US20010026440A1 - EMI reduction device and assembly - Google Patents

Abstract

An EMI reduction device coupled between to a printed circuit board (PCB) assembly and a heat sink is herein disclosed. The PCB assembly includes a processor core that is the source of unintentional electromagnetic interference (EMI). The EMI reduction device is attached to a heat sink which is positioned over the processor core such that it capacitively couples emissions from the processor core to a grounding plane resident in the PCB assembly, thereby reducing the unintentional EMI. Simultaneously, the EMI reduction device is able to maintain thermal contact with the heat sink.

Description

FIELD OF THE INVENTION
• The present invention relates generally to electronic systems. More particularly, the invention relates to an electromagnetic interference (EMI) reduction device.[0001]
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.[0002]
• 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.[0003]
• 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[0004]heat sink100 coupled to a groundedelectronic device102 that is mounted onto a printed circuit board (PCB)104. In this example, the groundedelectronic device102 is a processor core having a silicon logic die106. Athermal compound108, such as a dielectric material, is placed between theheat sink100 and the groundedelectronic device102 so that thermal contact is maintained and the heat generated by thedevice102 is transferred to theheat sink100.
• [0005]Mounting fences110 are positioned on four opposite sides of the PCB104 in order to ground theheat sink100. Themounting fences110 are attached to theheat sink100 and thePCB104, which acts in this case as a ground plane. Themounting fences110 provide a Faraday shield around thedevice102 in order to shield the EMI energy generated from the clock circuitry internal to theprocessor102 from damaging adjacent components and from releasing EMI energy outside of theheat sink100.
• Additional EMI suppression is provided by four sets of grounding pads[0006]112a-112don thePCB104 that surround theprocessor102, as shown in FIG. 2. The grounding pads112 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.[0007]
• 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.[0008]
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.[0009]
• 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.[0010]
• 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.[0011]
• 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.[0012]
• 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.[0013]
BRIEF DESCRIPTION OF THE DRAWINGS
• For a better understanding of the nature and objects of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which:[0014]
• FIG. 1 is a schematic view of a prior art grounded heat sink assembly for a printed circuit board;[0015]
• FIG. 2 is a top plan view of the printed circuit board used in the grounded heat sink assembly shown in FIG. 1;[0016]
• FIG. 3 is a perspective view of a processor module in accordance with a first embodiment of the present invention;[0017]
• FIG. 4 is an exploded view of the components of the processor module shown in FIG. 3;[0018]
• FIG. 5 illustrates the processor module positioned on a printed circuit board as part of computer system in accordance with the embodiments of the present invention;[0019]
• FIG. 6 is a front perspective view of the printed circuit board assembly shown in FIGS. 3 and 4;[0020]
• FIG. 7 is a top plan view of the printed circuit board shown in FIG. 6;[0021]
• FIG. 8 is a perspective view of the bottom side of the EMI clip shown in FIGS. 3 and 4;[0022]
• FIG. 9 is a top plan view of the EMI clip positioned onto the printed circuit board assembly in accordance with a first embodiment of the present invention;[0023]
• FIG. 10 is a perspective view of the heat sink shown in FIGS. 3 and 4 in accordance with the embodiments of the present invention;[0024]
• FIG. 11 is cross-sectional view of the processor module taken along plane AA shown in FIG. 3;[0025]
• FIG. 12 is a rear perspective view of the heat sink with the EMI clip attached thereto in accordance with the first embodiment of the present invention;[0026]
• FIG. 13 is an isometric view of a printed circuit board having spring fingers mounted onto the PCB assembly in accordance with a second embodiment of the present invention; and[0027]
• FIG. 14 is an isometric view of a printed circuit board having spring washers in accordance with a third embodiment of the present invention.[0028]
• Like reference numerals refer to corresponding parts throughout the several views of the drawings.[0029]
DETAILED DESCRIPTION OF THE INVENTION
• FIGS. 3 and 4 depict a[0030]processor module120 having aPCB assembly122, an EMI clip orreduction device124, aheat sink126, and a pair of heat sink retention clips128a-128b. ThePCB assembly122 includes aprocessor core130. TheEMI clip124 is positioned onto thePCB assembly122 surrounding theprocessor core130 and is used to efficiently shield the EMI emissions generated from theprocessor core130. TheEMI clip124 is removably mounted to theheat sink126. Theheat sink126 is thermally coupled to thePCB assembly122 and is used to transfer the heat from the electronic components to theheat sink126. A pair of heat sink retention clips128 is used to fasten theheat sink126 to thePCB assembly122.
• The[0031]processor module120 can be removably mounted onto a motherboard or other type of circuit board. For example, as shown in FIG. 5, theprocessor module120 is positioned onto amotherboard127 through aprocessor retention assembly129. Themotherboard127 can be part of a computer system, subsystem, or the like.
• Preferably, the[0032]PCB assembly122 is a Single Edge Contact Cartridge 2 (SECC2) manufactured by the Intel Corporation. The SECC2 supports “slot” type processors. A slot is a connector that is resident on a motherboard and which supports the Intel P6 microprocessor bus. Previously, microprocessors were mounted onto a motherboard through a socket. In a slot-type processor, the processor is placed on a PCB known as a single edge contact (SEC). The SEC has pins on an edge of the card which is inserted into a 242-pin slot on the motherboard . This pin construction is otherwise known as an edge finger connection.
• A more detailed discussion of the SECC2 can be found in Intel Corporation, Single Edge Contact Connector 2 (S.E.C.C.2) Thermal Interface Material Functional Requirements, Order No: 244458-001, November 1998; Intel Corporation, Single Edge Contact Cartridge 2 (S.E.C.C.2) Heat Sink Attachment and Heat Sink Functional Requirements Order Number: 244456-001, Nov. 23, 1998; and Intel Corporation, S.E.C.C.2 Heat Sink Installation and Removal Process, Order Number 244454-001, December 1998 which are hereby incorporated by reference as background information.[0033]
• FIG. 6 illustrates the[0034]SECC2122. There is shown a printedcircuit board132 and acover134. ThePCB132 is preferably a substrate constructed of a multilayer plastic laminate, such as a plastic land grid array or organic land grid array processor core substrates. Thesubstrate132 has an edge finger connection. In addition, thesubstrate132 includes a ground plane. Thecover134 is used to protect thePCB132 and is removably mounted to thecover134 through a set of snaps136. However, it should be noted that the technology of the present invention is not constrained to the SECC2, to any particular circuit board, or to any particular circuit board layout.
• FIG. 7 illustrates the[0035]PCB132 in further detail. There is aprocessor core130 that includes a silicon logic die138. ThePCB132 includes four attachment holes140a-140dthrough which the heat sink retention clips128 are fit so that the heat sink can be attached to thePCB assembly122. In addition, surrounding the logic die138, there are other electronic components, such as but not limited to cache memory, resistors, capacitors, and the like. However, in this embodiment of thePCB132, there are no grounding pads.
• FIG. 8 illustrates the[0036]EMI clip124. TheEMI clip124 has rectangular-shaped frame including adie aperture150, four mounting clip tabs152a-152d, four attachment holes154a-154d, and four spring contact fingers156a-156d. Thedie aperture150 is of a rectangular shape and is located in the center of theEMI clip124. TheEMI clip124 is positioned over the logic die138 so that thedie aperture150 allows the logic die138 to be in direct physical contact with the surface of theheat sink126.
• The four mounting clip tabs[0037]152a-152dare used to attach theEMI clip124 to theheat sink126. The mounting clip tabs152a-152dare located on the outer periphery of theEMI clip124 and are situated in each corner of the top side of theEMI clip124.
• The mounting holes[0038]154a-154dare located on the surface of the top side of theEMI clip124 and each mounting hole154 is located at a particular corner of theEMI clip124. Each of the four mounting holes154a-154dare aligned with a respective mounting hole140 on thePCB132 and with a respective mounting hole on the heat sink. The retention clips128 are inserted through the mounting holes166 on theheat sink126 in a downward direction through the mounting holes154 on theEMI clip124 through the mounting holes140 on thePCB132.
• The four spring contact fingers[0039]156a-156dare situated on the bottom side of theEMI clip124 and allow theEMI clip124 to maintain physical contact on four sides of theprocessor core130. The contact fingers156 are the only portion of theEMI clip124 that is in physical contact with thePCB132. The spring fingers156 ensure that the frame of theEMI clip124 does not protrude directly onto any of the components of thePCB132. Direct contact with theprocessor substrate132 would damage the electronic components positioned under theEMI clip124. As such, each of the spring contact fingers156 is positioned on thePCB132 in an area where there are no electronic components.
• The shape, size and dimension of the[0040]EMI clip124 is tailored for theparticular processor substrate132 to which theclip124 is coupled. FIG. 9 illustrates the placement of theEMI clip124 onto aSECC2122. For this embodiment, theEMI clip124 is approximately 48.97 mm by 54.35 mm. Thedie aperture150 is 19 mm by 18 mm and is constructed such that there is a 6 mm distance between the outer edge of the processor aperture to the outer edge of the processor logic die. It should be noted that the present invention is not constrained to anEMI clip124 having the particular shape, size and dimensions described herein and one skilled in the art can easily modify the design of theEMI clip124 to suit other PCBs or the like.
• The electromagnetic bandwidth of the processor core can extend to include multiple times the advertised processor clock speed. In the case of an Intel Pentium II processor with an internal clock speed of 500 MHz, the EMI bandwidth can extend to 2500 MHz or more. The[0041]EMI clip124 reduces portions of that bandwidth with varying shielding effectiveness and can vary depending on the mechanical dimensions of theEMI clip124. Another modifier to EMI reduction is the mechanical pressure that exits both on theheat sink126 and on theprocessor core130. The higher the pressure the more effective the capacitive coupling becomes and hence the EMI reduction.
• Preferably, the[0042]EMI clip124 is constructed of thin steel sheet metal. However, the present invention is not constrained to any particular type of metal and can be constructed of any material, such as but not limited to stainless steel, beryllium copper, phospher bronze, hardened steel, spring steel, and the like.
• Referring to FIG. 12, the bottom side of the[0043]heat sink126, that side which touches thePCB132, is coated with a dielectric layer that ensures that theEMI clip124 does not short the electronic components mounted on the surface of thePCB132. An example of such a dielectric coating is powder paint, in particular, the epoxy power coatings based on epoxy resins.
• The placement of the[0044]EMI clip124 over theprocessor core130 and in the manner described above is effective in reducing the EMI emissions from theprocessor core130. Although, theEMI clip124 is not in direct electrical contact with theprocessor core130, its close proximity to theprocessor core130 capacitively couples these emissions to the ground plane of thesubstrate132 and through a shorter grounding path. As a result, the emissions are minimized or eliminated directly at theprocessor core130.
• FIGS. 10 and 11 illustrates the[0045]heat sink126. Theheat sink126 has a number offins160, abase portion162, and two skirts164a-164b. Thefins160 project outwardly and upwardly from thebase portion162. Thebase portion162 extends over and covers theprocessor core130 thereby providing the maximum amount of thermal contact area between theprocessor core130 and theheat sink126. The two skirts164a-164bextend laterally and downwardly from thebase portion162 and extends over and covers opposite sides of thePCB assembly122.
• The[0046]fins160 are eliminated where the heat sink retention clips128 are positioned (see FIGS. 3 and 4) into two groves situated on the top surface of the base portion of theheat sink126. The groves include mounting holes166 that enable the heat sink retention clips128 to fit through the top surface of thebase portion162. The retention clips128 are inserted through the mounting holes166 on theheat sink126 in a downward direction through the mounting holes154 on theEMI clip124 through the mounting holes140 on thePCB132.
• The heat sink retention clips[0047]128 are made from a flexible material that is non-electrically conductive, such as plastic. The heat sink retention clips128 are secured at one end in any appropriate manner, and in the present example by barbed ties. Preferably, the heat sink retention clips128 are those heat sink retention clips designed for the SECC2 and which are manufactured by ITW Fastex.
• FIG. 12 illustrates the bottom side of the[0048]base portion162 of theheat sink126. There is shown theEMI clip124 mounted to thebase portion162 of theheat sink126. Each mounting clip152 is fitted over onto a corresponding groove170 on the base portion of theheat sink126.
• The[0049]heat sink126 is made of aluminum that is chromate conversion coated and is electrically conductive. Athermal compound168, such as a dielectric layer, is placed on the bottom side of thebase portion162 of theheat sink126. This dielectric layer is used to provide high dielectric capacity and to provide a high thermal conductivity between the processor logic die138 and theheat sink126. An example of such a compound is MCM-STRATE® manufactured by Power Devices, Inc. However, the present invention is not constrained to any particular type of material and other materials having the same properties can also be used.
• The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. In other instances, well known circuits and devices are shown in order to avoid unnecessary distraction from the underlying invention. Thus, the foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, obviously many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.[0050]
• In particular, one skilled in the art can alter the EMI clip described above to include additional spring fingers in order to provide more capacitive contacts between the EMI clip and the substrate. In another alternate embodiment, the EMI clip can be used in conjunction with grounding pads that are positioned on the substrate. In yet another embodiment, the EMI clip can be made an integral part of the heat sink and not a separate device. Furthermore, the EMI clip can be made without the dielectric coating on the bottom surface thereby making electrical contact with the PCB. Alternatively, the EMI clip can be applied to a processor that is mounted directly onto a motherboard and is not constrained to edge finger connection devices.[0051]
• FIG. 13 illustrates a second embodiment of the present invention. In this embodiment, there is no[0052]EMI clip124 rather spring fingers180a-180dmounted directly onto the printedcircuit board132′. Preferably, a spring finger180 is mounted on each side surrounding theprocessor core130′. As shown in FIG. 13, there are four spring fingers180a-180dmounted directly onto thecircuit board132′. However, the number of spring fingers180 and their position of thecircuit board132 is not a limitation of this embodiment.
• A heat sink is mounted directly over the[0053]circuit board132′ and coupled with retention clips in the manner described above. The bottom surface of the heat sink will be in direct physical contact with the processor die138′ and the top surface of the spring fingers180. As such, the emissions from the processor die138′ are capacitively coupled to the ground plane in thecircuit board132′ and suppressed directly at the source of the emissions.
• FIG. 14 illustrates a third embodiment of the present invention. In this embodiment, the ground plane of the[0054]circuit board132″ is extended to and through the mountingholes140a″-140d″ on thecircuit board132″. A spring washer182a-182dis mounted in each mountinghole140a″-140d″. A heat sink is mounted over the top surface of thecircuit board132″.
• The spring washers[0055]182 are formed of an electrically conductive material, such as but not limited to thin sheet steel metal and are shaped as a compression coil. The spring washers182 serve to ground the emissions from theprocessor core130″ while ensuring that the placement of the heat sink on top of thecircuit board132″ does not damage theprocessor core130″ and the electronic components mounted thereon.
• Each spring washer[0056]182 has a hollow core that is wide enough to allow a retention clip to fit through it thereby allowing the heat sink to be fastened to thecircuit board132″ in the manner described above. When the heat sink is fastened to thecircuit board132″, the spring washers182 are compressed by the weight of the heat sink and the bottom surface of the heat sink is in direct physical contact with the processor die138″ and the top surface of the spring washers182. As such, the emissions from the processor die138″ are grounded to the ground plane in thecircuit board132″ and suppressed directly at the source of the emissions.

Claims (15)

In the claims:

1. An electronic assembly apparatus, comprising:

a circuit board having a top surface and a bottom surface, the circuit board including an electronic device emanating electromagnetic emissions;

an EMI reduction device that is coupled to the circuit board, the EMI reduction device is positioned around the outer periphery of the electronic device, the EMI reduction device touches the circuit board at contact points that surround the electronic device, the EMI reduction device has no physical contact with the electronic device; and

a thermal dissipation device that is coupled to the EMI reduction device and the circuit board, the thermal dissipation device is positioned over the top surface of the circuit board;

wherein the thermal dissipation device dissipates heat generated from the circuit board;

wherein the EMI reduction device reduces electromagnetic emissions generated from the electronic device.

2. The apparatus of

claim 1

,

the EMI reduction device including at least one mounting clip for attaching the EMI reduction device to the thermal dissipation device.

3. The apparatus of

claim 1

,

the EMI reduction device including a plurality of spring fingers that surround the electronic device and maintain physical contact with the circuit board.

4. The apparatus of

claim 1

,

the EMI reduction device including an aperture that is positioned over the electronic device;

wherein the aperture enables the electronic device to maintain thermal contact with the thermal dissipation device.

5. The apparatus of

claim 1

,

the thermal dissipation device having a top surface and a bottom surface, the bottom surface including a thermal compound;

wherein the thermal compound is coupled to the electronic device.

6. The apparatus of

claim 1

,

the EMI reduction device having a top surface and a bottom surface, the bottom surface having a dielectric layer.

7. An EMI reduction device for use with an electronic device mounted on a circuit board, comprising:

a frame that is coupled to the circuit board, the frame surrounds peripheral portions of the electronic device, the frame is not in physical contact with the electronic device, the frame having a top surface and a bottom surface; and

a plurality of spring fingers that are coupled to the circuit board, the spring fingers are positioned onto the circuit board at points that surround the electronic device.

8. The apparatus of

claim 7

, comprising:

an aperture that surrounds the electronic device;

wherein the aperture enables the electronic device to maintain thermal contact with a thermal dissipation device.

9. The apparatus of

claim 7

, comprising:

a fastener that is coupled to a thermal dissipation device;

wherein the fastener enables the EMI reduction device to be detachably mounted to a thermal dissipation device.

10. The apparatus of

claim 7

,

wherein the electronic device is a processor.

11. The apparatus of

claim 7

,

wherein the bottom surface of the frame is non-electrically conductive.

12. The apparatus of

claim 7

,

wherein the top surface of the frame is electrically conductive.

13. A method for reducing EMI emissions emanating from an electronic device having a ground plane, said method comprising the steps of:

surrounding the outer periphery of the electronic device with a frame that has no direct physical contact with the electronic device;

contacting the frame to the ground plane at contact points that surround the electronic device; and

capacitively coupling the EMI emissions from the electronic device to the ground plane.

14. The method of

claim 13

, comprising the steps of:

placing a thermal dissipation device over the electronic device; and

dissipating heat generated from the electronic device through the thermal dissipation device.

15. The method of

claim 14

, comprising the step of:

attaching the frame to the thermal dissipation device.

Images