Electromagnetic and Radio frequency interference (EMI/RFI) shield design is complex and multi-faceted, combining electrical and mechanical engineering concepts. In practice, shielding is very much an iterative process in which controlling one variable impacts another, leading to even more changes in the design.
One aspect of shield design is knowing which external signals are most critical to keep from interfering with the circuits inside. This is important from the standpoint of material selection, both for the shield structure and any subsequent plating. Knowing the frequency and amplitude of critical signals and how they behave with conductive metals helps you determine the conductivity and thickness required to achieve adequate shielding effectiveness.
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Skin effect
One way to describe shield performance is with a phenomenon called skin effect. Circuits use either direct current (DC) or alternating current (AC) as a power source, but only alternating currents exhibit skin effect.
AC currents tend to crowd into the top layers of a conductor, closest to the surface, increasing current density in that area of the material. This, in turn, increases the impedance of the conductor and lowers it’s conductivity, thus reducing the effectiveness of the shield. The higher the frequency the more pronounced the effect.
Ways to counter this include increasing the shield’s surface area (thereby providing more space for currents to flow), using a denser or even solid top layer (i.e. without apertures or very tightly woven mesh), or adding a more highly conductive plating.
Skin depth
Another aspect of signal behavior relative to shield material is called skin depth. It describes how far into the material a given wave will penetrate. This chart shows skin depth for various metals at different frequencies. For example, at a frequency of 1 GHz, gold has a skin depth of 2.36 micrometers. Skin depth takes into account:
- wave frequency (in Hz)
- wave amplitude (in dB)
- material conductivity
- material permeability
You can see how skin depth is calculated here.
For a given material, each unit of skin depth attenuates approximately 9 dB of wave amplitude. By using a material thickness that exceeds the skin depth for a given wave, you can control for penetration while also minimizing excess thickness. Remember also that due to skin effect, waves will concentrate in the upper layers of a conductor, so that helps ensure the topmost layers are where most absorption/penetration occurs.
Here’s an example from All About Circuits that shows how skin depth works in practice. “Consider RF signals for WiFi or Bluetooth, which operate at 2.4 GHz. Using the calculator, we see that the skin depth with a copper conductor is 1.331 micrometers. This means that even with a very thin (e.g., 30 AWG) wire, only a tiny fraction of the wire is carrying a significant amount of current.”
Skin depth is one of several important criteria in determining the type of material, thickness, or plating used for a stamped part. A material that is too thin will let waves in and possibly resonate or interfere with signal reception or transmission, but too thick may end up adding unnecessary weight, expense, or both.
In reality, most shields need to control for more than one variable (such as multiple signals at different frequencies or the need to trap signals inside the shield), so it’s usually not as simple as adjusting thickness for only one type of wave. And even though thin material can save costs, the finished stamped components must also have sufficient physical strength for their intended purpose, so you may need to use a material thicker than strictly what skin depth indicates. It all goes to show that in EMI/RFI shielding, each design is unique and one size doesn’t necessarily cover all situations. Custom precision stamping is a great way to manufacture the exact shield you need for your application – we can help, so connect with us today.