EMI-attenuating air ventilation panel

EMI-attenuating air ventilation panel for electrical and electronic systems. An opening through a metallic panel is realized using an extrusion method in order to obtain a tube leaving the opening at one end. The periphery of the opening at the other side of the extruded tube has a smooth edge in order to improve air circulation. A multitude of such extruded holes are critically placed on the panel to form an array. The panel in turn is mounted in an electrical or electronic system to allow air cooling while the extruded tubes attenuate EMI radiation to and from electrical circuits.

FIELD OF THE INVENTION 
The present invention generally relates to electrical and electronic 
apparatus. More particularly, the invention relates to apparatus for 
cooling an electrical or electronic system, and for shielding 
electromagnetic interference (EMI) radiation generated by the system 
during operation thereof. 
BACKGROUND OF THE INVENTION 
In today's competitive electronics marketplace, there is a demand for 
higher frequency computer components while consumers demand smaller 
packaging. These requirements have lead to much higher temperatures within 
the electronic package, requiring more cooling. Higher frequency computer 
components are also generating more electromagnetic interference or EMI in 
addition to the heat which, if not properly shielded, can interfere with 
other electronic equipment by way of radiation or conduction. 
Consequently, electronic manufacturers often face a design trade-off 
between cooling the package and shielding for EMI, since EMI radiation 
typically escapes through air holes commonly used for cooling purposes. It 
should efficiently be noted that radiations generated by electronic 
packages can include electromagnetic energy of wavelengths along various 
points of the spectrum such as radio frequency interference. As used 
herein, the term electromagnetic interference (EMI) refers to interfering 
electromagnetic energy of any wavelength. 
Printed circuit boards used in various types of electronic equipment are 
typically mounted within a housing structure. During operation of the 
circuit board, EMI radiation is generated within the board, emanates 
therefrom, and must be substantially prevented from escaping outwardly 
through the housing structure. One solution to the above problem is to 
provide the housing with the necessary EMI radiation shield by coating the 
interior of the housing with a metallic material which is brought into 
contact with a grounding portion of the electronic circuits, such as the 
ground plane of the circuit board disposed within the housing. Other types 
of EMI shields positioned around the circuit board may be alternatively 
used. For instance it has been realized in the art that a foil shield, 
placed around the electronic circuitry and connected to ground would 
reduce EMI radiations. Typically aluminum or copper is used for such 
shields. However, such aluminum and copper foil shields, while somewhat 
effective, have proven to be costly to manufacturers. Moreover, none of 
the above solutions facilitate air circulation or improve cooling 
capabilities of the system. 
In order to allow air circulation while reducing EMI, arrays of small flat 
holes have been used to shield against EMI, but the holes had to be so 
small that dust and lint would easily collect and clog the holes. In 
another solution, stacked plates of holes arranged into an array have been 
used, but the weight and cost of this solution make it unattractive. Yet 
another solution known in the art uses screen meshes, but this solution 
creates a high impedance air flow and presents a great risk of lint and 
dust build-up. Honeycomb vents have also been used for the same purpose, 
but the cost is prohibitive. 
Accordingly, it is an object of the present invention to attenuate EMI 
radiation entering or leaving electronic packages to an acceptable level. 
It is a further object of the invention to produce an EMI radiation 
shielding device which allows for low impedance airflow through the 
package for cooling purposes. It is a further object of the invention to 
lower the cost compared to traditional EMI solutions. It is a still 
further object to reduce problems related to lint and dust collection 
around cooling holes. 
SUMMARY OF THE INVENTION 
In an embodiment, the invention includes an EMI-attenuating air ventilation 
panel for an electronic device enclosure. This air ventilation panel is 
typically made of an electrically conductive panel such as a metallic 
panel, and includes at least one air ventilation hole in it. Around the 
periphery of the air ventilation hole a tube is electrically and 
mechanically connected to the panel. The axis of the tube is approximately 
perpendicular to the panel. The ventilation panel has consequently two 
distinguishable sides, respectively referred to as upstream and downstream 
airflow sides. The downstream airflow side is the side where the tube is 
attached to the panel, and the upstream airflow side is the other side of 
the panel. As indicated by their names, the airflow preferably flows from 
the upstream air flow side, through the tube, and out to the downstream 
air flow side. It should also be noted that both the air ventilation hole 
and the electrically conductive tube typically have a circular cross 
section. The most common conductive material used for the manufacturing of 
the panel is metal. The tube leaving a ventilation hole from around its 
perimeter is formed with extruded metal. 
In another embodiment, the invention includes an EMI-attenuating air 
ventilation panel for an electronic device enclosure with a plurality of 
air ventilation holes formed in it. Each of these air ventilation holes 
has an electrically conductive tube which is both electrically and 
mechanically coupled to the panel at the periphery of the air ventilation 
hole. The axis of the tube extends away from the panel in a direction 
approximately normal to the panel. Both the air ventilation hole and the 
tube have typically a circular cross section. The side of the ventilation 
panel where the tubes are connected is referred to as the downstream 
airflow side. The other side is the upstream airflow side. An air 
ventilation hole has a smooth edge around its periphery on the upstream 
airflow side of the electrically conductive panel. This facilitates air 
circulation through the tube. A typical material used for the panel is a 
zinc-plated steel sheet having a thickness of approximately 1 mm. The 
electrically conductive tubes can then be extruded from the sheet of 
zinc-plated steel. The length-to-diameter ratio of each of the 
electrically conductive tubes is between approximately 0.5 and 1.0. Each 
of the tubes has a length of approximately 3.5 mm, and a diameter of 
approximately 4.8 mm. The relative spacing of the air ventilation holes is 
approximately 8.5 mm center-to-center. 
In a further embodiment, the invention includes an electronic device 
enclosure having improved air ventilation and EMI attenuation 
characteristics. This enclosure includes a casing; an EMI-attenuating air 
ventilation panel made with an electrically conductive panel having a 
plurality of air ventilation holes formed in it; and a cooling fan for 
circulating air into the casing through the air ventilation holes. Each of 
the ventilation holes has an electrically conductive tube which is 
electrically and mechanically coupled to the panel at the periphery of the 
air ventilation hole with the axis of the tube extending away from the 
panel in an approximately perpendicular direction relative to the panel. 
The air ventilation holes and the tubes have typically a circular cross 
section. The side of the panel with the tubes is referred to as the 
downstream airflow side, with the other side being the upstream airflow 
side. The periphery of a hole has a smooth edge on the upstream airflow 
side in order to facilitate airflow from the upstream airflow side, 
through the tube, and out to the downstream airflow side. A typical 
material used for the panel is a zinc-plated steel sheet having a 
thickness of approximately 1 mm. The electrically conductive tubes can 
then be extruded from the sheet of zinc-plated steel. The 
length-to-diameter ratio of each of the electrically conductive tubes is 
between approximately 0.5 and 1.0. Each of the tubes has a length of 
approximately 3.5 mm, and a diameter of approximately 4.8 mm. The relative 
spacing of the air ventilation holes is approximately 8.5 mm 
center-to-center.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in the drawings for purposes of illustration, the invention is 
embodied in a novel air ventilation panel for cooling and EMI containment. 
A system according to the invention provides substantial EMI attenuation 
entering or leaving packages on a circuit board; while reducing airflow 
impedance through the package and therefore improving the cooling 
capabilities of the system. Existing solutions such as honeycomb vents, 
screen meshes, stacked perforated plates and various others have typically 
several disadvantages such as allowing a difficult airflow, being prone to 
dust and lint collection and clogging, heavy weight, and high 
manufacturing costs. 
An EMI-attenuating air ventilation panel according to the present invention 
provides a multitude of extruded air ventilation holes for EMI containment 
and cooling. The resulting panel is simple and cost-effective. It provides 
an efficient EMI containment solution at a lower cost and adds only a low 
impedance to the airflow through the system. Additional advantages include 
reducing product weight, and minimizing the risk of inadequate cooling due 
to lint and dust collection. 
Depicted in FIG. 1 is an oblique view of an EMI-attenuating air ventilation 
panel 1 for an electronic device enclosure. The ventilation panel includes 
an electrically conductive panel 10 having an air ventilation hole 12 
formed in it. FIG. 2 is a side view of the ventilation panel and depicts 
an electrically conductive tube 14 which is electrically and mechanically 
coupled to the electrically conductive panel at the periphery 15 of an air 
ventilation hole. The electrically conductive tube 14 illustrated in the 
preferred embodiment has a cylindrical interior wall surface 21. The axis 
of this electrically conductive tube extends away from the electrically 
conductive panel in a direction approximately normal to the electrically 
conductive panel. FIG. 3 shows a side of the ventilation panel referred to 
as the upstream airflow side 16. Similarly, the opposite side of the 
ventilation panel which is viewed in FIG. 4 is referred to as the 
downstream airflow side 18. As depicted in the figures, a smooth surface 
20 joins the upstream airflow side of the electrically conductive panel 
with the inner surface 21 of the electrically conductive tube. The air 
ventilation hole and the electrically conductive tube both have typically 
a circular cross section. The most common material used for the 
EMI-attenuating air ventilation panel is usually metal. Accordingly, the 
electrically conductive tube is made with extruded metal. 
Another embodiment is depicted in FIGS. 5-7, showing an EMI-attenuating air 
ventilation panel 3 for an electronic device enclosure with a plurality of 
air ventilation holes formed in it. Each of these air ventilation holes 
has an electrically conductive tube which is both electrically and 
mechanically coupled to the panel at the periphery of the air ventilation 
hole. The axis of the tube extends away from the panel in a direction 
approximately normal to the panel. Both the air ventilation hole and the 
tube have typically a circular cross section. FIG. 6 illustrates the side 
of the ventilation panel where the tubes are connected to the panel around 
each air ventilation hole, which is referred to as the downstream airflow 
side. The other side of the ventilation panel is the upstream airflow 
side, depicted in FIG. 5 and FIG. 7. An air ventilation hole has a smooth 
edge around its periphery on the upstream airflow side of the electrically 
conductive panel. This facilitates air circulation through the tube. A 
typical material used for the panel is a zinc-plated steel sheet having a 
thickness of approximately 1 mm. The electrically conductive tubes can 
then be extruded from the sheet of zinc-plated steel. The 
length-to-diameter ratio of each of the electrically conductive tubes is 
usually between approximately 0.5 and 1.0. Each of the tubes has typically 
a length of approximately 3.5 mm, and a diameter of approximately 4.8 mm. 
The relative spacing of the air ventilation holes is approximately 8.5 mm 
center-to-center. 
In yet another embodiment, the invention includes an electronic device 
enclosure 2 having improved air ventilation and EMI attenuation 
characteristics as depicted in FIG. 8. This enclosure includes a casing 
22; an EMI-attenuating air ventilation panel 3 made with an electrically 
conductive panel having a plurality of air ventilation holes formed in it; 
and a cooling fan 24 for circulating air into the casing through the air 
ventilation holes. Each of the ventilation holes has an electrically 
conductive tube which is electrically and mechanically coupled to the 
panel at the periphery of the air ventilation hole with the axis of the 
tube extending away from the panel in an approximately perpendicular 
direction relative to the panel. The air ventilation holes and the tubes 
have typically a circular cross section. The side of the panel with the 
tubes is referred to as the downstream airflow side, with the other side 
being the upstream airflow side. The periphery of a hole has a smooth edge 
on the upstream airflow side in order to facilitate airflow from the 
upstream airflow side, through the tube, and out to the downstream airflow 
side of the panel. The smooth edge improves the circulation of air through 
the tube, compared to an opening with sharp edges. A typical material used 
for the panel is a zinc-plated steel sheet having a thickness of 
approximately 1 mm. The electrically conductive tubes can then be formed 
in the sheet of zinc-plated steel using an extruding operation. Through an 
iterative process, the optimum hole spacing can be determined for any 
given material and set of constraints. For example, in the case of 1 mm 
thick zinc-plated steel, the optimal length-to-diameter ratio of each of 
the electrically conductive tubes is determined to be between 
approximately 0.5 and 1.0 in order to allow the greatest number of holes 
for a given area. Accordingly, each of the tubes has a length of 
approximately 3.5 mm, and a diameter of approximately 4.8 mm. These tubes 
act as a collection of waveguide below cutoff attenuators. The cross 
sectional dimensions of do not support propagation of electromagnetic 
energy below a wavelength related to those dimensions. Consequently, the 
EMI is effectively attenuated by the length of the tube leaving the air 
ventilation hole, and the panel is nearly opaque to signals below that 
cutoff frequency. The relative spacing of the air ventilation holes is 
approximately 8.5 mm center-to-center. 
As discussed above, the EMI-attenuating air ventilation panel described in 
this disclosure provides a superior barrier for radiated electromagnetic 
emissions when compared to a conventional "flat" hole array structure. 
This design provides extruded hole walls creating a conductive tube at 
each hole location. The tube acts as a waveguide below cutoff, resulting 
in significant signal attenuation at high frequencies. This technique 
mimics the performance of honeycomb filter assemblies at a much lower cost 
basis. Extensive testing has confirmed the superiority of the present 
invention. One such test was performed with two sample panels having a 
six-inch by six-inch array of holes. One sample containing conventional 
holes was used as the control element in this test. The other sample 
contained the extruded hole design described in this disclosure, and is 
represented as the panel 30 in FIG. 9. The tests were conducted using a 
custom-built shielding effectiveness tester (SET) 32. This test fixture 
features near-field source excitation with mode stirring at both the 
source 34 and the antennae 36. The frequency range of the test represents 
the maximum flat response region of the SET fixture (290 MHZ to 2 GHZ). 
All readings are relative to a normalized baseline where no test sample is 
in place. Testing in the SET fixture demonstrates a shielding 
effectiveness of 25 dB for the flat hole structure and a superior 40 dB 
for the extruded structure. A 15 dB increase in shielding effectiveness 
translates to more than a five to one signal reduction. This method of 
testing is representative of actual computer product implementations of 
EMI shielding solutions. Good correlation has been demonstrated between 
SET testing and testing performed in FCC qualified chambers. The FCC is a 
U.S. regulatory body requiring that manufacturers of certain classes of 
electronic equipment test and certify that radio frequency emissions from 
that equipment are within prescribed limits. Based on these tests, the 
disclosed EMI-attenuating air ventilation panel featuring extruded tubes 
offers superior shielding performance over more conventional "flat" 
structures. 
From the foregoing it will be appreciated that the EMI-attenuating air 
ventilation panel provided by the invention represents a significant 
advance in the art. The present invention provides a single 
cost-effective, easy, and efficient way to satisfy two demanding 
requirements of most electrical and electronic systems: Cooling and EMI 
attenuation. An air ventilation panel embodying the invention attenuates 
EMI radiation entering or leaving the package to an acceptable level, 
while allowing for low impedance airflow through the system since no 
significant resistance is imposed to the airflow within the system. In 
addition to its elegant simplicity, the invention lowers the cost and the 
weight of the final product compared to traditional EMI solutions. 
Moreover, since the extruded holes can be larger and still contain EMI 
radiation, the problem of lint and dust collection around the holes is 
also eliminated. A panel embodying the present invention has the 
additional advantages of being light-weight, easy to manufacture, easy to 
assemble and disassemble in a system, and easy to clean. 
While the invention has been described in detail in relation to a 
particular embodiment thereof, this description is intended to be 
illustrative only. It will be obvious to those skilled in the art that 
many modifications can be made to the described embodiment without 
departing from the spirit and scope of the invention, and that such 
modifications will remain within the scope of the following claims.