Material EMI Performance Comparisons Fortunately, all EMI shielding product performance levels are expressed in a common format. They are represented in a two axis curve, plotting frequency range against attenuation. The frequency range as shown on the X axis is not linear, but rather is shown as a log table, usually ranging from 1MHz to 40GHZ for E-field materials and from 1Hz to 300 KHz for H-field materials.
The attenuation level, which is also known as the shielding effectiveness, is expressed in decibels (dB) and is shown on the Y axis with a range of 0 to 120. The vertical attenuation values are linear, but each numerical increase of 3 dB represents a reduction of the interference signal by one-half. In other words, a material that is represented by the value 20dB is not twice as good as a material represented by the value 10dB, but approximately 8 times better. (Example: If at 10dB the interference leakage had an arbitrary value of 1.0, then at 13 dB it would be 0.5 or half of the original value, at 16dB it would be half again or 0.25; at 19dB it would be half of the 16dB value or 0.13).
The above example illustrates that a material with a 19 dB value would leak less than 13% of the energy than a material having a 10 dB value. Obviously, higher is better, but if your requirement is to achieve a 40dB noise reduction, just recognize that you don’t need to double that value to be safe. If you can buy a 60dB material for the same cost as a 40dB material (your requirement), then by all means do so. Remember not to go overboard; each 3dB increase is reducing the leakage factor by another 50 percent.
When looking at EMI shielding attenuation performance curves you should be suspicious of curves that have only two data points. Technically, you need at least three data points to create a curve. The fewer data points you see may indicate that the supplier is trying to cover up some serious weak spots. You should see at least 6-7 data points at a series of frequencies. A curve like that tells you the trend in which the shielding effectiveness is falling or climbing.
Don’t be scared off of a material because the curve has a few significant dips. There are other factors known as harmonics that can cause these dips. Lay a ruler across the attenuation curve and try to hit at many data points as possible. This will show you the performance trend. Again, too few data points should make you question the material’s true EMI performance.
Evaluating EMI shielding materials can be like buying a new car. To truly find what you need, you must cut through all the flashy sales brochures and the emotional sales pitches. You need to determine your true requirement, be that a basic utility vehicle, a sedan or a flashy red sports car. The decision should not be based solely on shielding effectiveness. Your specific shielding application will be defined by a series of performance characteristics your system requires.
It is always a good idea to make a list of the EMI performance characteristics that are important to your application before you begin shopping for EMI shielding.
Below are some typical questions you should consider;
- Is this a new system in development or a retrofit of an existing one?
- What are the environmental parameters the product will have to operate in?
- e., Chemical, Mechanical, Electrical & Thermal concerns
- How much material flexibility do I need?
- e., Is your system static or in motion?
- Do I know what the EMI interference frequency range is?
- Do I know how much Attenuation I need to drop below the upset threshold?
- Is the material form conducive to my application?
- e., Will you need a tape, pull-thru tube or wrap-around tube?
Once you can answer these questions you can make an educated purchasing decision. EMI shielding is generally viewed as expensive because all too often it is an afterthought. Buyers will always want to purchase the cheapest product they can. Purchasing cheaper EMI shielding materials up front can cost your company a lot of money if the product purchased makes the production department’s life a living hell! There is an old adage that applies very well to adding EMI shielding. “It’s not what it cost you coming in your door; it’s what it cost you going out your door.” Labor and overhead (not the raw materials being procured), are the biggest costs to a company.
When specifying a particular EMI product, be sure you understand how and where it will be installed and with what tooling. Also consider the level of expertise your installers have. To achieve the highest and most consistent EMI performance from one system to the next it is desirable to design the performance into the parts as much as possible and minimize the amount of creativity required by production personnel.
To learn more and to read through the EMI-101 5-part series, check back here on the “Engineers Knowledgebase”.