Honeycomb air vent panels are used to help cool electronics with airflow and while maintaining electromagnetic interference (EM) shielding.
The most important factors of honeycomb air vent panels are shielding effectiveness and airflow, which are both directly related to the cell size. Both factors have a large impact on the size, material, and design of the vent panel. While you will need to balance the tradeoffs between airflow and EMI shielding needs, you should also ask yourself if air particle filtration is needed as well.
Durability and the possibility of corrosion in harsh environments have an impact on design. Honeycomb is described in terms of two different size categories: cell size and thickness.
Specifies the width of the individual cells. The honeycomb is hexagonal, and the cell size is the measurement across each cell and the thickness is the measurement of the entire sheet of honeycomb material.
Cell size can have an impact on airflow, but it can also have an impact on shielding effectiveness. As a general trend, the smaller the cell size, the lower the airflow or the greater the pressure drop across the air vent panel.
At the same time, the smaller the cell size, the greater the shielding effectiveness because the smaller cell size limits the amount of electromagnetic energy able to pass through the openings. The key is to find the middle ground, the largest opening to allow enough air-flow to cool the system, but also the smallest cell size to block the undesired frequencies and obtain the required shielding.
The thickness of the air vent panel also has an impact on both the airflow and the shielding effectiveness. If the cell size were kept the same, the lower the thickness of the vent panel, the greater the allowable airflow.
The decreased airflow is caused by the surface friction of the air flowing through the honeycomb cells. However, reducing the vent thickness will also reduce the attenuation capabilities of the honeycomb. Again, the key is to try and find the middle ground between good air-flow (less pressure drop) and shielding effectiveness.
Aluminum honeycomb is made from thin ribbons of bent aluminum that are adhered together using a non-conductive adhesive. The points at which the ribbons come together are known as nodes and can cause EMI shielding leakage. With single-layer honeycomb vents, there is actual directional EMI shielding. This is known as the polarization principle.
It’s also important to note that this is only the case with aluminum honeycomb vents because, with brass and steel vent construction, the nodes are welded together and therefore are inherently conductive.
One way to reduce the directionality of attenuation in vent panels is by using what Parker Chomerics calls an OMNI CELL construction. This means that the second layer of aluminum honeycomb is stacked on top of the first at a 90-degree angle.
The directionality is offset by the second layer and the new panel should have nearly equal attenuation in both directions. One small drawback is that the two layers will reduce airflow across the new vent panel.
Platings and coatings
OMNl-CELL can be a great option in many cases, but for applications where an OMNI-CELL construction will not work because of space or high attenuation needs, you can achieve the same effect by plating the honeycomb. The plating of aluminum honeycomb will bridge the non-conductive node, and eliminate the directional effect of the honeycomb.
It’s also a much more thorough coverage, which results in better shielding. Platings will protect the vents from corrosion and standard wear and tear. Electroless nickel is one of the most common plating options, as is a chromate conversion coating.
And lastly, EMI vents can be coated with aesthetic paints to match any enclosure design. This includes CARC paints and common military color patterns.
This article was produced by Parker Chomerics. JHC Specialised Solutions are the exclusive authorised distributor for Parker Chomerics in Australia and New Zealand. Should you require any further information or wish to discuss a customised solution for your application, please contact JHC on +61 2 9531 7905 or firstname.lastname@example.org