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EMI
Shielding Principles [top]
The importance
of wave impedance is shown by an electromagnetic wave encountering
an obstacle such as a metal shield (see Figure 3). If the
impedance of the wave differs greatly from the natural impedance
of the shield, much of the energy is reflected and the rest
is transmitted across the surface boundary, where absorption
in the shield further attenuates it. Because most metals have
an intrinsic impedance of only milliohms, less low impedance
H-field energy is reflected and more is absorbed. This is
because the metal is more closely matched to the impedance
of the field. This is also why it is difficult to shield against
magnetic fields. On the other hand, the wave impedance of
electric fields is high, so most of the energy is reflected
for this case. At higher frequencies, typically over 10 MHz,
EMI shielding is governed mostly by absorption
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Figure
3
Attenuation of EMI by a Shield |
Shielding
effectiveness of metallic enclosures is not infinite, because
the conductivity of all metals is finite. They can, however,
approach very large values. Because metallic shields have less
than infinite conductivity, part of the field is transmitted
across the boundary and supports a current in the metal, as
illustrated in Figure 4. The amount of current flow at any depth
in the shield, and the rate of decay is governed by the conductivity
of the metal, its permeability, and the frequency and amplitude
of the field source. The residual current appearing on the opposite
face is the one responsible for generating the field which exists
on the other side. |
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current density in a metal shield is not affected by the shield’s
thickness. A secondary reflection occurs at the far side of
the shield for all thicknesses. The only difference with thin
shields is that a large part of the re-reflected wave may appear
on the front surface. This wave can add to or subtract from
the primary reflected wave depending upon the phase relationship
between them. For this reason, a correction factor appears in
shielding equations to account for reflections from the far
surface of a thin shield. |
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Figure
4
Variation of Current Density
with Thickness for Electrically Thin Wall |
E
= Electric Field Strength
J = Current Density
i = initial
t = transmitted |
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EMI
Gasketing [top]
A gap
or seam in a shield will allow electromagnetic fields to radiate
through the shield, unless the current continuity can be preserved
across the gaps. The function of an EMI gasket is to preserve
continuity or current flow in the shield. If a gasket is made
of material identical to the walls of the shielded enclosure,
the current density in the gasket will be the same. (This
assumes it could perfectly fill the slot, which is not possible
due to mechanical considerations.)
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The
flow of current through a shield including a gasket interface
is illustrated in Figure 5. Electromagnetic leakage through
the seam can occur in two ways. First, the energy can leak through
the material directly. The gasket material shown in Figure 5
is assumed to have lower conductivity than the material in the
shield. The rate of current decay, therefore, is less in the
gasket, resulting in more current flow on the far side of the
shield. This increased flow causes a larger leakage field to
appear on the far side. Second, leakage can occur at the interface
between the gasket and the shield. If an air gap exists at the
interface, the flow of current will be diverted to the points
or areas in contact. A change in the direction of the flow of
current alters the current distribution in the shield as well
as in the gasket, which lowers shielding performance. A high
resistance joint does not behave much differently than an open
seam. It simply alters the distribution of current somewhat.
A current distribution for a typical seam is shown in Figure
5. Lines of constant current spaced at larger intervals indicate
less flow of current. It is important in gasket design to make
the electrical properties of the gasket as similar to the shield
as possible, maintain low impedance interface surfaces, and
avoid air gaps which also increase joint resistance. |
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Shielding
and EMI Gasket Equations [top]
As described above, electromagnetic waves incident upon a
discontinuity will be partially reflected, and partly absorbed
by the material. The effectiveness of the shield is the sum
total of these two effects, plus a correction factor to account
for reflections from the back surfaces of the shield.
Reductions
in field strength are determined by the frequency, the shielding
material’s conductivity, thickness and permeability, and by
the distance between the radiating source and the EMI shield.
How well a shield reduces the energy of (attenuates) a radiated
electromagnetic field is referred to as its shielding effectiveness,
or SE. The standard unit of SE measurement is the decibel,
or dB. The decibel value is the ratio of two measurements
of electromagnetic field strength taken before and after shielding
is in place. Every 20 dB increase in SE represents a tenfold
reduction in EMI leakage through a shield. A 60 dB shield
reduces field strength by a factor of 1,000 times (e.g., from
5 volts per meter to 5 millivolts/meter).
The overall
expression for shielding effectiveness is written as:
SE = R + A + B (1)
where
SE is the shielding effectiveness
R is the reflection factor
A is the absorption factor, and
B is the correction factor to account for reflections from
the far boundary
All values are expressed in dB (decibels) Reflection loss
(R) includes reflections at both surfaces of the shield, and
is dependent upon the relative mismatch between the incoming
wave impedance and the frequency of the impinging wave, as
well as upon the electrical parameters of the shielding material
itself. The equations for the three principal fields are given
by the following expressions:
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where:
RE,
RH, and RP are the reflection terms for the
electric, magnetic, and plane wave fields expressed in dB
G is the relative conductivity
referred to copper
f is the frequency in
Hz
m is the relative
permeability referred to free space
r1
is the distance from the source to the shield in inches
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The
absorption term (A) is the same for all three waves and is
given by the expression:
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where:
A is the absorption or penetration loss expressed in dB, and
t is the thickness of the shield in mils.
The correction
factor (B) can be mathematically positive or negative
(in practice it is always negative), and becomes insignificant
when A>6 dB. It is usually only important when metals
are thin, and at low frequencies (i.e., below approximately
20 kHz).
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A
plot of reflection and absorption loss for copper and iron is
shown in Figure 6. This illustration gives a good physical representation
of the behavior of the component parts of an electromagnetic
wave. It also illustrates why it is so much more difficult to
shield magnetic fields than electric fields or plane waves.
Note: in Figure 6, copper offers more shielding effectiveness
than iron in all cases except for absorption loss. This is due
to the high permeability of iron. These shielding numbers are
theoretical, hence they are very high (and unrealistic) practical
values.
When only electric field or plane wave protection is required,
reflection is the important factor to consider in the design.
If magnetic shielding is required, particularly at frequencies
below 10 kHz, it is customary to neglect all terms in equation
(1) except the absorption
term A.
Nomographs are available from Chomerics to aid designers in
determining absorption and magnetic field reflection losses
directly. These nomographs are based on the previously shown
equations used in determining shielding effectiveness. Commercial
equipment typically needs shielding of 40-60 dB from 30 MHz
to 1 GHz, and often 10 GHz. (See also Chomerics’ EMI Shielding
for Military/Aerospace Electronics Engineering Handbook.) |
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EMI
Gasket Types [top]
Conductive
EMI gaskets must conform intimately to enclosure mating surfaces.
They provide current continuity in shielding systems by reducing
resistance across the seams. EMI gaskets are generally made
of metal, metal combined with elastomer materials, or metallized
fabric over foam cores. All-metal gaskets include knitted
wire mesh made in different metals for cost-performance choices.
By knitting wire mesh around elastomer cores, metal gaskets
can better meet mechanical design needs such as enhanced compressibility.
Another type of metal gasket, typically used in door seams
(where shear forces are present), features linear rows of
beryllium copper spring fingers, or spirals.
Foam-core gaskets covered by conductive fabric, yarn or metal
foil provide a large deflection range with modest closure
force. Another approach for low closure force applications
is a hollow silicone tube with a conductive surface coating.
Conductive
elastomer gaskets, which contain metal-plated particles, provide
excellent shielding characteristics. Such gaskets can be extruded
and cut to length, typically for preventing EMI leakage around
the perimeters or between sections of electronic enclosures.
Co-extruded EMI gaskets combine a conductive elastomer with
a non-conductive silicone environmental seal. Conductive elastomers
can also be die-cut from sheet material, or molded into intricate
shapes. Robotically applied form-in-place gasketing systems
precisely dispense conductive elastomer compounds on electronic
housings. Specially formulated conductive elastomers can be
molded onto thin-wall plastic spacer frames to provide grounding
of circuit boards in small enclosures such as cellular phone
housings. Conductive elastomers are also being molded directly
onto the inner surface of large plastic housing covers in
place of plating or other types of metallizing. These shielded
covers feature integral conductive elastomer walls, which
eliminate the need for EMI gaskets. When form-stable gaskets
are impractical for an application, conductive adhesives and
sealants can often be applied. A variety of formulations are
available for providing either rigid or flexible shielding
solutions. These materials can also be used for bonding EMI
gaskets to flange surfaces.
EMI
Gasket Selection [top]
The most
critical features to consider when evaluating EMI gaskets
are: shielding effectiveness, operating environmental conditions,
fitting the physical properties of the gasket with the packaging
design, electrical stability, and the installed cost. Chomerics’
inPHormTM product selection software provides a convenient,
comprehensive system for engineers to create an application
profile. The inPHorm program recommends one or more products
that will best fit the application needs. On-line technical
data is provided for hundreds of EMI shielding products (see
Inside Front Cover).
Shielding Effectiveness
As described in the preceding pages, determining a system’s
overall shielding needs involves understanding the radiated
emission spectrum of the equipment, and the specifications
the unit must meet. Various EMI gasket types provide different
levels of shielding effectiveness across the frequency range
of 10 kHz to 20 GHz.
Operating
Conditions
One reason so many gasket variations exist is a result of
trying to provide a best fit for the different operating environmental
conditions. Exposure to high and low temperatures, wind and
rain, salt spray from the ocean, solvents used in cleaning
and plating, and other conditions can severely affect the
life of a gasket.
Mechanical
Requirements
The primary goal of a shielding gasket is to seal openings
in an electronic enclosure to prevent transmission of EMI.
Improper design of the seal or enclosure mating flanges can
result in a failure to meet this goal. Several mechanical
design issues must be considered for the proper mating of
an EMI gasket with the flanges of an electronic enclosure.
Among the most important are compression-deflection and compression
set.
- Compression-Deflection
EMI gaskets require some amount of compressive force to
function properly. As a result of this load the material
will decrease in height (deflect). The magnitude of the
decrease is proportional to the applied load up to the elastic
limit or the point at which the material yields or ruptures.
Many commercial-grade gaskets must deflect 30-40% under
low closure forces to properly maintain contact with mating
flanges.
- Compression
Set
If a gasket material is subjected to a compressive force
for an extended time, some deflection remains when the load
is removed. Compression set is an important property in
designs where the gasket will be compressed and released
regularly
while in service, such as in enclosure doors and access
panels.
Electrical
Stability
EMI gaskets provide conductive pathways that electrically
bond system components to a common ground. The gaskets serve
as low impedance conductors to ensure the reliability of an
enclosure’s shielding. For example, an EMI gasket will provide
continuity between the housing shield and other system ground
points. Gasket conductivity and electrical stability are critical
to system performance and shielding integrity.
Installed
Cost
The method of installing an EMI gasket can be a major factor
in determining EMI shielding costs. Gaskets can be installed
using pressure-sensitive adhesives, fasteners, or epoxies,
or by press-fitting into grooves. Some gaskets are molded
in place on the enclosure flange or on a plastic spacer frame.
The installed cost for an EMI gasket includes the cost of
the gasket, together with labor and other manufacturing costs.
A gasket applied without fasteners or adhesives, for instance,
may offer lower installed cost than an EMI gasket purchased
at a lower price.
Where
EMI Shielding is Needed
Requirements for EMI shielding abound in computers, medical
devices, telecommunications, and many other types of electronic
equipment. As new emission and immunity requirements are placed
on these devices, the importance of shielding grows.
Among the typical applications for EMI shielding are the following:
Enclosures
Plastic and other non-conductive materials used for lightweight
housings can be metallized with sprayable conductive paints,
thin-film metal coatings or plating. Laminates of metal foil
and plastic film can be formed and die-cut into shadow shields,
ground planes or Faraday cages. Metal housings for electronic
systems provide inherent levels of EMI shielding, dependent
on factors such as metal type, flange design, and seam and
aperture treatment.
Apertures
Doors, cable ports, vents, windows, access panels and other
openings in an otherwise shielded electronic package are pathways
for radiated EMI. A variety of gaskets and specialized conductive
materials are available for preserving shielding around door
seams and the perimeters of other openings. Shielding vents
and windows are designed to reduce the amount of EMI passing
through such apertures.
The amount of EMI leakage through an opening is a function
of the maximum dimension of the opening at a given frequency.
A long, narrow slit, regardless of width, like the gap around
the edge of a door, will leak much more radiation than a round
hole of the same area. The imperfect joints between panels
or covers and enclosure walls are typical “slots” where EMI
can efficiently escape or enter a shielded enclosure. Conductive
EMI gaskets inserted between panel mating surfaces will provide
low resistance across the seam, preserve current continuity
of the enclosure, and provide the necessary shielding.
Cables
Signal-carrying cables can act as antennas to radiate EMI.
Conversely, false signals can occur when EMI couples into
a cable. A number of shielding products are available for
reducing EMI problems on both internal and external cables.
Grounding
Issues
Shielding against EMI emissions is commonly provided by a
conductive enclosure. The separate parts of the enclosure
must be electrically bonded together and grounded for the
shielding to work. Disruptions in the electrical continuity
between parts adversely affect shielding performance. Proper
grounding of PCBs and shielding enclosure components is also
a method for reducing board-generated EMI. However, improper
or ineffective grounding may actually increase EMI emission
levels, with the ground itself become a major radiating source.
Many Chomerics shielding materials can be used for providing
conductive grounding paths.
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Gasket
Mounting Choices [top]
Our various
EMI gasket mounting techniques offer designers cost-effective
choices in both materials and assembly. These options offer
aesthetic choices and accommodate packaging requirements such
as tight spaces, weight limits, housing materials and assembly
costs. Most Chomerics gaskets attach using easily repairable
systems. Our Applications Engineering Department or your local
Chomerics representative can provide full details on EMI gasket
mounting. The most common systems are shown here with the available
shielding products. |
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Pressure-Sensitive
Adhesive
Quick, efficient attachment strip
- Conductive
Elastomers
- SOFT-SHIELD
- POLASHEET
- SPRING-LINE
- POLASTRIP
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Friction
Fit in a Groove
Prevents over-deflection of gasket Retaining groove required
- Conductive
Elastomers
- MESH
STRIP
- POLASTRIP
- SOFT-SHIELD
- SPRINGMESH
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Adhesive
Compounds
Conductive or non-conductive spot bonding
- Conductive
Elastomers
- MESH
STRIP
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Robotically
Dispensed
Form-in-Place
Conductive Elastomer
Chomerics’ Cho-Form™
automated technology applies
high quality conductive elastomer gaskets to metal or plastic
housings. Manufacturing options include Chomerics facilities,
authorized Application Partners, and turnkey systems. |
Friction
Fit on Tangs Accommodates thin walls, intricate
shapes
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Spacer
Gaskets
Fully customized, integral conductive elastomer and plastic
spacer provide economical EMI shielding and grounding in small
enclosures. Locator pins ensure accurate and easy installation,
manually or robotically.
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Clip-On
Gaskets
Require knife edge mounting flange.
- Conductive
Elastomers
- METALKLIP
- CLIP-SHIELD
- SOFT-SHIELD
5000
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Rivets/Screws
Require integral compression stops Require
mounting holes on flange
- Conductive
Elastomers
- SHIELDMESH
- SPRING-LINE
- COMBO
STRIP
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Frames
Extruded aluminum frames and strips add rigidity.
Built-in compression stops for rivets and screws.
- Conductive
Elastomers
- Mesh
Strip
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