Screened vents balance cooling with EMC
Ventilation openings often allow more in to them than just cool air; electromagnetic waves from surrounding devices can pass through the openings and cause interference.
As the density of electronic components increases on PCBs so too does the heat that requires dissipation.
And when these boards are stacked within an electronics cabinet, the internal temperature can be in excess of 70C, which can cause potential breakdown of component performance.
Efficient airflow is the key to dissipating heat within a confined space, and the best method is ventilation to a cooler environment outside.
However, ventilation openings often allow more in to them than just cool air; electromagnetic waves from surrounding electronic devices can pass through the openings and cause interference to the electronics within.
This interference can have a hindering effect to performance that can in the case of medical or aerospace environments be potentially fatal.
Most cabinets, boxes or cases are constructed of metal panels and these will make up the majority of the shielding you need.
But any gaps, slits or windows on the unit will all be potential outlets for emission; a logical approach to sealing these emission points with conductive material will eliminate most problems.
Many test houses can give a detailed breakdown of emission points during testing, but repeat visits to test houses for rechecking can become costly so early elimination of obvious emission points is essential at the design stage.
Sources of emission can range from air intake slits, card and disc entry slots or data terminal points etc Such potential sources of emission need to be sealed with conductive gaskets.
Even very small gaps can allow emission or reception of unwanted interference.
And panels that appear flush to a frame can have gaps large enough for emissions.
In cases such as this, a conductive gasket(s) needs to be used to fill the holes.
If your design incorporates an air vent, unwanted radiation introduced through the aperture can usually be blocked by using a metal honeycomb vent, again with a soft conductive gasket, without compromising the much needed airflow.
LEDs, LCDs or VDU displays can be a source of EMI radiation, a window made from conductive coated plastic or laminated plastic/mesh/plastic can be placed over the display to reduce this emission.
If you are using a plastic moulding or nonmetallic case there are three basic options for successful shielding.
Try to locate the exact component within the box that is emitting the radiation, and shield this specifically (using a formed metal box, or laminate).
If this cannot be done thought must be given to coating the inside of the box with a conductive material, such as custom shaped adhesive copper foil parts, that wrap around the inside of the box, and connect to a common earth.
If the internal coating of the case still appears unsatisfactory then a final option is to replace all or part of the case with a metal equivalent.
Two recent products from Laird Technologies are excellent examples.
MaxAir is an environmentally friendly vent panel product line, and these RoHS compliant panels provide high levels of airflow and EMI protection for electronic enclosures and cabinets used in the military and telecommunications industries.
The nickel-copper plated polycarbonate honeycomb material provides a rigid medium, eliminating the need for costly frame designs.
This allows for greater airflow through the entire honeycomb surface and ease of installation as a result of its press-to-fit assembly.
MaxAir offers designers a lightweight, environmentally friendly and dent resistant alternative to traditional metal vent panels and this product benefits designers via its superior durability, increased airflow and smoother installation at competitive pricing.
The US patented MaxAir vent panels are available in varying densities of material to meet specific levels of rigidity requirements.
UL94V0 versions also are available.
The honeycomb cell size can be 3.18 or 6.35mm in standard thicknesses of 6.35 and 12.7mm.
Also from Laird Technologies are Recyclable Clean Copper gaskets - a new range of environmentally friendly, copper-based shielding gaskets that offer a beryllium-free EMI solution for use in a wide range of slotted applications.
This product protects against electromagnetic emissions and enhances elasticity and performance.
Laird Technologies continues to consider customers' needs as copper prices fluctuate.
Currently, Recyclable Clean Copper alloys cost about 10% less than copper beryllium (CuBe) alloys.
Additionally, Recyclable Clean Copper contains 13% less copper by weight and is therefore less affected by increasing variations in copper prices than similar products that contain CuBe.
Moreover, the Recyclable Clean Copper gaskets meld strong stability with tensile strength to achieve high levels of thermal and electrical conductivity and elasticity making it suitable for utilisation in both grounding and shielding applications at a rate that is comparable to traditional metal EMI shields.
Additionally, shielding performance is similar to other copper alloys, with values over 100dB shielding effectiveness readily achieved.
Recyclable Clean Copper shielding gaskets extend the company's continuing commitment to protecting the environment.
This alternative material also exhibits better stress relaxation properties than many other solutions, allowing it to maintain pressure longer at contact points resulting in higher end product quality and longer system life spans for automotive, computer, military and telecommunications devices.
Recyclable Clean Copper is fully compliant with the EU RoHS Directive 2002/95/EC and alleviates the environmental, safety and segregation concerns associated with the traditional use and recycling of beryllium-based copper alloys.
This gasket material is characterised by superior yield strength (up to 100MPa) compared with other common copper alloys.
Key applications for this product include navigation and infotainment systems, NAS or SAN storage systems, high-end servers and communications display units, as well as basestations, handsets and network enclosures.
The flameproof product is targeted at high-volume designs, and custom stampings are available on request.
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