Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
1999-05-26
2001-04-17
Thompson, Gregory (Department: 2835)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C165S080300, C174S034000, C361S715000, C361S818000
Reexamination Certificate
active
06219239
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to electronic systems. More particularly, the invention relates to an electromagnetic interference (EMI) reduction device.
BACKGROUND OF THE INVENTION
A trend in the electronics industry is to generate smaller and faster electronic devices. As a result, these devices consume more power and hence, produce more heat. Excessive heat can cause significant damage to an electronic device thereby reducing its life. For this reason, various techniques are employed to eliminate or dissipate the heat generated from the electronic device. A heat sink is one such technique. A heat sink is thermally coupled to the electronic device and as such, allows the heat to flow from the electronic device through the heat sink to the surrounding open space.
In addition, the operation of these electronic devices is accompanied by the generation of electromagnetic radiation or energy. The emissions of the electromagnetic radiation can cause significant electronic interference or noise with other surrounding electronic circuitry. To reduce such disturbances, the source of these emissions is usually suppressed. However, the EMI suppression problem is further complicated when the electronic device is coupled to a heat sink. The heat sink acts as an antenna for the EMI energy thereby amplifying the EMI energy.
A common solution for suppressing EMI energy is to ground the heat sink.
FIG. 1
illustrates an exemplary grounded heat sink. There is shown a heat sink
100
coupled to a grounded electronic device
102
that is mounted onto a printed circuit board (PCB)
104
. In this example, the grounded electronic device
102
is a processor core having a silicon logic die
106
. A thermal compound
108
, such as a dielectric material, is placed between the heat sink
100
and the grounded electronic device
102
so that thermal contact is maintained and the heat generated by the device
102
is transferred to the heat sink
100
.
Mounting fences
110
are positioned on four opposite sides of the PCB
104
in order to ground the heat sink
100
. The mounting fences
110
are attached to the heat sink
100
and the PCB
104
, which acts in this case as a ground plane. The mounting fences
110
provide a Faraday shield around the device
102
in order to shield the EMI energy generated from the clock circuitry internal to the processor
102
from damaging adjacent components and from releasing EMI energy outside of the heat sink
100
.
Additional EMI suppression is provided by four sets of grounding pads
112
a
-
112
d
on the PCB
104
that surround the processor
102
, as shown in FIG.
2
. The grounding pads
112
a
-
112
d
minimize or ground EMI noise generated by the switching of all the component pins under maximum capacitive load. Thus, the combination of the grounding pads and the grounded beat sink dissipates the heat from the processor core and grounds the EMI energy generated by the processor core.
As the internal clock speed of processors increase, these processors will require more extensive EMI reduction or grounding. In some cases, the grounded heat sink approach may not be suitable to effectively eliminate the EMI emissions since this approach has a longer grounding path. As such, grounding may need to be performed closer to the silicon logic die in order to create a shorter grounding path that effectively shields the EMI energy emanating from the processor core.
Accordingly, there is a need for an EMI reduction technique that can minimize or eliminate EMI energy generated from a processor core closer to the source of the emissions and in such a manner that is cost efficient and easy to manufacture.
SUMMARY OF THE INVENTION
The present invention pertains to an EMI reduction device that is coupled between a printed circuit board (PCB) assembly and a heat sink. The PCB assembly includes a processor core that is the source of a large amount of electromagnetic interference. In a first embodiment, the EMI reduction device is fastened to a heat sink which is positioned over the processor core such that it capacitively couples the emissions from the processor while maintaining thermal contact with the heat sink.
In a first embodiment, the EMI reduction device has a electrically-conductive rectangular frame that includes a die aperture positioned in the center of the device, four mounting clip tabs, and four spring contact fingers. The die aperture allows the processor logic die to be in direct physical contact with the bottom surface of the heat sink thereby maintaining thermal contact between the processor logic die and the heat sink. The four mounting clip tabs are positioned on the outer periphery of the reduction device and are used to clip the EMI reduction device to the heat sink.
The spring contact fingers are located on each of the four sides of the processor. The spring contact fingers are the only portion of the EMI reduction device that maintains physical contact with the PCB assembly. This is to ensure that the EMI reduction device does not protrude directly onto the PCB assembly yet remains in close proximity to the processor. As such, the EMI reduction device is not in direct electrical contact with the PCB assembly. The EMI reduction device grounds the processor emissions by capacitively coupling them to the ground plane present in the PCB assembly.
A pair of heat sink retention clips is used to secure the heat sink to the PCB assembly. The heat sink retention clips are inserted through mounting holes on the heat sink in a downward direction through the mounting holes of the EMI clip and on the PCB assembly.
The placement of the EMI clip in close proximity with the processor is beneficial since it creates a shorter grounding path thereby effectively reducing the EMI energy emanating from the processor directly at the source. In addition, the EMI clip is able to accomplish this while maintaining thermal contact with the heat sink. Furthermore, performing the EMI reduction at the processor is more economical than incorporating an EMI reduction technique into a chassis assembly that houses the PCB assembly.
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patent: 5136120 (1992-08-01), Craft et al.
patent: 5241453 (1993-08-01), Bright et al.
patent: 5278351 (1994-01-01), Herrick
patent: 5311408 (1994-05-01), Ferchau et al.
patent: 5357404 (1994-10-01), Bright et al.
patent: 5428508 (1995-06-01), Pronto
patent: 5566052 (1996-10-01), Hughes
patent: 5740013 (1998-04-01), Roesner et al.
patent: 5880930 (1999-03-01), Wheaton et al.
patent: 6043983 (2000-03-01), Taylor et al.
patent: 6053771 (2000-04-01), Hood, III et al.
Mark I. Montrose; “EMC and the Printed Circuit Board, Design, Theory, and Layout Made Simple”, pp. 70-74, 1998, IEEE; Order No. PC5756.
Chan Bertram Kim Cheong
Gardner Susannah
Mellberg Hans T.
Hewlett--Packard Company
Thompson Gregory
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