EMI reduction device and assembly

Details EMI reduction device and assembly EMI reduction device and assembly

2001-01-16

2002-05-07

Thompson, Gregory (Department: 2835)

Electricity: electrical systems and devices

Housing or mounting assemblies with diverse electrical...

For electronic systems and devices

C165S080300, C174S034000, C361S710000, C361S715000, C361S818000

Reexamination Certificate

active

06385048

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 heat 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
One aspect of the present invention provides an electronic assembly. The electronic assembly includes a circuit board including a surface and an electronic device mounted on the surface, a thermal dissipation device positioned over the surface of the circuit board and thermally contacting the electronic device, an Electromagnetic Interference (EMI) reduction device interposed between the circuit board and the thermal dissipation device, and at least one retention clip passing through the EMI reduction device and securing the thermal dissipation device to the circuit board. The EMI reduction device includes at least one compressible spacing element and contacts the circuit board and the thermal dissipation device.
In one embodiment, the at least one compressible spacing element contacts the circuit board and the thermal dissipation device.
In one embodiment, the at least one compressible spacing element has a hole defined therein. As such, the at least one retention clip passes through the hole of the at least one compressible spacing element.
In one embodiment, the at least one compressible spacing element is formed of an electrically conductive material. In one embodiment, the at least one compressible spacing element contacts a ground plane of the circuit board.
In one embodiment, the at least one compressible spacing element contacts the circuit board at a point spaced from the electronic device. As such, the EMI reduction device has no physical contact with the electronic device.
In one embodiment, the at least one compressible spacing element is a spring washer. In one embodiment, the at least one compressible spacing element includes a compressible coil.
In one embodiment, the at least one compressible spacing element includes a spring finger projecting from a surface of the EMI reduction device. As such, the EMI reduction device contacts the circuit board via the spring finger. In one embodiment, the EMI reduction device has an aperture defined therein. As such, the thermal dissipation device maintains thermal contact with the electronic device through the aperture.
In one embodiment, the electronic device is a processor.
Another aspect of the present invention provides a method of reducing Electromagnetic Interference (EMI) emissions emanating from and dissipating heat generated by an electronic device mounted on a circuit board. The method includes positioning a thermal dissipation device over the circuit board, interposing an EMI reduction device including at least one compressible spacing element between the circuit board and the thermal dissipation device, and securing the thermal dissipation device to the circuit board with at least one retention clip. As such, positioning the thermal dissipation device includes thermally contacting the electronic device with the thermal dissipation device, interposing the EMI reduction device includes contacting the circuit board and the thermal dissipation device with the EMI reduction device, and securing the thermal dissipation device includes passing the at least one retention clip through the EMI reduction device.
The present invention provides an EMI reduction device which is interposed between a printed circuit board (PCB) assembly and a heat sink. The PCB assembly includes a processor core which is the source of a large amount of electromagnetic interference. The EMI reduction device grounds the processor emissions by capacitively coupling them to the ground plane present in the PCB assembly.


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Mark I. Montrose; “EMC and the Printed Circuit Board, Design, Theory, and Layout Made Simple”, pp. 70-74, 1998, IEEE; Order No. PC 5756.

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