Electromagnetic interference/radio frequency interference conducting strip

An aluminum foil strip is provided with adhesive strips on one face surrounding the strip of foil. The strip of foil is then positionable and attachable to an electronic cabinet frame over exposed raw metal areas for intimate electrical contact when the cabinet door is closed. This strip of foil provides electrical continuity between the frame and door as well as a seal against EMI/RFI at the cabinet/door interface.

FIELD OF THE INVENTION 
This invention relates to techniques for electrically interconnecting the 
movable portions of an electronic systems cabinet or container with the 
fixed portions of the cabinet or container and more specifically, to the 
material and device used for electrically interconnecting of the two 
portions of the cabinet. 
BACKGROUND OF THE INVENTION 
High performance electronic equipment such as computers and particularly 
the processor portion of a computer must be shielded from radio frequency 
interference and from electromagnetic interference in order to reliably 
operate. Electromagnetic interference/radio frequency interference 
(EMI/RFI) interferes with the signals on the different lines leading to 
and from the computer and within the computer processor itself, since the 
lines leading to and from the computer act as antennas and an electrical 
current may be generated in those electrical conductors as a result of 
being a receiver for EMI/RFI. 
In order to protect the electronic device, from the EMI/RFI and to improve 
its performance, the electronic device is typically enclosed within a 
metal cabinet which in turn is grounded. The metal cabinet acts as a 
receiver for the EMI/RFI and the resulting signals generated in the 
cabinet are then grounded and thereby prevented from effecting the 
reliable and efficient operation of the electronic system contained 
therein. 
Additionally, the electronics of the electronic system generate EMI/RFI 
interference. There exists requirements for an EMI/RFI generating device 
to be equipped with shielding to prevent the propagation of such 
interference. 
Typically, cabinets are a self-contained item having five sides including a 
bottom, three sides and a top and additionally a door or closure attached 
on the open side. The top and bottom may contain a perforated sheet to 
allow for air flow in and out of the cabinet. The cabinet may have a door 
or closure adjacent to each other, on opposite sides or wherever access is 
needed to the cabinet. 
In order to prevent leakage, either incoming or outgoing, of EMI/RFI 
through the gap between the cabinet enclosure and the door, many attempts 
have been made to conductively connect the door and the cabinet and at the 
same time seal the gap between the two metal pieces. In order to 
effectively shield the contents of the cabinet from EMI/RFI, the door, in 
addition to being connected through the hinges, must be efficiently 
connected electrically to the cabinet itself around the opening to the 
cabinet. When a large system computer cabinet frame is assembled, for 
example, it is painted with a conductive paint to provide a conductive 
path to ground and also to provide corrosion protection. 
Tin plated copper strips with a silver filled conductive pressure sensitive 
adhesive and a protective strip that may be removed from the adhesive to 
allow its installation are commercially available. These strips are 
applied over the bare metal of the frame, typically before, painting. The 
conductive pressure sensitive adhesive material carried by the tin/copper 
strips relies upon silver fill material in the conductive pressure 
sensitive adhesive for conductivity between the frame and the tin plated 
copper strip. The conductivity of the adhesive can vary with the amount of 
pressure applied to the adhesive during application. 
The cost of this material is exceptionally high due to the silver content. 
The strip is typically applied to the bare metal of the frame and 
positioned around the periphery of the opening to the cabinet such that 
when the door of the cabinet closes and a resilient electrically 
conductive gasket material carried by the door is engaged in face-to-face 
contact with the tin plated copper strip, the electrical path between the 
door through the electrically conductive gasket, the tin plated copper 
foil and the conductive pressure sensitive adhesive is completed to the 
bare metal of the frame. 
An example of such an electrically conductive tape is found in the IBM 
Technical Disclosure Bulletin, Vol. 14, No. 2, Jul., 1971, page 474. 
A further example of efforts to address the problem of electromagnetic 
compatibility, i.e., shielding for EMI, is described and disclosed in the 
IBM Technical Disclosure Bulletin, Vol. 14, No. 2, Jul., 1971, page 518. 
U.S. Pat. No. 4,977,296 to Hemming, discloses a metal foil with adhesive 
strips placed parallel to each other along the surface of the metal foil 
thus causing a ridge/valley contour forming a multiplicity of small wave 
guides which are effective in attenuating electromagnetic energy by virtue 
of their physical characteristics. The shielding tape disclosed by Hemming 
is utilized to join together adjacent panels of shielding material which 
have been positioned on a fixed wall or structure. 
It is an aspect of this invention to be able to establish electrical 
continuity between the closure of an electronic systems cabinet and the 
electronic systems cabinet itself. 
It is a further aspect of the invention that the continuity between the 
closure and the frame of the cabinet is not dependent upon a conductive 
adhesive material, but rather upon highly conductive metal foils. 
SUMMARY OF THE INVENTION 
An electrically conductive foil of a highly conductive metal is provided 
with strips of adhesive material or beads of adhesive material in regions 
proximate the edges of the foil. These beads or strips may then be 
adhesively engaged, by force or pressure, with the frame of the cabinet 
which will contain the electronic system or computer. The frame of the 
cabinet when fabricated and painted, is provided with an exposed raw metal 
region completely surrounding the opening to the cabinet. The foil and its 
adhesive are positioned such that one face of the exposed foil, the 
exposed foil being between the adhesive regions, is positioned directly 
over the raw metal of the frame which was intentionally left unpainted. 
The adhesive acts purely as a holding element to maintain the foil properly 
positioned in overlying face-to-face relationship with the exposed raw 
metal region. The foil will be forced into intimate face-to-face contact 
and thus establish electrical conductivity with the frame when the cabinet 
door is closed. The cabinet door carries, in electrical continuity with 
the cabinet material, a gasket which is fabricated of conductive material. 
For example, a highly carbon or metallically loaded rubber material may be 
used as such a gasket. Other synthetic materials may also be used to carry 
the carbon or metallic particle loading. When the door of the cabinet is 
closed, the gasket will forcibly engage itself, under compressive forces, 
with the exposed surface of the foil while at the same time forcing the 
inner surface of the foil against the raw metal surface of the cabinet 
frame. 
The compressive forces of the gasket onto the foil will establish 
electrical conductivity between the door or closure of the cabinet and the 
frame of the cabinet. At the same time, since the foil will extend 
completely around the opening and will be engaged and compressed between 
the gasket and the frame at all points, an effective EMI/RFI seal is 
established. 
It can be seen that the shortcomings of the prior art are overcome and the 
specific aspects of the invention accomplished by the conductive strip and 
adhesive material described in summary form above. 
A better understanding of the invention may be obtained from the drawings 
and the detailed description of the preferred embodiment below.

Detailed Description of the Preferred Embodiment of the Best Mode for 
Carrying Out The Invention 
Referring now to FIG. 1, cabinet 10 is shown with the cabinet enclosure 12 
surrounding a frame 18. 
Cabinet 10 is suitable for accepting and containing an electronic system 
such as a computer. The frame 18 is typically formed of steel such as 
steel tubing. The cabinet enclosure 12 is preferably sheet metal. The 
metal sheet of the enclosure 12 is typically painted with a highly 
conductive paint, as is the frame 18. 
For example, the paint may be sprayed onto all surfaces of the structure 
and then heated to fuse the paint into a continuous protective coating and 
at the same time, due to its conductive nature, provide conductivity on 
all surfaces of the enclosure 12 and frame 18. 
In order to close the cabinet and to protect the contents from EMI/RFI, a 
door 14 is provided. The door 14 may likewise be fabricated from sheet 
metal and may include a frame 20 or, alternatively, the frame 20 may be 
made by bending and forming the sheet metal of door 14 to provide the 
frame member 20. 
In any event, the door 14 is movably mounted with respect to the cabinet, 
preferably by use of hinges 16. 
As shown in FIG. 1, the only electrical contact and conductive paths 
between door 14 and frame 18 is through hinges 16 which, at best, is 
unreliable, when the door 14 is closed, there still is a remaining gap 
between the frame portion 20 of door 14 and frame 18. Such a gap is 
typically sufficient to allow undesired quantities of EMI/RFI to enter the 
cavity of the cabinet 10 and to interfere with the reliable and proper 
operation of the computer system mounted therein. 
Referring now to FIG. 4, it is a preferred embodiment of the best mode for 
carrying out the invention, that frame 18 is fabricated from a steel 
material, preferably square or rectangular cross-section tubular steel, 
and welded together. After the frame 18 has been fabricated and covered 
with sheet metal 22. The sheet metal 22 is attached to the exterior of 
frame 18, by any of several conventional ways, such as with screws, bolts 
or welds. 
The sheet metal 22, for example sheet steel, utilized for the exterior 
surfaces 12 must be preferably highly conductive and act as a partial 
EMI/RFI shield in and of itself. The exterior sheet metal 22 will be 
electrically grounded to the frame by conventional attaching techniques, 
such as bolting, screwing or welding. 
The entire structure is typically painted with a conductive paint and may 
be painted using a liquid or powder type material which is sprayed onto 
the surfaces and then heated to a temperature causing it to fuse into a 
continuous protective coating on the metal as well as a continuous 
electrically conductive shield on the metal. During the painting step, an 
unpainted region 26, which is defined by boundaries 24 in FIG. 4, is 
provided by masking that region prior to painting. Masking can be 
accomplished in many different ways, but a preferred method is to use a 
strip of magnetic rubber material which will magnetically adhere to the 
frame 18 and prevent the paint from reaching region 26. 
Once the painting has occurred and prior to firing, the magnetic strips may 
be removed exposing the raw steel of the frame 18. 
The unpainted region 26 then becomes the major contact and electrical 
continuity point for the completion of the EMI/RFI shield. 
The cabinet 10 is closed with a door 14 which may be mounted on the cabinet 
frame 18 by any conventional means such as, for example, hinge 16. 
The door 14 is made of an electrically conductive sheet metal which may 
contain a frame member 20 or preferably may be fabricated by means of 
bending and joining the sheet metal of the door 14. This frame 20 provides 
rigidity to the door and at the same time provides a mounting surface for 
an electrically conductive gasket 36. Electrically conductive gasket 36 is 
attached to the bare metal on the door 14. The bare raw metal region 38 of 
the door 14 is left exposed during the painting process in the same manner 
that the unpainted region 26 of frame 18 was left exposed. The remainder 
of the surfaces of the door 14 are painted typically with a decorative 
coating, then fired or fused to complete the coating of paint on the door 
while at the same time permanently adhering the door 14 and the paint 
layer. The electrically conductive gasket 36 may be one of any of several 
types. Several types include carbon filled compressible rubber strips 
where the carbon particles provided the paths of continuity; or 
alternatively, a metal sphere or particle loading into the rubber matrix 
in such quantities that particles will be in contact with adjacent 
particles thus forming conductivity paths through the rubber gasket. 
The flexible gasket 36 may be attached to the bare metal 38 with a metal 
clip, selective bonding to insure the conductive gasket remains in contact 
with the bare metal in the cover, provide a recess in the cover to press 
fit the gasket, or other means that provides a conductive path between the 
gasket and door. 
When the door 14 is moved from its open to closed position the cabinet door 
14 will assume a face-to-face closed position with frame 18. Electrical 
continuity provided by the hinges 16 in FIG. 4 is insufficient to reliably 
connect the door 14 to frame 18 for purposes of EMI/RFI shielding. 
The electrically conductive foil 40 is illustrated in FIG. 2 with pressure 
sensitive adhesive strips 42 exploded away from the surface. Pressure 
sensitive adhesive strips 42 are positioned along the edges of foil 40 and 
may be of the type having a release sheet engaged with the exposed 
surfaces which may then be subsequently removed when the foil is to be 
applied. 
The foil 40 is a high purity aluminum foil which will oxidize over time but 
where the oxidation will be insufficient to create a conductivity problem 
and introduce any significant resistance to the electrical surface between 
the door 14 and the cabinet frame 18. The foils is preferably of the type 
which is specified in ASTM STANDARD DESIGNATION B373-90. The foil should 
be dry annealed and 0 tempered. 
An alternative embodiment is illustrated in FIG. 3 where the foil 40 has 
deposited thereon, around its periphery, beads of a pressure sensitive 
adhesive 44. The pressure sensitive adhesive in the case of FIG. 3 may be 
deposited from a dispenser or nozzle in a molten or semi-molten state and 
allowed to cool prior to positioning and application of the foil 40 to the 
frame 18. In the case of FIG. 2, the pressure sensitive adhesive may take 
the form of a tape 42 such as a double-face adhesive tape 42. The tape 42 
need not be conductive nor need the adhesive component to be conductive 
inasmuch as the foil 40 will be relied upon for electrical continuity and 
the adhesive is only provided for positioning and retention. 
Referring again to FIG. 4, foil 40 is shown positioned over the unpainted 
regions 26 on frame 18. Between the edges of the foil 40 and the painted 
portions of frame 18, resides the pressure sensitive adhesive strip 42 if 
the embodiment shown in FIG. 2 is utilized, or the bead of pressure 
sensitive adhesive material 44 in the case where the embodiment of FIG. 3 
is utilized. This arrangement with the adhesive 42 or 44 between the inner 
surface of foil 40 and the painted region of frame 18 holds the foil 40 in 
position over the unpainted region 26 to provide protection to the raw 
steel portion 26 of frame 18. The foil 40 will retard rust and corrosion 
of the unpainted region 26. 
When door 14 is closed to completely enclose the electronic system or 
computer within cabinet 10, the electrically conductive gasket 36 will be 
brought into face-to-face abutting relationship with foil 40 and its 
exterior surface. As the door is forced toward frame 18, the compression 
of the electrically conductive gasket 36 will provide a force which will 
displace foil 40 until its inner surface is in face-to-face abutment with 
the unpainted region 26 of frame 18. At this point, electrical continuity 
exists between door 14 through conductive gasket 36, foil 40 to the frame 
18. Accordingly, any EMI/RFI which is received or picked up by door 14 
will then be conducted through the metal of door 14 to the conductive 
gasket 36. Conductive gasket 36, being in intimate face-to-face engagement 
with foil 40, will then allow the electrical current generated by the 
EMI/RFI to be conducted directly to the foil 40 and then to frame 18 for 
grounding. 
By utilizing a high purity, highly electrically conductive foil 40, an 
improved interface between the conductive gasket 36 and unpainted surface 
26 of frame 18 is accomplished. The foil 40, under pressure from gasket 
36, will conform to and engage the unpainted region 26 over a very large 
area thus reducing electrical resistance. Likewise, engagement of foil 40 
with the conductive gasket 36 will be accomplished over a large area due 
to the pliability of foil 40 and the resilient and compressive nature of 
gasket 36. 
While this particular embodiment of the invention has been described with 
respect to a cabinet 10 and a door 14, it is equally applicable for 
connecting the face panels or other frame portions of electronic 
assemblies with the frame 18. Where the frame 18 may be engaged by a 
portion of the electronic assembly inserted therein, the continuity 
between the outer frame member of the computer or electronic assembly may 
be similarly established. The electronic assembly may have attached to it 
a conductive gasket material, and the foil as described and shown in FIGS. 
2 or 3 may be attached at strategic locations to the bare metal portion of 
the frame which would have been left unpainted for this purpose, thereby 
establishing continuity between the different shielding elements of the 
electronic assembly and the frame enclosure, adding further protection 
against EMI/RFI. 
The elimination of an adhesive material from the area between the metal 
foil 40 and the exposed metal of the frame 18 improves the electrical 
conductivity between the foil 40 and the frame 18 by eliminating a 
substantial mass of material which is not conductive, as was practiced in 
the prior art. The electrically conductive particles which must be added 
to the adhesive material to provide conductivity greatly increases costs 
and tends to reduce the adhesion capability of the pressure sensitive 
adhesive since the area that the pressure sensitive adhesive engages, 
either the foil 40 or the frame 18 is reduced by the amount of physical 
contact between the conductive particle loading and either the foil 40 and 
the frame 18. Accordingly, adequate adhesion is also secured with a 
smaller area of adhesive by utilizing the described approach Referring to 
FIG. 5, the gasket 50 may be attached to the door 14 by metal conductive 
spring clip 52. If the door 14 is fabricated of an extruded metal or sheet 
metal, a flange 54 may be formed into the door 14. The surface of the 
flange 54 is left unpainted for electrical continuity. 
The invention described above may be modified or have changes made to it 
within the scope of the invention as defined by the attached claims.