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 is formed in the strip to improve compressibility and thus enhance EMI shielding.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a Divisional of U.S. application Ser. No. 09/934,279, filed on Aug. 21, 2001 now U.S. Pat. No. 6,621,000. 

   BACKGROUND 
   The disclosures herein relate generally to computer systems and more particularly to a perforated gasket for providing an electromagnetic interference seal for a computer chassis enclosure. 
   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&#39;s electromagnetic interference (EMI) requirement and well as electrostatic discharge susceptibility. 
   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. 
   The bigger/taller the gasket profile, or cross-section, the greater it&#39;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. 
   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. 
   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 can cause the computer to fail EMI or ESD requirements. 
   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. 
   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 
   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. 
   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 
       FIG. 1  a diagrammatic view illustrating an embodiment of a computer system. 
       FIG. 2  is perspective view illustrating an embodiment of a chassis in an open position. 
       FIG. 3  is a perspective view illustrating the chassis in a closed position. 
       FIG. 4  is another perspective view illustrating the chassis in the open position. 
       FIG. 5  is a further perspective view illustrating the chassis in the closed position. 
       FIG. 6  is another perspective view illustrating the chassis in the open position. 
       FIG. 7  is a perspective view illustrating a sealing gasket in a tongue and groove engagement of a portion of the, chassis. 
       FIG. 8  is a view illustrating a gasket including a plurality of equidistantly spaced holes. 
       FIG. 9  is a view illustrating a gasket including a plurality of variably spaced holes. 
       FIG. 10  is a view illustrating a gasket including a plurality of variably sized holes. 
       FIG. 11  is a partial view illustrating a gasket having a round hole. 
       FIG. 12  is a partial view illustrating a gasket having a rectangular hole. 
       FIG. 13  is a partial view illustrating a gasket having a hexagonal hole. 
       FIG. 14  is a perspective view illustrating a chassis utilizing a sealing gasket as disclosed herein. 
       FIG. 15  is a graphical view comparing gasket compression curves. 
       FIG. 16  is a graphical view comparing gasket compression curves. 
   

   DETAILED DESCRIPTION 
   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 . 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. 
   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 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 . 
   The hinge connection  36  permits the top portion  34  to pivot to an open position  0  about 90° 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° as desired. 
   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  56   a  and a release button  56   b  which permits latch member  56   a  to disengage from base portion  28 . 
   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 . 
   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  to pivot relative to the base metal portion  30  between the open position  0  and the closed position C, as described above. 
   The metal base portion  30  includes a pair of opposed base sidewalls  30   a ,  30   b ,  FIGS. 5 and 6 , and the metal top portion  38  includes a pair of opposed top sidewalls  38   a ,  38   b . The sidewalls  30   a ,  30   b , respectively matingly engage the sidewalls  38   a ,  38   b . Preferably, the base sidewalls  30   a ,  30   b  include a tongue  31  and the top sidewalls  38   a ,  38   b  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® by Advanced Performance Materials, Inc. (APM) of St. Louis, Mo. 
   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 . 
   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, P 1 , between the holes  112  to vary the compressibility of the gasket. 
   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  112   a ,  112   b , each being of a different size such as sizes S 1  and S 2 , respectively. 
   In addition, the holes  112 ,  FIGS. 11-13  can be of variable cross-sectional shapes. A hole  112   c ,  FIG. 11 , is of a circular cross-section, a hole  112   d ,  FIG. 12 , is of a rectangular cross-section, and a hole  112   e ,  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. 
   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 the holes 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. 
   In  FIG. 14 , a chassis  120  includes a chassis body  122  and a pair of chassis covers  124   a ,  124   b  which are pivotally attached to body  122 . Gaskets  35  may be selectively positioned along edges  126  of cover  124   a  for engagement with edges  127  of chassis body  122 . Also, additional gaskets  35  are selectively positioned along edges  130  of cover  124   b  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  124   a  and along edge  134  of cover  124   b , so that these gaskets  35  engage when covers  124   a  and  124   b  are closed on the chassis body  122  such that edges  132  and  134  overlap. 
     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. 
     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. 
   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&#39;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 (˜&lt;30%), and the conductive path is significantly longer as well. A rectangular gasket presents a larger contact throughout it&#39;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&#39;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. 
   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). 
   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. 
   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. 
   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.) 
   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. 
   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 embodiments 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.