in one room and sensors are placed in the other. To further remove any external signals, shielding lead may be used.

Conclusion

Testing the effectiveness of EMI shielding is essential to ensure the perfect working of electronic devices in any given context.

Given the various materials, shapes, sizes, and environments associated with each unique device, we can use one of these four tests to measure how well a shielding is likely to block EMI.

Test to Measure Shielding Effectiveness	When to Use
Open Field Test	To test finished electronic products in a normal usage environment
Coaxial Transmission Line Test	For planar materials
Shielded Box Test	When frequencies are less than 500 MHz
Shielded Room Test	No other way to eliminate ambient noise