U.S. patent application number 10/442602 was filed with the patent office on 2003-10-30 for perforated emi gasket.
This patent application is currently assigned to Dell Products L.P.. Invention is credited to Hailey, Jeffrey C., Jensen, Ralph W., Mills, Richard S..
Application Number | 20030201111 10/442602 |
Document ID | / |
Family ID | 25465290 |
Filed Date | 2003-10-30 |
United States Patent
Application |
20030201111 |
Kind Code |
A1 |
Jensen, Ralph W. ; et
al. |
October 30, 2003 |
Perforated EMI gasket
Abstract
A computer chassis includes a first metal portion and a second
metal portion. A mating edge connection is provided between the
first and second portions. A gasket is mounted in the edge
connection. The gasket includes a compressible strip of
electromagnetic interference (EMI) limiting material. A pattern of
holes are formed in the strip to improve compressibility arid thus
enhance EMI shielding.
Inventors: |
Jensen, Ralph W.; (Austin,
TX) ; Mills, Richard S.; (Austin, TX) ;
Hailey, Jeffrey C.; (Austin, TX) |
Correspondence
Address: |
HAYNES AND BOONE, LLP
901 MAIN STREET, SUITE 3100
DALLAS
TX
75202
US
|
Assignee: |
Dell Products L.P.
Round Rock
TX
|
Family ID: |
25465290 |
Appl. No.: |
10/442602 |
Filed: |
May 21, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10442602 |
May 21, 2003 |
|
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|
09934279 |
Aug 21, 2001 |
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Current U.S.
Class: |
174/394 ;
425/149 |
Current CPC
Class: |
G06F 1/182 20130101;
H05K 9/0015 20130101 |
Class at
Publication: |
174/35.0GC |
International
Class: |
H02G 003/00 |
Claims
What is claimed is:
1. A gasket comprising: a compressible strip of electromagnetic
interference (EMI) limiting material; and a pattern of apertures
formed in the strip.
2. The gasket as defined in claim 1 wherein the apertures are
equidistantly spaced apart.
3. The gasket is defined in claim 1 wherein the apertures are
spaced apart by variable distances.
4. The gasket as defined in claim 1 wherein the apertures are
variably sized.
5. The gasket as defined in claim 1 wherein the apertures have a
circular cross-section.
6. The gasket as defined in claim 1 wherein the apertures have a
rectangular cross-section.
7. The gasket as defined in claim 1 wherein the apertures have a
hexagonal cross-section.
8. A chassis comprising: a metal chassis first portion; a metal
chassis second portion; a mating edge connection between the first
portion and the second portion; a gasket mounted in the edge
connection, the gasket including a compressible strip of
electromagnetic interference (EMI) limiting material; and a pattern
of apertures formed in the strip.
9. The gasket as defined in claim 8 wherein the apertures are
equidistantly spaced apart.
10. The gasket as defined in claim 8 wherein the apertures are
variably spaced apart.
11. The gasket as defined in claim 8 wherein the apertures are
variably sized.
12. The gasket as defined in claim 8 wherein the apertures have a
circular cross-section.
13. The gasket as defined in claim 8 wherein the apertures have a
rectangular cross-section.
14. The gasket as defined in claim 8 wherein the apertures have a
hexagonal cross-section.
15. A computer system comprising: a chassis having an internal
computer component cavity defined therein; a microprocessor mounted
in the chassis; a storage coupled to the microprocessor; a video
controller coupled to the microprocessor; a memory coupled to
provide storage to facilitate execution of computer programs by the
microprocessor; a first portion of the chassis formed of a metal
portion; a second portion of the chassis formed of a metal portion;
a plurality of computer components mounted in the first portion of
the metal chassis; a mating edge connection between the first and
second portions of the metal chassis; and a gasket mounted in the
edge connection, the gasket including a compressible strip of
electromagnetic interference (EMI) limiting material; and a pattern
of apertures formed in the strip.
16. The gasket as defined in claim 15 wherein the apertures are
equidistantly spaced apart.
17. The gasket is defined in claim 15 wherein the apertures are
variably spaced apart.
18. The gasket as defined in claim 15 wherein the apertures are
variably sized.
19. The gasket as defined in claim 15 wherein the apertures have a
circular cross-section.
20. The gasket as defined in claim 15 wherein the apertures have a
rectangular cross-section.
21. The gasket as defined in claim 15 wherein the apertures have a
hexagonal cross-section.
Description
BACKGROUND
[0001] The disclosures herein relates generally to computer systems
and more particularly to a perforated gasket for providing an
electromagnetic interference seal for a computer chassis
enclosure.
[0002] There is a widespread problem of trying to close, or fill,
gaps in chassis enclosures, especially removable-cover seams. The
ability to close these gaps is essential in order to pass the FCC's
electromagnetic interference (EMI) requirement and well as
electrostatic discharge susceptibility.
[0003] Conductive foam gaskets have proven to be the most robust
and cost effective solution to providing an EMI seal. However,
traditional foam gaskets pose a number of problems.
[0004] The bigger/taller the gasket profile, or cross-section, the
greater it's range of compression. However, the problem is further
complicated by cover and chassis geometry. Firstly, a foam gasket
is selected that, theoretically, gives the required range of
compression, given the theoretical tolerances (and theoretical
forces). But if this gasket generates forces, which either deform
the covers so subsequent gaps are created, or the net forces are
too high for ergonomic requirements, then a larger gasket is
selected that generates less force for a given range of
compression. Most often, both tolerances and actual forces
contribute to the problem, invariably due to design changes and
variance in the parts throughout the product design/development
cycles. However the chassis design must be revised to accommodate
the larger volume gasket, if possible. Often the space is simply
not available. In thin rack servers this is the case because the
residual height of the gasket after maximum allowable compression
must be accommodated and that space is not available. When
engineers initially "pad" their designs with excessive gasket
volumes, the computer designs as a whole will be subsequently
degraded from lost volume or other geometric/space conflicts.
Whole-programs maybe abandoned or disabled due to this practice.
Therefore, any solution that incrementally reduces the compressive
forces relative to range of compression for a gasket helps
tremendously.
[0005] Two other solutions are commonly used to solve the above
problems; custom spring fingers and wire mesh gaskets. Custom
spring fingers are far more expensive (if made from Beryllium
Copper or Phosbronze) or not as resilient as foam core gaskets.
Additionally, spring fingers are not as robust in terms of customer
access as they can easily hang up on passing objects, getting
permanently deformed or broken off. Wire mesh gaskets have an
inherent problem with having to be sealed at their ends to prevent
unraveling. This causes the ends to be too stiff, thereby
countering the high compliance given by the middle sections. Also,
there is much more difficulty in adhering them to the covers or
chassis as there are no continuous surfaces to apply a contact
adhesive. This lack of continuous contact surfaces also causes the
wire mesh to be of less value in term of radio frequency (RF)
attenuation or electrostatic discharge (ESD) conductivity.
[0006] Chassis designers face another general problem concerning
gap closure; non-uniform distortion of covers. Parts deflections
under load (aside from coil springs) produce various complex
deflection curves. This deflection curve, all too often, causes
covers to bow away from the chassis to the point where a gap
develops along the seam. Even a miniscule gap of a few thousandths
of an inch ca cause the computer to fail EMI or ESD
requirements.
[0007] An additional problem encountered is that a linear gasket
provides a force/unit length proportional to the compression in the
same unit length. In many cases, the compression is severely uneven
over the length that the gasket is being used. For example, on a
hinged door with a latch on the outside edge, there would be much
more compression (and more force) toward the hinge and toward the
latch than there would be in the center of the door. Using a
standard gasket tends to deform such a door, and potentially does
not provide enough force to electrically seal the door in the
center. What is ideally needed is a gasket that provides a varying
force-compression curve along its length. Again, in the case of a
latched door, it would provide more force in the center, and less
toward the hinge and latch, optimally providing a constant force
per unit length while the door is closed and latched.
[0008] Therefore, what is needed is a gasket that provides EMI
shielding and generates less force than a traditional gasket, and
that has the ability to vary the force provided along the length of
the gasket.
SUMMARY
[0009] One embodiment, accordingly, provides an EMI shielding
gasket which reduces the closure force between the chassis closure
surfaces and provides enhanced EMI shielding. To this end, a gasket
includes a compressible strip of EMI limiting material. A pattern
of apertures is formed in the strip.
[0010] A principal advantage of this embodiment is that a more
consistent linear sealing force is provided along the seam between
the chassis closure surfaces.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 a diagrammatic view illustrating an embodiment of a
computer system.
[0012] FIG. 2 is perspective view illustrating an embodiment of a
chassis in an open position.
[0013] FIG. 3 is a perspective view illustrating the chassis in a
closed position.
[0014] FIG. 4 is another perspective view illustrating the chassis
in the open position.
[0015] FIG. 5 is a further perspective view illustrating the
chassis in the closed position.
[0016] FIG. 6 is another perspective view illustrating the chassis
in the open position.
[0017] FIG. 7 is a perspective view illustrating a sealing gasket
in a tongue and groove engagement of a portion of the chassis.
[0018] FIG. 8 is a view illustrating a gasket including a plurality
of equidistantly spaced holes.
[0019] FIG. 9 is a view illustrating a gasket including a plurality
of variably spaced holes.
[0020] FIG. 10 is a view illustrating a gasket including a
plurality of variably sized holes.
[0021] FIG. 11 is a partial view illustrating a gasket having a
round hole.
[0022] FIG. 12 is a partial view illustrating a gasket having a
rectangular hole.
[0023] FIG. 13 is a partial view illustrating a gasket having a
hexagonal hole.
[0024] FIG. 14 is a perspective view illustrating a chassis
utilizing a sealing gasket as disclosed herein.
[0025] FIG. 15 is a graphical view comparing gasket compression
curves.
[0026] FIG. 16 is a graphical view comparing gasket compression
curves.
DETAILED DESCRIPTION
[0027] In one embodiment, computer system 10, FIG. 1, includes a
microprocessor 12, which is connected to a bus 14. Bus 14 serves as
a connection between microprocessor 12 and other components of
computer system 10. An input device 16 is coupled to microprocessor
12 to provide input to microprocessor 12.
[0028] Examples of input devices include keyboards, touchscreens,
and pointing devices such as mouses, trackballs and trackpads.
Programs and data are stored on a mass storage device 18, which is
coupled to microprocessor 12. Mass storage devices include such
devices as hard disks, optical disks, magneto-optical drives,
floppy drives and the like. Computer system 10 further includes a
display 20, which is coupled to microprocessor 12 by a video
controller 22. A system memory 24 is coupled to microprocessor 12
to provide the microprocessor with fast storage to facilitate
execution of computer programs by microprocessor 12. It should be
understood that other busses and intermediate circuits can be
deployed between the components described above and microprocessor
12 to facilitate interconnection between the components and the
microprocessor.
[0029] A chassis 26, FIG. 2, is provided to support all or most of
the components of system 10, as set forth above. Chassis 26
includes a base portion 28 formed of a metal portion 30 and a
cosmetic cover 32. A top portion 34 of chassis 26 is pivotally
connected to base portion 28 at a hinge connection generally
designated 36. Top portion 34 includes a metal portion 38 and a
cosmetic cover 40. The base portion 28 includes a base surface 42.
The cosmetic cover 40 includes a top surface 46 and an endwall 48.
The base portion 28 forms part of a cavity 50 in chassis 26 for
containing a plurality of first computer components 52, and the top
portion 34 forms another part of the cavity 50 for containing a
plurality of second computer components 54.
[0030] The hinge connection 36 permits the top portion 34 to pivot
to an open position 0 about 90.degree. relative to base portion 28,
and to pivot to a closed position C, FIG. 3, wherein the top
portion 34 and base portion nest together to define the cavity 50.
It is understood that the open position 0 may be more or less than
90.degree. as desired.
[0031] A pair of side panels 72, FIGS. 3 and 4, of top cosmetic
cover 40 are configured to nest with a complimentary configured
pair of side panels 74 of base cosmetic cover 32 when chassis 26 is
in the closed position C. When closed, the top portion 34 is
automatically secured to the base portion 28 by a releasable latch
56, extending from each side panel 72 of top portion 34, which
includes a latch member 56a and a release button 56b which permits
latch member 56a to disengage from base portion 28.
[0032] Pivotal movement of top portion 34, FIG. 2, relative to base
portion 28 is assisted by the hinge connection 36 including a pair
of arcuate guides 58 attached to base portion 28. A groove 60 in
guides 58 receives a pin 62 attached to top portion 34 for sliding
movement in guides 58.
[0033] In FIG. 5, the metal chassis is illustrated including the
metal base portion 30 and the metal top portion 38. The hinge 36 is
also illustrated including one of the arcuate guides 58, including
groove 60, in the metal base portion 30, and one of the pins 62
attached to the metal top portion 38. This enables the top metal
portion 38 is to pivot relative to the base metal portion 30
between the open position 0 and the closed position C, as described
above.
[0034] The metal base portion 30 includes a pair of opposed base
sidewalls 30a, 30b, FIGS. 5 and 6, and the metal top portion 38
includes a pair of opposed top sidewalls 38a, 38b. The sidewalls
30a, 30b, respectively matingly engage the sidewalls 38a, 38b.
Preferably, the base sidewalls 30a, 30b include a tongue 31 and the
top sidewalls 38a, 38b include a groove 33, see also FIG. 7. A
gasket 35 is compressed into groove 33 so that a potentially
harmful adhesive may not be required to maintain the gasket 35 in
place. Thus, when the tongue 31 seats in groove 33, tongue 31 is
sealingly engaged with gasket 35. Gasket 35 is preferably a fabric
over foam EMI gasket sold under the name Foam Tite.RTM. by Advanced
Performance Materials, Inc. (APM) of St. Louis, Mo.
[0035] In FIG. 8, gasket 35 includes a compressible strip of EMI
limiting material such as discussed above. FIGS. 8 and 9
respectively illustrate examples of rectangular and D-shaped
gaskets. A pattern of perforations such as holes 112 are formed
through gasket 35.
[0036] The pitch P of holes 112, i.e. the center-to-center distance
between adjacent holes 112 may be consistent or may vary along a
length L of the gasket 35. FIG. 8 illustrates a consistent pitch P
whereas, FIG. 9 illustrates a variable pitch P, P1, between the
holes 112 to vary the compressibility of the gasket.
[0037] Also, FIG. 10 illustrates that compressibility can be varied
by varying the size of the holes 112 as is illustrated by a
plurality of holes 112a, 112b, each being of a different size such
size S1 and S2, respectively.
[0038] In addition, the holes 112, FIGS. 11-13 can be of variable
cross-sectional shapes. A hole 112c, FIG. 11, is of a circular
cross-section, a hole 112d, FIG. 12, is of a rectangular
cross-section, and a hole 112e, FIG. 13, is of a hexagonal cross
section. A rotary die can be used to punch holes in gasket 35, as
the gasket 35 is fed through the die.
[0039] The embodiments disclosed herein can be applied to any sort
of continuous cross-section (D-shaped, square, C-fold . . . etc.)
gasket material such as metalized fabric--foam core or conductive
extruded elastomers. In general, any shaped hole can be put into
the gasket to maximize the desired effect such as minimal forces or
maximum conductivity, etc. Also, the pitch of hole the perforations
can be varied in order to match the deflection curve of the cover
seams; as well as, in combination with the above variations in hole
pattern.
[0040] In FIG. 14, a chassis 120 includes a chassis body 122 and a
pair of chassis covers 124a, 124b which are pivotally attached to
body 122. Gaskets 35 may be selectively positioned along edge 126
of cover 124a for engagement with edges 127 of chassis body 122.
Also, additional gaskets 35 are selectively positioned along edges
130 of cover 124b for engagement with edges 131 of chassis body
122. In addition, gaskets 35 (not viewable in FIG. 14) are
positioned along edge 132 of cover 124a and along edge 134 of cover
124b, so that these gaskets 35 engage when covers 124a and 124b are
closed on the chassis body 122 such that edges 132 and 134
overlap.
[0041] FIG. 15 illustrates the big improvement in force/length
reduction for a given gasket cross-section, when it is perforated
according to the present disclosure. At point D the perforated
gasket is 3 times softer than the non-perforated one. The
difference in conductivity at this point is only 4.4 milliohm-Ft.
The gaskets represented here are a perforated and a non-perforated
74011 gasket from Chromerics.
[0042] FIG. 16 illustrates the comparison between the FIG. 15
perforated gasket and a somewhat smaller/shorter gasket. Although
they both share a very similar Conductivity Vs Compression curve,
the comparison of Force Vs Compression shows that the smaller
(non-perforated) gasket at point B generates about 3.3 times as
much force as the bigger/taller Perforated Gasket (which is 0.055"
taller than the shorter gasket!). The gaskets represented here are
a perforated 74011 gasket from Chromerics and a 4212 gasket from
APM.
[0043] The reasons that the perforations do not adversely affect
gasket performance is threefold. Firstly, the perforations allow a
much larger sized (height/cross-section) gasket to be used for a
given application (as stated above). Therefore the net contact area
between cover and gasket may be substantially increased. Secondly,
the conformability of the perforated gaskets are much better than
their non-perforated counterparts along their length (as stated
previously), and, in how well they flatten out. A regular
non-perforated gasket will very often wrinkle or fold along it's
periphery as it is compressed. This both reduces the contact area
between cover and chassis, and also increases the length of the
conductive path going from cover to chassis. This wrinkling/folding
effect increases the contact resistance and conductive resistance
of the gasket especially for rectangular cross sections. In fact,
the primary (or only) reason there are D-shaped gaskets, verses
rectangular, is in an attempt to produce softer more compliant
gaskets. However, the D-shaped cross section generates only a small
contact area in the lower range of compression (.about.<30%),
and the conductive path is significantly longer as well. A
rectangular gasket presents a larger contact throughout it's
compression (and a shorter conductive path), but because of the
high forces they generate, as well as the aforementioned problems,
the D-shaped gaskets are often (perhaps more often) used. However,
when perforated, in accordance with these embodiments, the
rectangular cross sections are ideal for use in nearly all
applications. Thirdly, because the type of gasket in these
embodiments only conducts thru it's skin (metal plated fabric or
metal foil) the contact area, along the centerline of the gasket,
contributes little to the gaskets conductivity and hence can be
removed without much impact, provided sufficient area is left to
make conductive contact.
[0044] By removing large amounts of core material the gaskets are
made much softer. These embodiments can be utilized on conductive
elastomer type gaskets as well, and on various gasket cross
sections. The preferred embodiment includes circular perforations
with a ratio of open holes per gasket length of 0.687 (running
along a centerline C of the gasket). The larger this ratio the
softer the gasket. The above ratio tested to be good for ESD
conductivity and EMI attenuation while vastly reducing cover forces
(approximately 3 times softer).
[0045] In the event that an adhesive is used, the perforations
should be formed in the gaskets prior to laminating the PSA
(pressure sensitive adhesive) along the length of the gaskets. The
perforations could be placed by any number of means used in
standard hole punching technology, however the preferred embodiment
of the hole punching method would be to use a rotary die tool which
would also have continuous rotary means for applying the PSA after
the hole punching.
[0046] As can be seen, the principal advantages of these
embodiments are that they reduce the closure force on a metal
fabric/foil wrapped foam core gasket by providing holes along the
length of the gasket. The hole geometry can be varied to maximize
effect. Additionally, the hole geometry can be varied along the
length of the gasket to provide a variable force/compression curve
to compensate for the geometry of the parts being closed by the
gasket.
[0047] Additionally, the perforated gasket is much more
compliant/conformable along its length compared with its
non-perforated counterpart. This is in terms of maintaining
continuous contact surfaces along its length over an obstacle in
the chassis, or cover surfaces (screw heads, rivets, steps in
sheet-metal lap joints, etc.)
[0048] The perforated gasket provides a generic form of EMI/ESD
gasket with the lowest forces possible, via perforations along its
length (with improved electrical/mechanical performance). That
gasket also provides a means of precisely controlling the force
output of an EMI gasket via varying pitch and/or size, and/or
shape, of perforation holes along the length of gasket.
[0049] Although illustrative embodiments have been shown and
described, a wide range of modification, change and substitution is
contemplated in the foregoing disclosure and in some instances,
some features of the embodiment may be employed without a
corresponding use of other features. Accordingly, it is appropriate
that the appended claims-be construed broadly and in a manner
consistent with the scope of the embodiments disclosed herein.
* * * * *