Honeycomb Air Ventilation Panels - The Polarity Principle
Honeycomb air ventilation panels are used in applications where superior electromagnetic interference (EMI) shielding must be incorporated with heat dissipation in the form of airflow. Every vent panel has a variety of design features, each providing benefits to end customers based on specific application needs. These design features can include framing, plating/painting, gasketing, and vent size control.
An often overlooked but highly important phenomenon to consider when designing EMI vent panels is that of polarity.
Polarity - Polarity is the difference between EMI shielding of honeycomb in different X/Y directions
What this means is that honeycomb vents can have differences in shielding effectiveness, sometimes as great at 50 dB, depending on the direction of the electromagnetic waves.
For example, a basic aluminium honeycomb vent may provide shielding of 70 dB in the horizontal direction while only providing shielding of 25 dB in the vertical direction. This characteristic is due to the manufacturing process of standard aluminium honeycomb vent panels.
Basic aluminium honeycomb is created using thin ribbons of aluminium that are bonded using a non-conductive adhesive. Polarity is associated with seam leakage caused by the non-conductive bonds from cell to cell created during the manufacturing process of adhering aluminium ribbons together to make the honeycomb. While thin, this non-conductive gap is the cause of difference in shielding effectiveness (SE). It is important to note that polarity is only an issue for aluminium vents, not for steel, stainless steel, and brass honeycomb due to a different manufacturing process (steel and brass honeycomb use a welding process, eliminating the non-conductive gap). The below graph demonstrates the significant difference in shielding effectiveness in the horizontal and vertical directions.
Fortunately, there are several solutions to combat this polarity issue:
Layered vents
With the addition of a second layer of honeycomb, offset at a 90-degree angle, the polarisation effect can be dramatically reduced. The Chomerics term for these layered vents is Omni Cell. By rotating the second layer of honeycomb 90 degrees, RF wave interaction in both the X and Y axes are combated by the seam orientation of each layer of honeycomb. This means that while the electromagnetic waves may pass through one layer of the honeycomb, the offsetting layer will not allow them to pass through the entire vent assembly. Of note, airflow through the vent is not significantly impacted, allowing for enough heat dissipation.
While the maximum shielding effectiveness of the Omni Cell vents is nearly identical compared to that of a single layer vent, the directional consistency is instantly noticeable. There is no longer a difference in the horizontal and vertical shielding effectiveness, with the offsetting layers eliminating the polarity effect.
Plating
Electroless nickel plating is an ideal plating option to combat polarity on aluminium vents. The nickel plating covers the non-conductive bonds and eliminates seam leakage between aluminium ribbons. The nickel plating electrically connects the aluminium ribbons which overcome the non-conductive adhesive. Not only does nickel plating effectively eliminate the polarity effect, it increases the durability of the vents and improves their lifespan in harsh environments.
As with Omni Cell vents, the polarisation effect is eliminated with the vent exhibiting nearly no difference in shielding between horizontal and vertical testing. A properly plated vent will also increase the SE of the entire honeycomb array, creating conductive contact between every individual aluminium ribbon in the assembly. The nickel plating also improves the electrical connection of the honeycomb to the frame if the plating process is done after assembly.
Based on the above graphs, a conclusion can be made about the techniques used in eliminating the polarity effect. Since the plating process can eliminate the polarisation effect AND increase the SE, this approach is most common. Omni Cell construction is effective if it meets the desired shielding effectiveness level. It is rare to see a nickel plated Omni Cell vent.
Individual project specifications such as airflow requirements, shielding performance, environmental exposure, budget and a myriad of others will drive the design process of EMI shielding honeycomb ventilation panels, but it is important to know about principles such as polarity in making final considerations.
