Shielding: The Hole Problem

Effective EMI shielding continues to challenge electronics manufacturers with constantly increasing device frequencies and proportionally decreasing wavelengths. Unfortunate for manufacturers and end users alike, shorter wavelengths mean that unwanted noise escapes through even the smallest of openings or holes in electronic enclosures.

For the most part, the maximum shielding effectiveness (SE) of an enclosure is determined by the material used to construct it. But many other design characteristics, such as seams, apertures, and penetrations, play equally important roles in reducing the overall SE.

The two principal SE loss mechanisms are the energy reflection from the surface of the enclosure and the absorption within the enclosure material. If the RF source is external to the shield, then the reflected RF energy propagates away from the enclosure and adds to the SE. If the enclosure contains the RF source, the reflected RF energy always is inside the enclosure and does not add fully to the overall SE.

SE calculations for an enclosure containing an emitter are very simple since the absorption loss provides most of the shield attenuation. The attenuation for the enclosed emitter is approximated by the absorption loss:

AdB = Ka t

where: AdB = absorption loss

Ka = a constant: 3.338 (t in mils); 131,4 (t in mm)

t = thickness (mils or meters)

µ = relative permeability

s = relative conductivity

F = frequency in megahertz

SE calculations for an enclosure containing a receptor are much more complex because both the absorption and reflection losses provide attenuation. The attenuation relationship for the emitter external to the shield can be generalized and approximated by:

SEdB = AdB + RdB

where: AdB = absorption loss

RdB = magnetic, electric, or planewave reflection loss

Far field (planewave) conditions for d ³ (l 2p ) are:

RPdB =108.1 – 10 log10 (m F/s )

where: RPdB = reflection loss in the planewave

m = relative permeability

d = separation distance

s = relative conductivity

F = frequency in megahertz

Similar equations apply for the near-field (electric and magnetic) conditions. The relationship among electric, magnetic, and planewave fields depends upon the source impedance and the separation distance.

Apertures

The SE of an aperture is a function of its size. The worst-case SE based on the radiation efficiency of a slot antenna provides a simple model for calculating the worst-case SE of an aperture.

Some error results from its use, but it is adequate for design because the SE is greater than the calculation indicates. SE of a single aperture with slot opening length is given by:

SEdB = 20 log l /2 L – 20 log n

where: L = length of slot (meters) and L > width; L >> thickness and L £ l /2

l = wavelength in meters

n = number of apertures within l /2

This equation can be solved in terms of the length to determine what size aperture is required for a given attenuation.

To achieve acceptable attenuation values at a frequency of 100 MHz, typical for high-speed digital devices, apertures should not exceed 10 mm. The type of shielding used and how it is applied are determined largely by the function of the aperture. Apertures are required for displays, fans, keyboards, ventilation, connectors, and parting seams.

Except for doors and access panels, the largest apertures are windows or ventilation panels. Windows or vent panels made from knitted or woven wire screening can be used up to about 150 MHz. Beyond that point, the skin effect across the material becomes excessive, and it begins to degrade in SE.

A better solution is perforated metal materials, once commonly used in microwave ovens but now available in optical grades. As an alternative, the knitted or woven screen can be plated to improve SE. To avoid the visual distraction from moiré patterns that results from superimposing one repetitive image over another, consider shielding the PCBs or the back side of the CRT itself.

For shielded ventilation panels, honeycomb materials provide the best airflow with the best attenuation characteristics. The honeycomb is constructed of small waveguides assembled in parallel. This produces a material that is approximately 97% open area and has a single cell attenuation value of:

AdB = 30 t/d

where: d = diameter of a cell tube

t = thickness of the material, such as length of cell tube

In addition, honeycomb materials can be constructed in drip-proof and vision-blocking configurations for special applications without appreciable change in airflow characteristics.

Shielding considerations for doors, accesses, and ventilation panels are similar; however, doors and access panels are solid, and vent panels contain openings for airflow. Normally, doors and access panels are smaller than the enclosure on which they are mounted. An exception to this is the PCB-mounted enclosure. From a design standpoint, it can be regarded as a small enclosure mounted on an access panel.

The SE of the shielding panel or material depends upon how well it is sealed into the enclosure. An access or ventilation panel that is not adequately bonded to the enclosure often will behave like an antenna structure. Grounding the panel at only one point generally will prevent it from acting like an antenna, but it will not eliminate leakage from the rest of the seam.

For example, even though honeycomb has an SE of 110 dB at 100 MHz, if it is improperly installed over a 6″ (152,4-mm) muffin fan, the leakage around the perimeter can limit the attenuation to approximately 20 dB. The same is true of perforated or expanded metal materials.

The best methods for sealing the perimeters of shielded apertures, in order of effectiveness, are welding, brazing, soldering, and riveting. Unfortunately, these methods preclude easy access for maintenance and repairs. Installing closely spaced screws or clamps permits, but does not facilitate, field removal. As a result, enclosure manufacturers generally use RF gasket materials to maintain contact across mating surfaces to eliminate the need for fasteners or to at least permit spacing these fasteners farther apart.

Three Seam Configurations

Isolated

The isolated seam can best be described as a butt joint with no overlap. An example is the top and bottom seams between blank panels mounted in a relay rack. Because there is no overlap, conventional gasket materials are very difficult to use, and the preferred sealing method is conductive foil tape placed over the seam. This configuration frequently is used in lightweight spacecraft and satellite applications.

Compression

The compression seam tends to be the most frequently used, especially for existing enclosures that are being converted to shielded enclosures. This configuration also is intended to be a static joint. In this application, panels overlap the perimeter of the apertures and can be sealed with any type of gasket material.

Since the gasket-material compression forces are normal to the panel, periodic threaded fasteners or clamps must be used around the perimeter to maintain the RF seal. As enclosure configurations become smaller and more complex, it is difficult to fabricate conventional gaskets that can be used effectively.

Shear

The shear seam is the only dynamic configuration so it must be treated differently than the other two. This type of joint is constructed in several configurations, such as pan-edge, knife-edge, longitudinal, or modified knife-edge. These designs align the mechanical forces parallel to the panel surface and can eliminate the need for fasteners. Metal finger-gasket materials normally are used for this application. Besides having the highest SE, this type of joint is self-cleaning.

Additional Considerations

PCB Shields

If the enclosure has many complex apertures or the device is small, often it is more effective to provide the shielding at the PCB level. As shielded enclosures are made smaller, the problems associated with mechanical tolerances, aperture leakage, and penetrations are reduced.

PCB shields can be made from metal or laminates. PCB shields also are used for intra-system interference problems, such as crosstalk between digital and analog circuits, power supply emissions, and emissions from high-speed clocks and circuits.

Laminates

A laminate is a combination of conductive and insulating materials in thin layers. These materials can be die-cut, folded, or manufactured and formed in many different configurations. Not only can laminates be used as PCB shields, they also can be used in place of plated plastics, conductive coatings, and other similar shields. Laminates also may be used as I/O panel shields, electrostatic shields, and ground planes.

Materials Characteristics Chart

The Materials Characteristics Chart that accompanies this article is an effective tool in determining the appropriate shielding material to use for your applications. It defines criteria for electrical, mechanical, and environmental concerns. The chart provides some typical applications and gives the features and benefits of various shielding materials. For a more complete list of materials characteristics, visit EE’s web site at www.nelsonpub.com/ee/.

About the Authors

Ron Brewer is vice president of EMC Technical Services at Instrument Specialties. The NARTE-certified EMC engineer has more than 25 years experience in the EMC/ESD/Tempest engineering field. Mr. Brewer serves on three technical committees, holds several patents in the EMC field, and has earned B.S.E. and CEd degrees from the University of Michigan.

Gary Fenical has been employed by Instrument Specialties for more than 12 years and currently is a senior EMC engineer at the company. He is the only American invited to attend meetings of the EMCIT/EMCEL committee on Europe’s EMC Directive. Mr. Fenical is a NARTE-certified EMC engineer and a member of the IEEE-EMC Society and SMRI.

Instrument Specialties, P.O. Box A, Broad St., Delaware Water Gap, PA 18327, (717) 424-8510.

Copyright 1998 Nelson Publishing Inc.

July 1998

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