U.S. patent application number 10/264057 was filed with the patent office on 2003-03-20 for methods of manufacturing emi shields.
This patent application is currently assigned to Amesbury Group, Inc.. Invention is credited to Kaplo, Joseph J., Rapp, Martin L..
Application Number | 20030051891 10/264057 |
Document ID | / |
Family ID | 25505835 |
Filed Date | 2003-03-20 |
United States Patent
Application |
20030051891 |
Kind Code |
A1 |
Rapp, Martin L. ; et
al. |
March 20, 2003 |
Methods of manufacturing EMI shields
Abstract
An electromagnetic interference shield for use around access
panels and doors in electronic equipment enclosures includes a base
and profile manufactured from an electrically nonconductive solid
material such as a thermoplastic resin polymer. An electrically
conductive layer of a metallized fabric is bonded to the profile to
provide effective shielding and grounding functions. The underlying
polymer provides elastic compliancy and resiliency to the shield.
The shield may be divided into flexible fingers which can be
rectangular or nonlinear shaped. The nonlinear shaped flexible
fingers form nonlinear shaped slits between the fingers which
provides improved EMI shielding by reducing the amount of EMI
transmission that can pass through the shield. The electromagnetic
interference shield may also be made from an electrically
conductive material.
Inventors: |
Rapp, Martin L.; (Grover,
MO) ; Kaplo, Joseph J.; (Hampton, NH) |
Correspondence
Address: |
TESTA, HURWITZ & THIBEAULT, LLP
HIGH STREET TOWER
125 HIGH STREET
BOSTON
MA
02110
US
|
Assignee: |
Amesbury Group, Inc.
Amesbury
MA
|
Family ID: |
25505835 |
Appl. No.: |
10/264057 |
Filed: |
October 3, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10264057 |
Oct 3, 2002 |
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09530406 |
Aug 3, 2000 |
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09530406 |
Aug 3, 2000 |
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PCT/US98/23058 |
Oct 30, 1998 |
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PCT/US98/23058 |
Oct 30, 1998 |
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08962417 |
Oct 31, 1997 |
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6294729 |
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Current U.S.
Class: |
174/360 ;
174/355 |
Current CPC
Class: |
Y10T 29/49067 20150115;
Y10S 277/92 20130101; H05K 9/0015 20130101; H05K 9/0016
20130101 |
Class at
Publication: |
174/35.0GC |
International
Class: |
H05K 009/00 |
Claims
What is claimed is:
1. A method of manufacturing a shield for shielding electromagnetic
interference from passing through a seam between a first
electrically conductive body and a second electrically conductive
body, the method comprising the steps of: forming a base and a
profile of a solid material in a predetermined configuration; and
dividing the profile into a plurality of generally independently
flexible fingers forming nonlinear slits therebetween.
2. A method according to claim 1 wherein the solid material is
electrically conductive.
3. A method according to claim 2 wherein the solid material
comprises a copper beryllium alloy.
4. A method according to claim 1 wherein: the solid material is
electrically nonconductive; and the method further comprises the
step of disposing an electrically conductive layer on the
profile.
5. A method according to claim 4 wherein the forming step is
accomplished by extrusion.
6. A method according to claim 5 wherein the forming step and the
disposing step are accomplished by in-line crosshead extrusion.
7. A method according to claim 4 wherein the electrically
conductive layer comprises a metallized fabric bonded to the
profile.
8. A method according to claim 1 wherein the formed nonlinear slits
are selected from the group consisting of arcuate slits,
interdigitated slits, chevron-shaped slits, and combinations
thereof.
9. A method of manufacturing a shield for shielding electromagnetic
interference from passing through a seam between a first
electrically conductive body and a second electrically conductive
body, the method comprising the steps of: disposing an electrically
conductive layer on an electrically nonconductive solid sheet
material; forming the sheet into a base and a profile of a
predetermined configuration; and dividing the profile into a
plurality of generally independently flexible fingers forming
nonlinear slits therebetween.
10. A method according to claim 9 wherein the forming step is
accomplished by thermoforming.
11. A method according to claim 9 wherein the electrically
conductive layer comprises a metallized fabric.
12. A method according to claim 9 wherein the formed nonlinear
slits are selected from the group consisting of arcuate slits,
interdigitated slits, chevron-shaped slits, and combinations
thereof.
13. A shield for shielding electromagnetic interference from
passing through a seam between a first electrically conductive body
and a second electrically conductive body, the shield comprising: a
base for securing the shield to the first body; and a plurality of
generally independently flexible fingers attached to the base for
contacting the second body, wherein the plurality of fingers form
nonlinear slits therebetween.
14. A shield according to claim 13 wherein the fingers are
electrically conductive.
15. A shield according to claim 14 wherein the fingers comprise a
copper beryllium alloy.
16. A shield according to claim 13 wherein the base and fingers are
formed integrally.
17. A shield according to claim 13 wherein: the fingers are
electrically nonconductive; and the shield further comprises an
electrically conductive layer disposed on the fingers.
18. A shield according to claim 17 wherein the electrically
conductive layer comprises a metallized fabric bonded to the
fingers.
19. A shield according to claim 13 wherein the fingers are attached
to the base by a hinge.
20. A shield according to claim 13 further comprising an adhesive
strip attached to the base.
21. A shield according to claim 13 wherein the base includes a
return.
22. A shield according to claim 21 wherein the return includes
barbing.
23. A shield according to claim 13 wherein the formed nonlinear
slits are selected from the group consisting of arcuate slits,
interdigitated slits, chevron-shaped slits, and combinations
thereof.
24. A method of manufacturing a shield for shielding
electromagnetic interference from passing through a seam between a
first electrically conductive body and a second electrically
conductive body comprising the steps of: forming a base and a
profile of an electrically nonconductive solid material in a
predetermined configuration; and disposing an electrically
conductive layer comprising a metallized fabric on the profile.
25. A method according to claim 24 wherein the forming step is
accomplished by extrusion.
26. A method according to claim 24 wherein the forming step and the
disposing step are accomplished by in-line crosshead extrusion.
27. A method according to claim 24 wherein the metallized fabric is
bonded to the profile.
28. A method according to claim 24 further comprising the step of
dividing the profile into a plurality of independently flexible
fingers.
29. A method of manufacturing a shield for shielding
electromagnetic interference from passing through a seam between a
first electrically conductive body and a second electrically
conductive body comprising the steps of: disposing an electrically
conductive layer comprising a metallized fabric on an electrically
nonconductive solid sheet material; and forming the sheet into a
base and a profile of a predetermined configuration.
30. A method according to claim 29 wherein the forming step is
accomplished by thermoforming.
31. A method according to claim 29 wherein the metallized fabric is
bonded to the sheet material.
32. A method according to claim 29 further comprising the step of
dividing the profile into a plurality of independently flexible
fingers.
33. A shield for shielding electromagnetic interference from
passing through a seam between a first electrically conductive body
and a second electrically conductive body, the shield comprising: a
base for securing the shield to the first body; a profile of an
electrically nonconductive solid material attached to the base for
contacting the second body; and an electrically conductive layer
comprising a metallized fabric disposed on the profile.
34. A shield according to claim 33 wherein the base and the profile
are formed integrally.
35. A shield according to claim 33 wherein the metallized fabric is
bonded to the profile.
36. A shield according to claim 33 wherein the profile comprises a
plurality of independently flexible fingers.
37. A shield according to claim 33 wherein the profile is attached
to the base by a hinge.
38. A shield according to claim 33 further comprising an adhesive
strip attached to the base.
39. A shield according to claim 33 wherein the base includes a
return.
40. A shield according to claim 39 wherein the return includes
barbing.
Description
TECHNICAL FIELD
[0001] The present invention relates to electromagnetic
interference ("EMI") shields and, more specifically, to EMI finger
shields forming nonlinear slits between adjacent fingers, whether
the shields are manufactured from metal or from an electrically
nonconductive material and clad with an electrically conductive
layer.
BACKGROUND
[0002] During normal operation, electronic equipment generates
undesirable electromagnetic energy that can interfere with the
operation of proximately located electronic equipment due to EMI
transmission by radiation and conduction. The electromagnetic
energy can by of a wide range of wavelengths and frequencies. To
minimize the problems associated with EMI, sources of undesirable
electromagnetic energy may be shielded and electrically grounded.
Shielding is designed to prevent both ingress and egress of
electromagnetic energy relative to a housing or other enclosure in
which the electronic equipment is disposed. Since such enclosures
often include gaps or seams between adjacent access panels and
around doors, effective shielding is difficult to attain because
the gaps in the enclosure permit transference of EMI therethrough.
Further, in the case of electrically conductive metal enclosures,
these gaps can inhibit the beneficial Faraday Cage Effect by
forming discontinuities in the conductivity of the enclosure which
compromise the efficiency of the ground conduction path through the
enclosure. Moreover, by presenting an electrical conductivity level
at the gaps that is significantly different from that of the
enclosure generally, the gaps can act as slot antennae, resulting
in the enclosure itself becoming a secondary source of EMI.
[0003] Specialized EMI gaskets have been developed for use in gaps
and around doors to provide a degree of EMI shielding while
permitting operation of enclosure doors and access panels. To
shield EMI effectively, the gasket should be capable of absorbing
or reflecting EMI as well as establishing a continuous electrically
conductive path across the gap in which the gasket is disposed.
Conventional metallic gaskets manufactured from copper doped with
beryllium are widely employed for EMI shielding due to their high
level of electrical conductivity. Due to inherent electrical
resistance in the gasket, however, a portion of the electromagnetic
field being shielded induces a current in the gasket, requiring
that the gasket form a part of an electrically conductive path for
passing the induced current flow to ground. Failure to ground the
gasket adequately could result in radiation of an electromagnetic
field from a side of the gasket opposite the primary EMI field.
[0004] In addition to the desirable qualities of high conductivity
and grounding capability, EMI gaskets should be elastically
compliant and resilient to compensate for variable gap widths and
door operation, yet tough to withstand repeated door closure
without failing due to metal fatigue. EMI gaskets should also be
configured to ensure intimate electrical contact with proximate
structure while presenting minimal force resistance per unit length
to door closure, as the total length of an EMI gasket to shield a
large door can readily exceed several meters. It is also desirable
that the gasket be resistant to galvanic corrosion which can occur
when dissimilar metals are in contact with each other for extended
periods of time. Low cost, ease of manufacture, and ease of
installation are also desirable characteristics for achieving broad
use and commercial success.
[0005] Conventional metallic EMI gaskets, often referred to as
copper beryllium finger strips, include a plurality of cantilevered
or bridged fingers forming linear slits therebetween. The fingers
provide spring and wiping actions when compressed. Other types of
EMI gaskets include closed-cell foam sponges having metallic wire
mesh knitted thereover or metallized fabric bonded thereto.
Metallic wire mesh may also be knitted over silicone tubing. Strips
of rolled metallic wire mesh, without foam or tubing inserts, are
also employed.
[0006] One problem with metallic finger strips is that to ensure a
sufficiently low door closure force, the copper finger strips are
made from thin stock, for example on the order of about 0.05 mm
(0.002 inches) to about 0.15 mm (0.006 inches) in thickness.
Accordingly, sizing of the finger strip uninstalled height and the
width of the gap in which it is installed must be controlled to
ensure adequate electrical contact when installed and loaded, yet
prevent plastic deformation and resultant failure of the strip due
to overcompression of the fingers. To enhance toughness, beryllium
is added to the copper to form an alloy; however, the beryllium
adds cost and is a concern since beryllium is carcinogenic. Due to
their thinness, the finger strips are fragile and can fracture if
mishandled or overstressed, resulting in thin sharp edges which are
a safety hazard to installation and maintenance personnel. Finger
strips are also expensive to manufacture, in part due to the costs
associated with procuring and developing tooling for outfitting
presses and rolling machines to form the complex contours required.
Changes to the design of a finger strip to address production or
performance problems require the purchase of new tooling and
typically incur development costs associated with establishing a
reliable, high yield manufacturing process. Notwithstanding the
above limitations, metallic finger strips are commercially accepted
and widely used. Once manufacturing has been established, large
quantities of finger strips can be made at relatively low cost.
[0007] Metallic mesh and mesh covered foam gaskets avoid many of
the installation and safety disadvantages of finger strips;
however, they can be relatively costly to produce due to the
manufacturing controls required to realize acceptable production
yields.
[0008] Another problem with conventional finger strips is that they
are not as effective in EMI shielding as clock speed of an
electronic product is increased. As clock speed is increased, the
wavelength of the EMI waves produced decreases. Accordingly, the
waves can penetrate smaller and smaller apertures in the enclosure
and in the EMI shield. At lower wavelengths, the slits formed in
the finger shields can act as slot antennae, permitting the passage
of EMI therethrough and the resultant shielding effectiveness of
the shields decreases. Conventional finger strips with linear slits
formed between the fingers are increasingly less effective in these
applications.
SUMMARY OF THE INVENTION
[0009] A metallized fabric clad polymer EMI shield overcomes many
of the limitations and disadvantages of conventional EMI shields.
One method of manufacturing a metallized fabric clad polymer shield
for shielding EMI from passing through a seam between first and
second electrically conductive bodies includes forming a base and a
profile of an electrically nonconductive solid material in a
predetermined configuration. The base is designed to secure the
shield to the first body while the profile is designed to contact
the second body. An electrically conductive layer is then disposed
on at least part of the profile so as to be interdisposed between
the profile and the second body upon installation of the shield in
a suitable gap of an electronic enclosure. In one exemplary
embodiment, the profile and base may be an extrusion of a polymer
such as polyvinyl chloride ("PVC"), a thermoplastic resin, and the
conductive layer may be a metallized fabric bonded to the profile
by a heat sensitive glue. The forming and deposition processes may
be separate or may be substantially contiguous. After extrusion and
cooling of the profile and base, the metallized fabric may be
bonded to the profile in a separate operation. Alternatively, by
employing an in-line crosshead extrusion method, the polymer base
and profile may be formed and substantially immediately thereafter,
the metallized fabric applied as a thermally activated glue-backed
tape. Resultant thermal energy in the extrusion activates the glue
on the fabric side of the tape, bonding the metallized fabric to
the profile. As a subsequent step in either manufacturing method,
the profile may be divided into a plurality of independently
flexible cantilevered or bridged fingers to compensate for variable
gap width along the length of the gap.
[0010] Another embodiment for manufacturing a metallized fabric
clad polymer EMI shield according to the invention includes
disposing an electrically conductive layer on an electrically
nonconductive solid sheet material and then forming the sheet into
a base and a profile of a predetermined configuration. The sheet
may be a polymer such as PVC, the conductive layer may be a
metallized fabric bonded to the sheet by a thermally activated
glue, and the profile and base may be formed by a thermal process
such as thermoforming. As a subsequent step in the manufacturing
method, the profile may be divided into a plurality of
independently flexible cantilevered or bridged fingers.
[0011] According to certain embodiments of the invention, a
metallized fabric clad polymer shield for shielding EMI from
passing through a seam between first and second electrically
conductive bodies includes a base for securing the shield to the
first body, a profile of an electrically nonconductive solid
material attached to the base for contacting the second body, and
an electrically conductive layer disposed on the profile.
[0012] The base and the profile may be formed integrally of the
same material by extrusion or of different materials by
co-extrusion. Alternatively, the base and the profile may be
similar or distinct materials joined together by bonding.
Additionally, a hinge of a material exhibiting different flexural
characteristics may be disposed between the base and the profile,
either by co-extrusion or bonding. As used herein, the term bonding
includes chemical processes such as those using glues or solvents,
as well as mechanical processes such as friction welding and
interlocking mechanical cross-sections.
[0013] To facilitate installing the EMI shield in an enclosure, an
adhesive strip may be attached to the base. Alternatively, the base
may include apertures for mechanical fasteners or a return for
insertion in a slot or for capturing a flange of the first body.
The return may also include barbing to stabilize the shield once
installed.
[0014] Further EMI shield effectiveness may be achieved using
nonlinear shaped EMI shielding fingers forming nonlinear slits
therebetween. Various embodiments of nonlinear fingers and slits
are contemplated, including those which are arcuate, sinusoidal,
interdigitated, chevron-shaped, and combinations thereof. By using
nonlinear fingers, the slits formed between the fingers do not form
linear apertures so that EMI transmission in a given direction
cannot line up with an entire slit length, but only a limited
portion of the slit length. For arcuate and sinusoidal slits, as
the radius of curvature is decreased, in the limit, the wavelength
of EMI waves capable of passing through the slits approaches slit
width, which can be substantially zero.
[0015] In one embodiment, the EMI shielding fingers with nonlinear
slits may be manufactured out of an electrically conductive
material such as a copper beryllium alloy or other metallic
material. In another embodiment, the EMI shielding fingers with
nonlinear slits may be manufactured out of a metallized fabric clad
polymer or other coated or clad material composition. In general,
the nonlinear slits may be employed in any EMI finger shield,
regardless of the material and method of manufacture, providing
greatly improved EMI shielding effectiveness for short and very
short wavelength radiation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The invention, in accordance with preferred and exemplary
embodiments, together with further advantages thereof, is more
particularly described in the following detailed description taken
in conjunction with the accompanying drawings in which:
[0017] FIG. 1A is a schematic plan view of a portion of an EMI
shield in accordance with one embodiment of the present
invention;
[0018] FIG. 1B is a schematic sectional view of the EMI shield
depicted in FIG. 1A taken along line 1B-1B;
[0019] FIG. 1C is an enlarged view of FIG. 1B rotated
counterclockwise ninety degrees and including the metallized
fabric.
[0020] FIG. 2 is a schematic sectional view of an EMI shield
disposed in a gap in accordance with a another embodiment of the
present invention;
[0021] FIG. 3 is a schematic sectional view of an EMI shield in an
uninstalled state in accordance with yet another embodiment of the
present invention;
[0022] FIG. 4 is a schematic block diagram representation of a
method of manufacture of an EMI shield by extrusion in accordance
with one embodiment of the present invention;
[0023] FIG. 5A is a schematic plan view of an EMI finger shield
with non-linear arcuate fingers forming arcuate slits in accordance
with one embodiment of the present invention;
[0024] FIG. 5B is a schematic plan view of an EMI finger shield
with non-linear sinusoidal fingers forming sinusoidal slits in
accordance with another embodiment of the present invention;
[0025] FIG. 6A is a schematic plan view of an EMI finger shield
with non-linear chevron fingers forming chevron slits in accordance
with yet another embodiment of the present invention;
[0026] FIG. 6B is a schematic plan view of an EMI finger shield
with non-linear zigzag fingers forming zigzag slits in accordance
with still another embodiment of the present invention;
[0027] FIG. 7A is a schematic plan view of an EMI finger shield
with non-linear interdigitated fingers forming corresponding slits
in accordance with yet still another embodiment of the present
invention; and
[0028] FIG. 7B is a schematic plan view of an EMI finger shield
with non-linear interdigitated fingers forming corresponding slits
in accordance with still a further embodiment of the present
invention.
DETAILED DESCRIPTION
[0029] FIGS. 1A and 1B are schematic plan and sectional views,
respectively, of a metallized fabric clad polymer EMI shield 10 in
accordance with one embodiment of the present invention. The shield
10 is formed from an electrically nonconductive solid material to
produce a substantially planar base 12 and a generally arcuate
profile 14 having an offset tip 16. The base 12 is configured with
a series of apertures 18 so that the base 12 can be secured to a
first body (not depicted) by a plurality of nuts and bolts,
self-tapping machine screws, rivets, or other mechanical
fasteners.
[0030] Referring now to FIG. 1C, an enlarged sectional view of the
EMI shield 10 is depicted, including an electrically conductive
layer of metallized fabric 20 disposed on the profile 14. The
relative thicknesses of the fabric 20 and the profile 14 in this
and other embodiments are for illustrative purposes only. In some
embodiments, as will be discussed in greater detail hereinbelow,
the thickness of the fabric 20 is typically substantially less than
that of the profile 14. The metallized fabric 20 is bonded to the
profile 14 with the fabric portion proximate the profile 14 and the
metallic portion remote therefrom. The fabric 20 extends from the
base 12 proximate the aperture 18 along the profile 14 and wraps
around the offset tip 16. Accordingly, when the EMI shield 10 is
compressed between first and second bodies at contact zones shown
generally as arrows B1 and B2, the fabric provides an electrically
conductive path therebetween. Since the EMI shield 10 is secured to
the first body solely at the base 12 in a bridge configuration, the
tip 16 is free to slide across the first body as the profile 14 is
compressed by the second body. By offsetting the tip 16 slightly,
the tip 16 slides readily over imperfections in the surface of the
first body. Accordingly, the profile 14 elastically deforms during
compressive loading, such as when the door is closed, instead of
being jammed and crushed.
[0031] As is apparent from the depiction in FIG. 1C, the plane of
the base 12 is offset from an end of the tip 16 so that when the
EMI shield 10 is secured to the first body, a predetermined elastic
deformation of the profile 14 results. This preloading of the EMI
shield 10 ensures that the tip 16 remains in intimate contact with
the first body, thereby providing a positive electrical ground
path. In other embodiments, the fabric 20 may extend along the base
12 covering the aperture 18 so that a redundant electrical ground
path exists when a fastener such as an aluminum rivet passes
through the fabric and aperture, securing the EMI shield 10 to the
first body. In yet another embodiment, the fabric may extend
further, wrapping around the base 12 so that the fabric 20 is
captured between the base 12 and the first body thereby providing a
more positive electrical ground path. For any of these embodiments,
the fabric 20 may stop short of the tip 16 or need not wrap around
the tip 16, if desired.
[0032] Referring again to FIG. 1A, the profile 14 of the EMI shield
10 may be divided at regular intervals into a plurality of
independently flexible fingers 22, each extending from the common
base 12. By subdividing the profile 14 in this manner, the EMI
shield 10 can provide effective shielding in a gap which varies in
width along its length. Reliefs 24 may be provided at the corners
of the fingers 22 to blunt the fingers 22 and minimize catching of
the tips 16 on surface imperfections in the first body.
[0033] Instead of having a compressive arcuate profile 14, an EMI
shield 100 may have a substantially planar profile 114 attached to
a substantially planar base 112 by a hinge 26 in a cantilevered
configuration so that the EMI shield 100 resembles a recumbent "W"
as depicted in FIG. 2. A metallized fabric 120 envelops the entire
exterior surface of the EMI shield 100 and wraps around respective
tips of both the base 112 and profile 114. The base 112 may be
attached to a first body 28 by a thin layer of adhesive 32 along a
portion thereof to retain the EMI shield 100 in a predetermined
position. The adhesive 32 may be electrically conductive. As a
second body 30 moves relative to the first body 28 varying the
width of a gap G formed therebetween, the hinge 26 elastically
deforms to keep the profile 114 in contact therewith. The V-shaped
configuration of the hinge 26 is designed to accommodate a
relatively large range of motion between the first and second
bodies 28, 30 while maintaining contact for electrical ground and
shielding purposes; however, any of a variety of hinge
configurations may be used. While the fabric 120 is depicted as
being bonded to the entire exterior surface of the EMI shield 100,
for ease of manufacture the fabric 120 need not be bonded to the
hinge 26. A sufficient amount of fabric 120 should be provided in
the area of the hinge 26, however, so as not to restrict the full
range of compression of the EMI shield 100.
[0034] In this EMI shield 100, all of the base 112, the hinge 26
and the profile 114 are manufactured from the same polymer
material; however, different materials with different material
properties may be used as depicted in FIG. 3. The EMI shield 200
includes a substantially planar profile 214 which forms an acute
angle with a substantially planar base 212 in a cantilevered
configuration so that the EMI shield 200 resembles a recumbent "U."
Instead of terminating at a tip, however, the base 212 includes a
return 34 which forms a throat 36 to permit mounting the EMI shield
200 on a planar flange of the first body. The return includes
barbing 38 directed into the throat 36 to stabilize the EMI shield
200 once installed on the flange. The return 34 could alternatively
be configured for insertion into a slot in the first body. Barbing
38 extending in opposing directions from both sides of the return
may be used to stabilize the EMI shield 200 in such a
configuration.
[0035] The profile 214 is attached to the base 212 by a hinge 126,
but instead of being manufactured from the material used to make
the base 212 and the profile 214, the hinge 126 may be manufactured
of a more flexible material resistant to fatigue failure to extend
the life of the EMI shield 200. A metallized fabric 220 envelops
the entire exterior surface of the EMI shield 200, wrapping around
the tip of profile 214 and extending into the throat 36 to provide
a electrical ground path between the first and second bodies. While
the fabric 220 is depicted as being bonded to the entire exterior
surface of the EMI shield 200, the fabric 220 need not be bonded to
the hinge 126. A sufficient amount of fabric 220 should be provided
in the area of the hinge 126, however, so as not to restrict the
full range of compression of the EMI shield 200.
[0036] A cost effective method of manufacture of the various EMI
shields is by continuous extrusion of one or more selected polymers
using a screw extruder 40 having a die 42 with the desired shape of
the cross-section of the shield 10 as depicted schematically in
FIG. 4. In one embodiment, the shield 10 may be manufactured from
high temperature rated PVC for flame retardancy, although any
electrically nonconductive material with a suitable modulus of
elasticity may be employed. Flame retardancy is desirable, however,
since such EMI shields can be approved for installation in safety
certified electronic equipment enclosures. If a more flexible hinge
is desired, a compatible material such as polyester can be
coextruded with the profile and base to form the hinge. A suitable
polyester hinge material is Hytrel.TM., available from DuPont
located in Wilmington, Del. Techniques of coextrusion are well
known to those skilled in the art of polymer processing.
[0037] Once the polymer portion of the EMI shield 10 has been
formed to the desired configuration, an electrically conductive
layer is disposed on the profile 14 and any other portion of the
EMI shield 10 desired. One method employs a metallized fabric 20 in
tape form of suitable width backed with a thermally activated glue.
The glue may cover substantially the entire backing or solely
portions thereof such as along the edges. In a process known to
those skilled in the art as in-line crosshead extrusion, as the
polymer is being extruded through the die 42, the hot extrusion
passes through a second die 44 in which the metallized fabric tape
20 is mated thereto. The thermal energy in the extrusion and the
second die 44 activates the glue, bonding the metallized fabric 20
to the profile 14. Alternatively, the metallized fabric 20 can be
disposed on the extrusion in a separate operation, such as by
passing the formed polymer and metallized fabric tape 20 through a
heated die after the polymer has been cut to a desired length.
[0038] As used herein, the term metallized fabrics include articles
having one or more metal coatings disposed on woven, nonwoven, or
open mesh carrier backings and equivalents thereof. See, for
example, U.S. Pat. No. 4,900,618 issued to O'Connor et al., U.S.
Pat. No. 4,901,072 issued to Morgan et al.; U.S. Pat. No. 5,075,037
issued to Morgan et al., And U.S. Pat. No. 5,393,928 issued to
Cribb et al., the disclosures of which are herein incorporated by
reference. Metallized fabrics are commercially available in a
variety of metal and fabric carrier backing combinations. For
example, pure copper on a nylon carrier, nickel-copper alloy on a
nylon carrier, and pure nickel on a polyester mesh carrier are
available under the registered trademark Flectron.RTM. from
Advanced Performance Materials located in St. Louis, Mo. An
aluminum foil on a polyester mesh carrier is available from Neptco,
located in Pawtucket, R.I. Other suitable metals include silver and
tin. The choice of metal is guided, in part, by installation
conditions of the EMI shield. For example, a particular metal might
be chosen due to the composition of abutting body metal in the
enclosure to avoid galvanic corrosion of the EMI shield which could
increase electrical resistance and deteriorate electrical grounding
performance. Metallized tapes are desirable both for ease of
application as well as durability. Deposition of a metal layer
directly on the polymer profile would wear away relatively quickly
due to friction when compared to the metallized fabrics.
[0039] Once the metallized fabric has been bonded to the EMI
shield, other features can be formed. For example, the EMI shield
may pass through rotary drilling, stamping, or piercing tools to
divide the profile into independently flexible fingers or create
apertures in the base for mounting the shield. The EMI shield can
be cut to the desired length before or after these finishing
operations.
[0040] Instead of forming the polymer and then applying the
metallized fabric, according to some embodiments of the present
invention, the metallized fabric can be first bonded to either flat
or preformed polymer sheet stock. Then, the sheet stock can be
thermoformed to a final predetermined configuration by as known by
those skilled in the art. Thermoforming can be accomplished on
discrete pieces of sheet stock in a heated die or continuously by
passing the sheet stock through heated rollers to produce the
desired configuration. Additional manufacturing steps such as
generating the mounting apertures or dividing the profile into
fingers can be accomplished prior to, during, or after
thermoforming, as desired.
[0041] By relying on the metallized fabric to provide an
electrically conductive path and the underlying polymer to provide
elastic compliancy and resiliency, lower electrical resistance is
achievable since beryllium is not required to be added to copper
for toughness. As a result, the electrical conductivity and the
resultant EMI shielding capability of the EMI shield according to
this invention is improved over conventional copper beryllium
finger strips. Further, the polymer portion of the shield is less
brittle than all metal copper beryllium finger strips, resulting in
reduced handling and installation damage to the EMI shield,
improved safety, and extended service life.
[0042] In an exemplary embodiment, the thickness of an EMI shield
profile according to the invention may be between about 0.13 mm
(0.005 inches) or less and about 1.5 mm (0.060 inches) or more. In
a preferred embodiment, the thickness of the EMI shield profile may
be between about 0.25 mm (0.010 inches) or less and about 1.0 mm
(0.04 inches) or more. The metallized fabric may have a thickness
between about 0.1 mm (0.004 inches) or less and about 0.5 mm (0.02
inches) or more. These ranges are considered exemplary in nature
and specific dimensions for a particular application would depend
on the mechanical properties of the polymer material selected for
the profile and hinge, the overall configuration of the EMI shield,
and the electrical properties of the metallized fabric.
Accordingly, values outside these ranges are considered to be
within the scope of the invention.
[0043] An improved method of EMI shielding can be achieved by
modifying the configuration of the EMI shield fingers. Instead of
dividing the profile 14 of the EMI shield 10 into rectangularly
shaped independent fingers 22, according to some embodiments of the
present invention, the profile 14 22 may be divided into fingers of
other nonlinear shapes. As depicted in FIG. 5A, an EMI shield 310
having a profile 314 may be divided into arcuate-shaped,
independently flexible fingers 322, which form nonlinear arcuate
slits 350 therebetween. The width of the slits depicted in the
figures are exaggerated to facilitate depiction. In practice, slit
width can be substantially zero, with the profile being cut only,
without the removal of any material. By dividing the profile 314
into non-rectangular fingers, the EMI shield 310 can provide
improved EMI shielding, because an EMI transmission in a given
direction cannot be aligned with the entire slit length from the
base 312 to the tip 316. As with the EMI shield 10 with the
independently flexible fingers 22 of rectangular shape depicted in
FIG. 1A, the EMI shield 310 with arcuate shaped independently
flexible fingers 322 shown in FIG. 5A may also include the other
features, such as apertures 318 in a base 312 thereof and an offset
tip 316.
[0044] Instead of having arcuate-shaped independently flexible
fingers 322, an EMI shield 410 may be a version thereof, and have a
profile 414 divided into sinusoidal-shaped independently flexible
fingers 422, as depicted in FIG. 5B. The EMI shield 410 with
sinusoidal-shaped independently flexible fingers 422 also may
include a base 412 and apertures 418 formed therein. The radius or
radii of curvature of the arcuate and sinusoidal fingers 322, 422
may be selected, as desired, to minimize the passage of EMI waves
therethrough, as discussed hereinabove.
[0045] An EMI shield 510 with a profile 514 may be divided into
chevron-shaped independently flexible fingers 522 as depicted in
FIG. 6A. A base 512 may form apertures 518. Alternatively, an EMI
shield 610 may be a version thereof, with a profile 614 being
divided into zigzag shaped independently flexible fingers 622 as
depicted in FIG. 6B, with an associated base 612 and apertures
618.
[0046] An EMI shield 710 with a profile 714 may also be divided
into more complex shapes, such as independently flexible
interdigitated fingers 722 as depicted in FIG. 7A, with a base 712
and apertures 718.
[0047] Lastly, as depicted in FIG. 7B, an EMI shield 810 with a
profile 814 may combine the features of other fingers, such as the
zigzag fingers 622 of FIG. 6B and the generally rectangularly
interdigitated fingers 722 of FIG. 7A to form independently
flexible interdigitated fingers 822 with tapered portions.
[0048] The EMI finger shields having fingers of non-rectangular
shape may be manufactured according to any method and with any
material, including electrically conductive materials, such as
copper beryllium alloy, as well as metallized fabric clad
polymers.
[0049] While there have been described herein what are to be
considered exemplary and preferred embodiments of the present
invention, other modifications of the invention will become
apparent to those skilled in the art from the teachings herein. For
example, instead of the fingers being arcuate or planar, more
complex contours such as partially twisted fingers may be created
by thermoforming when the profile is divided into fingers or
thereafter. Additionally, the invention is not limited to linear
EMI shields, but rather includes shaped EMI shields such as those
in the form of annuli and other closed or open curves required for
specific applications. Still further, the non-linear slits formed
between adjacent fingers may be any combination of the shapes
disclosed herein, as well as other geometries and variants thereof.
For example, the slits may be chevron-shaped proximate the base and
arcuate proximate the tip. Moreover, the non-linear slits may be
formed in any EMI finger shield, irrespective of the method of
attachment to the enclosure to be shielded. For example, the shield
base may be bonded to the enclosure or may include a return with
barbing to attach to an enclosure flange. In general, all
combinations and permutations of the EMI shields disclosed herein
are considered to be within the scope of the invention. The
particular methods of manufacture of and geometries disclosed
herein are exemplary in nature and are not to be considered
limiting.
[0050] It is therefore desired to be secured in the appended claims
all such modifications as fall within the spirit and scope of the
invention. Accordingly, what is desired to be secured by Letters
Patent is the invention as defined and differentiated in the
following claims.
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