# Patent

## Title
Method and apparatus for grounding a heat sink in thermal contact with an electronic component using a grounding spring having multiple-jointed spring fingers

## Abstract

A grounding spring for electromagnetic interference (EMI) suppression is interposed between a heat sink and a printed circuit board (PCB). The grounding spring comprises a conductive material having an opening formed at its base through which the heat sink makes thermal contact with an electronic module mounted on the PCB. The base makes electrical contact with a peripheral surface of the heat sink, and multiple-jointed spring fingers extend from the base to make electrical contact with conductive pads on the PCB. During compression, the movement of each spring finger's tip is substantially limited to the z-axis. Accordingly, the final installed location of the tip can be precisely controlled even when the grounding spring must accommodate a wide variety of installed heights of the heat sink relative to the PCB. Preferably, the spring fingers terminate with a concave tip that is less susceptible to sliding off the conductive pads.

## Background
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates in general to the field of electronic packaging. More particularly, the present invention relates to electronic packaging that grounds a heat sink used to remove heat from an electronic component.
2. Background Art
Electronic components, such a microprocessors and integrated circuits, must operate within certain specified temperature ranges to perform efficiently. Excessive heat degrades electronic component performance, reliability, life expectancy, and can even cause failure. Heat sinks are widely used for controlling excessive heat. Typically, heat sinks are formed with fins, pins or other similar structures to increase the surface area of the heat sink and thereby enhance heat dissipation as air passes over the heat sink. In addition, it is not uncommon for heat sinks to contain high performance structures, such as vapor chambers and/or heat pipes, to further enhance heat transfer. Heat sinks are typically formed of metals, such as copper or aluminum.
Electronic components are generally packaged using electronic packages (i.e., modules) that include a module substrate to which the electronic component is electronically connected. In some cases, the module includes a cap (i.e., a capped module) which seals the electronic component within the module. In other cases, the module does not include a cap (i.e., a bare die module). In the case of a capped module, a heat sink is typically attached with a thermal interface between a bottom surface of the heat sink and a top surface of the cap, and another thermal interface between a bottom surface of the cap and a top surface of the electronic component. In the case of a bare die module, a heat sink is typically attached with a thermal interface between a bottom surface of the heat sink and a top surface of the electronic component. Heat sinks are attached to modules using a variety of attachment mechanisms, such as adhesives, clips, clamps, screws, bolts, barbed push-pins, load posts, and the like.
In addition to generating heat, electronic components also generate electromagnetic radiation. The electromagnetic radiation emitted by electronic components can cause electromagnetic interference (EMI) or noise in neighboring electronic components and systems. Regulatory bodies in the U.S. and in other countries set forth regulations that specify limits for EMI caused by electronic products. Because of these regulations and the negative effects of EMI, it is desirable to design electronic products so that EMI is suppressed (i.e., electromagnetic radiation generated within electronic products is minimized or effectively contained). EMI suppression is extremely important when an EMI producing electronic component is coupled to a heat sink. The heat sink acts as an antenna to further radiate the electromagnetic radiation generated by the electronic component.
It is not uncommon for a heat sink coupled to an EMI producing electronic component to be grounded for EMI suppression. For example, a grounding spring having an aperture that surrounds the EMI producing electronic component may electronically couple the heat sink and to a ground plane of a printed circuit board (PCB) on which the electronic component is mounted.
FIG. 1 illustrates, in exploded view, an exemplary heat sink assembly 100. A heat sink 102 includes a central surface 104, which is to be placed in thermal contact with a module having an EMI producing electronic component, such as a microprocessor. Heat sink 102 also includes a peripheral surface 106 on which is mounted a grounding spring 110. Grounding spring 110 includes a central aperture 1112, through which the module makes thermal contact with central surface 104 of heat sink 102. Grounding spring is made of an elastic and electrically conductive material. Four screws 120 extend through through-holes in grounding spring 110 and are threaded into threaded-holes in heat sink 102, to thereby secure and electrically connect grounding spring 110 to heat sink 102.
Eight spring fingers 114 extend from grounding spring 110. The spring fingers 114 are formed by cutting and bending grounding spring 110. Two spring fingers 114 extend from each side of aperture 112 so that the module will be surrounded thereby. The assembly 100 is attached to the module by dispensing a thermally conductive adhesive between the top of surface of the module and central surface 104 of heat sink 102. When heat sink assembly 100 is attached to the module, the tips of spring fingers 114 contact solder pads on the surface of the PCB on which the module is mounted. The solder pads are electrically connected to a ground plane of the PCB, and thus heat sink 102 is grounded thereto through the grounding spring 110.
Conventional grounding springs with single-jointed spring fingers, such as grounding spring 110 having single-jointed spring fingers 114, exhibit a number of disadvantages. Spring fingers 114, which are fragile because their length is long relative to their width, can be damaged during handling of heat sink assembly 100. In addition, the relatively long length of spring fingers 114 makes them susceptible to sliding off the solder pads on the surface of the PCB on which the module is mounted, both during and subsequent to the card assembly process (i.e., the process by which the heat sink assembly 100 is installed onto the PCB).
Another disadvantage of conventional grounding springs with single-jointed spring fingers is shown in FIG. 2, which illustrates, in perspective view, a portion of heat sink assembly 100 and a portion of a PCB 210 where the tip of one spring finger 114 engages PCB 210. As mentioned earlier with respect to FIG. 1, spring finger 114 extends from grounding spring 110, which is attached to peripheral surface 106 of heat sink 102. Typically, spring fingers 114 are relatively long to accommodate a range of installed heights of heat sink 102 relative to PCB 210. For example, the height at which heat sink 102 is installed relative to PCB 210 may differ from card assembly to card assembly due to mechanical production tolerances and the like. Unfortunately, as the installed height of heat sink 102 varies relative to PCB 210, the installed location of the tip of each spring finger 114 slides along the surface of PCB 210. Accordingly, it is difficult to precisely control the final installed location of the tips of spring fingers 114. As shown in FIG. 2, the final installed location of the tip of each spring finger 114 relative to PCB 210 will generally fall along an arc 212 (i.e., the length of spring finger 114 forms the radius of the arc). The longer the length of spring fingers 114, the more difficult it is to control the final installed location of the tips thereof.
It should therefore be apparent that a need exists for an enhanced mechanism for grounding a heat sink.
SUMMARY OF THE INVENTION
According to the preferred embodiments of the present invention, a grounding spring for electromagnetic interference (EMI) suppression is interposed between a heat sink and a printed circuit board (PCB). The grounding spring includes a plurality of multiple-jointed spring fingers, i.e., each spring finger has two or more joints. The grounding spring comprises a conductive material having an opening formed in a generally planar base portion thereof through which the heat sink makes thermal contact with an electronic module mounted on the PCB. The base portion of the grounding spring makes electrical contact with a peripheral surface of the heat sink, and the multiple-jointed spring fingers extend from the base portion of the grounding spring to make electrical contact with conductive pads on the PCB. Each multiple-jointed spring finger is configured so that movement of a tip thereof is substantially limited to the z-axis during compression. Accordingly, the final installed location of the tip can be precisely controlled even when the grounding spring must accommodate a wide variety of installed heights of the heat sink relative to the PCB. Preferably, the spring fingers terminate with a concave tip that makes it less susceptible to sliding off the conductive pads.
The foregoing and other features and advantages of the present invention will be apparent from the following more particular description of the preferred embodiments of the present invention, as illustrated in the accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
The preferred exemplary embodiments of the present invention will hereinafter be described in conjunction with the appended drawings, where like designations denote like elements.
FIG. 1 is a perspective exploded view of a conventional heat sink assembly having a grounding spring with single-jointed spring fingers.
FIG. 2 is a partial, perspective assembled view of the conventional heat sink assembly shown in FIG. 1 in an area where a single-jointed spring finger makes contact with a printed circuit board (PCB).
FIG. 3 is a perspective exploded view of a heat sink assembly having a grounding spring with double-jointed spring fingers according to the preferred embodiments of the present invention.
FIG. 4 is a partial, perspective enlarged view of the grounding spring shown in FIG. 3 in an area of a double-jointed spring finger according to the preferred embodiments of the present invention.
FIG. 5 is a partial, perspective assembled view of the heat sink assembly shown in FIG. 3 in an area where a double-jointed spring finger makes contact with a PCB.
FIG. 6 is side view of a circuit card assembly having a grounding spring with double-jointed spring fingers interposed between a heat sink and a PCB according to the preferred embodiments of the present invention.
FIG. 7 is a flow diagram of a method for grounding a heat sink in thermal contact with an electronic module using a grounding spring with multiple-jointed spring fingers according to the preferred embodiments of the present invention.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
1.0 Overview
In accordance with the preferred embodiments of the present invention, a grounding spring for electromagnetic interference (EMI) suppression is interposed between a heat sink and a printed circuit board (PCB). The grounding spring includes a plurality of multiple-jointed spring fingers, i.e., each spring finger has two or more joints. The grounding spring comprises a conductive material having an opening formed in a generally planar base portion thereof through which the heat sink makes thermal contact with an electronic module mounted on the PCB. The base portion of the grounding spring makes electrical contact with a peripheral surface of the heat sink, and the multiple-jointed spring fingers extend from the base portion of the grounding spring to make electrical contact with conductive pads on the PCB. Each multiple-jointed spring finger is configured so that movement of a tip thereof is substantially limited to the z-axis during compression. Accordingly, the final installed location of the tip can be precisely controlled even when the grounding spring must accommodate a wide variety of installed heights of the heat sink relative to the PCB. Preferably, the spring fingers terminate with a concave tip that makes it less susceptible to sliding off the conductive pads.
2.0 Detailed Description
With reference to the figures and in particular FIG. 3, there is depicted, in exploded view, a heat sink assembly 300 having a grounding spring 302 with double-jointed spring fingers 304 in accordance with the preferred embodiments of the present invention. The spring fingers 304 shown in FIG. 3 are “double-jointed” in that they have two joints. It should be appreciated, however, that the present invention is not limited to double-jointed spring fingers as shown FIG. 3. In accordance with the preferred embodiments of the present invention, the spring fingers may have any multiple-jointed configuration.
A heat sink 310 includes a central surface 312, which is to be placed in thermal contact with a module having an electromagnetic interference (EMI) producing electronic component, such as a microprocessor. Heat sink 310 also includes a peripheral surface 314 on which is mounted grounding spring 302. Grounding spring 302 includes a central aperture 306, through which the module makes thermal contact with central surface 312 of heat sink 310. Preferably, four screws 320 extend through through-holes in grounding spring 302 and are threaded into threaded-holes in heat sink 310, to thereby secure and electrically connect grounding spring 302 to heat sink 310. Any number or arrangement of screws may be used, or, alternatively, other attachment mechanisms (e.g., adhesives, welds, clips, clamps, bolts, barbed push-pins, load posts, and the like) may be used to secure grounding spring 302 between heat sink 310 and the printed circuit board (PCB) on which the module is mounted. For example, grounding spring 302 may be secured between heat sink 310 and the PCB by barbed push-pins that pass through through-holes in heat sink 310, grounding spring 302 and the PCB. In another alternative, grounding spring 302 may be secured between heat sink 310 and the PCB by load posts projecting from heat sink 310 and passing through through-holes in grounding spring 302, the PCB, a backside bolster, and a loading spring. Numerous other attachment mechanisms known in the art may be used. Those skilled in the art will appreciate that the spirit and scope of the present invention is not limited to any one attachment mechanism.
As illustrated, peripheral surface 314 of heat sink 310 is preferably recessed relative to central surface 312 of heat sink 310 to provide clearance for the heads of screws 320. However, those skilled in the art will appreciate that the mechanisms and apparatus of the preferred embodiments of the present invention apply equally regardless of the configuration of the heat sink.
Heat sink 310 is formed of a metal, such as aluminum, copper, or the like. As is typical, heat sink 310 is formed with fins, pins or other similar structures to increase the surface area and thereby enhance heat dissipation as air passes over the heat sink. In addition, heat sink 310 may contain high performance structures, such as vapor chambers and/or heat pipes, to further enhance heat transfer.
Grounding spring 302 is made of an elastically deformable and electrically conductive material, such as stainless steel, beryllium copper, phosphor bronze, hardened steel, spring steel, and the like. Preferably, grounding spring 302 is cut and bent from a sheet of 0.004′′ half-hard “301” stainless steel.
Preferably, eight double-jointed spring fingers 304 extend from grounding spring 302. Two double-jointed spring fingers 304 preferably extend form each side of central aperture 306 in grounding spring 302 so that the module will be surrounded thereby. However, any number or arrangement of double-jointed spring fingers 304 may be used. When heat sink assembly 300 is attached to the module, the tips of double-jointed spring fingers 304 contact conductive pads on the surface of the PCB on which the module is mounted. The conductive pads are electrically connected to a ground plane of the PCB, and thus heat sink 310 is grounded thereto through the grounding spring 302.
The heat sink assembly 300 is preferably attached to the module by dispensing a conventional thermally conductive adhesive between the top surface of the module and central surface 312 of heat sink 310. However, a thermally conductive adhesive need not be used. Instead, a thermally conductive interface such as a thermal gel, grease, paste, oil, pad or other high thermal conductivity material may be interposed between heat sink 310 and the module in the case where another attachment mechanism is used in lieu of thermally conductive adhesive to secure heat sink 310 relative to the PCB on which the module is mounted. For example, heat sink 310 may be secured relative to the PCB using barbed push-pins or load posts attachment mechanisms, as discussed above. However, because of the additional labor and parts required to utilize the other attachment mechanisms, electronics device manufacturers often prefer utilizing adhesive-mounted heat sinks in order to minimize production costs.
FIG. 4 illustrates, in a partial, perspective enlarged view, grounding spring 302 in an area of one of double-jointed spring fingers 304. Double-jointed spring finger 304 includes a first v-shaped joint 402 connecting a generally planar base portion 404 to a central finger portion 406, and a second v-shaped joint 408 connecting central finger portion 406 to a distal finger portion 410. In addition, distal finger portion 410 preferably includes a concave finger tip 416 extending in a direction substantially parallel to base portion 404. Preferably, base portion 404, central finger portion 406, distal portion 410, and concave finger tip 416 are cut and bent from a single piece of sheet metal to form a one-piece unit of unitary construction. For example, the base portion 404 may include a cut-out 412 from which central finger portion 406 is formed. Similarly, central finger portion 406 may include a cut-out 414 from which distal finger portion 410 and concave finger tip 416 are formed. The cut-out may be in the form of a hole between two legs as illustrated in FIG. 4, or, alternatively, the cut-out may be in the form of a notch adjacent to a single leg. In another alternative, double-jointed spring fingers 304 may be separately formed from base portion 404 and subsequently attached by a conventional fastening means such as welds, rivets, etc. In yet another alternative, double-jointed spring fingers 304 may be directly attached as individual units to the heat sink.
Preferably, distal finger portion 410 and concave finger tip 416 are about 2.0 mm wide, which is about the same width as the conventional grounding spring's single-jointed spring finger. Central finger portion 406 is preferably about 5.0 mm wide, which is much wider than the width of the conventional grounding spring's single-jointed spring finger. As discussed below, this wider stance makes the double-jointed spring finger in accordance with the preferred embodiments of the present inventions less fragile and more robust and stable than the conventional grounding spring's single-jointed spring finger.
FIG. 5 illustrates, in perspective view, a portion of heat sink assembly 300 and a portion of a PCB 510 where concave finger tip 416 of one double-jointed spring finger 304 engages PCB 510. As mentioned earlier with respect to FIG. 3, double-jointed spring finger 304 extends form grounding spring 302, which is attached to peripheral surface 314 of heat sink 310. As shown in FIG. 5, the final installed location of concave finger tip 416 of each of the double-joined spring fingers 304 relative to PCB 510 will generally fall along a line 512 (i.e., the movement of concave finger tip 416 is substantially limited to what is commonly referred to as the “z-axis” during compression). Accordingly, the final installed location of the tip can be precisely controlled even when the grounding spring must accommodate a wide variety of installed heights of the heat sink relative to the PCB. This compares favorably with single-jointed spring finger 114 of conventional grounding spring 110 shown in FIG. 2. As shown in FIG. 2, the final installed location of the tip of single-jointed spring finger 114 relative to PCB 210 will generally fall along an arc 212 (i.e., the length of single-jointed spring finger 114 forms the radius of the arc). This makes the final installed location of the tip of a conventional grounding spring's single-jointed spring finger difficult to precisely control when the conventional grounding spring must accommodate a wide variety of installed heights of the heat sink relative to the PCB.
Grounding springs in accordance with the preferred embodiments of the present invention exhibit several other advantages over conventional grounding springs. For example, a conventional grounding spring's spring fingers, which are fragile because their length is long relative to their width, can be damaged during handling of the heat sink assembly. Grounding springs in accordance with the preferred embodiments of the present invention are less susceptible to being damaged during handling of the heat sink assembly because the double-jointed spring fingers are not as fragile as the single-jointed spring fingers. For example, the narrowest portions of double-jointed spring fingers (i.e., distal finger portion 410 and concave finger tip 416) are shorter relative to their width as compared to the length of single-jointed spring fingers relative to their width. This also makes the tip of the double-jointed spring finger less susceptible to sliding off the conductive pads on the PCB. The relatively long length of a conventional grounding spring's single-jointed spring fingers makes them susceptible to sliding off the conductive pads, both during and subsequent to the card assembly process. For example, a conventional grounding spring's single-jointed spring fingers may slide off the conductive pads in the field subsequent to the card assembly process as the result of being subjected to shock or vibration, or as the result of being bumped by a human finger during maintenance. In this regard, the preferred embodiments of the present invention incorporate concave finger tip 416 that makes it even less susceptible to sliding off the conductive pads of the PCB.
FIG. 6 illustrates, in a side view, a circuit card assembly 600 having a grounding spring 302 with double-jointed spring fingers 304 interposed between a heat sink 310 and a PCB 605 in accordance with the preferred embodiments of the present invention. Only two double-jointed spring fingers 304 are shown in FIG. 6 for the sake of clarity. A bare die module 610 includes a module substrate 615, an electronic component such as a semiconductor chip 620, and an electronic connection 625. Electronic connection, which electrically connects PCB 605 to module substrate 615, may be a pin grid array (PGA), a ceramic column grid array (CCGA), a land grid array (LGA), or the like. Semiconductor chip 620 is thermally connected with heat sink 310 through a thermal interface 635, which is preferably a layer of thermally conductive adhesive that also secures heat sink assembly 300 relative to bare die module 610 and PCB 605. PCB 605 includes conductive pads, such as solder pads 640, connected to a ground plane (not shown) in PCB 605. Concave finger tips 416 of double-jointed spring fingers 304 engage solder pads 640, preferably in a “mating” ball-in-socket fashion to reduce the likelihood of concave finger tips 416 sliding off of solder pads 640. Solder pads 640 are located along the side edges of bare die module 610. Preferably, a backside bolster plate 645 is positioned adjacent PCB 605 with an insulating layer (not shown) interposed therebetween.
FIG. 7 is a flow diagram of a method 700 for grounding a heat sink in thermal contact with an electronic module using a grounding spring with multiple-jointed spring fingers according to the preferred embodiments of the present invention. Method 700 sets forth the preferred order of the steps. It must be understood, however, that the various steps may occur at any time relative to one another. An electronic module is soldered to a PCB (step 710). A grounding spring having a plurality of multiple-jointed spring fingers is attached to a heat sink to form a heat sink assembly (step 720). A thermally conductive adhesive is dispensed on the module (step 730). Finally, the heat sink assembly is attached to the module, with concave finger tips of the multiple-jointed spring fingers engaging conductive pads on the surface of the PCB (step 740).
One skilled in the art will appreciate that many variations are possible within the scope of the present invention. For example, the heat sink may be mounted by utilizing another attachment mechanism in lieu of a thermally conductive adhesive. Moreover, the grounding spring may be held in place between the heat sink and the PCB using another attachment mechanism in lieu of fastening the grounding spring to the heat sink with screws. In addition, other multiple-jointed spring fingers, such as triple-jointed spring fingers, may be utilized in lieu of double-jointed spring fingers. Thus, while the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that these and other changes in form and detail may be made therein without departing form the spirit and scope of the present invention.

## Claims

1. A heat sink grounding spring, comprising:
a base comprising a conductive material having a generally planar portion, wherein the generally planar portion of the base is configured to contact at least a portion of a peripheral surface of a heat sink;
a multiple-jointed spring finger comprising a conductive material extending from the generally planar portion of the base, wherein the multiple-jointed spring finger is double-jointed and includes a first v-shaped joint connecting the generally planar portion of the base to a central finger portion, and a second v-shaped joint connecting the central finger portion to a distal finger portion, and wherein the generally planar portion of the base includes a cut-out from which are formed the central finger portion, and the central finger portion includes a cut-out from which is formed the distal finger portion.

2. A heat sink grounding spring, comprising:
a base comprising a conductive material having a generally planar portion, wherein the generally planar portion of the base is configured to contact at least a portion of a peripheral surface of a heat sink;
a multiple-jointed spring finger comprising a conductive material extending from the generally planar portion of the base to form a generally Z-shaped spring element, wherein the multiple-jointed spring finger is double-jointed and includes a first v-shaped joint connecting the generally planar portion of the base to a central finger portion, and a second v-shaped joint connecting the central finger portion to a distal finger portion, and wherein the distal finger portion includes a concave finger tip extending in a direction substantially parallel to the generally planar portion of the base, the concave finger tip having a concave surface configured to receive a portion of a solder ball.

3. The heat sink grounding spring as recited in claim 2, wherein the multiple-jointed spring finger is configured so that movement of the concave finger tip thereof is substantially limited to a direction perpendicular to the generally planar portion of the base as the multiple-jointed spring finger is compressed toward the generally planar portion of the base.

4. A circuit card assembly, comprising:
a printed circuit board having an electronic module mounted on a surface thereof;
a heat sink disposed over the surface of the printed circuit board and in thermal contact with the module;
a grounding spring comprising a conductive material having an opening formed in a generally planar base portion thereof through which the heat sink and the module are in thermal contact with each other, wherein the base portion makes electrical contact with at least a portion of a peripheral surface of the heat sink, wherein a plurality of multiple-jointed spring fingers extend from the base portion to make electrical contact with conductive pads on the surface of the printed circuit board, wherein each of the multiple-jointed spring fingers is double-jointed and includes a first v-shaped joint connecting the base portion of the grounding spring to a central finger portion, and a second v-shaped joint connecting the central finger portion to a distal finger portion, and wherein the base portion of the grounding spring includes a plurality of cut-outs from which are formed the central finger portions, and each central finger portion includes a cut-out from which is formed the distal finger portion.

5. A method for grounding a heat sink in thermal contact with an electronic module, comprising the steps of:
providing a printed circuit board having an electronic module mounted on a surface thereof;
disposing a heat sink over the surface of the printed circuit card and in thermal contact with the module;
interposing a grounding spring between the printed circuit board and the heat sink, wherein the grounding spring comprises a conductive material having an opening formed in a generally planar base portion thereof through which the heat sink and the module are in thermal contact with each other, wherein the base portion of the grounding spring makes electrical contact with at least a portion of a peripheral surface of the heat sink, wherein a plurality of multiple-jointed spring fingers extend from the base portion of the grounding spring to form a plurality of generally Z-shaped spring elements that make electrical contact with conductive pads on the surface of the printed circuit board, wherein each of the multiple-jointed spring fingers is double-jointed and includes a first v-shaped joint connecting the base portion of the grounding spring to a central finger portion, and a second v-shaped joint connecting the central finger portion to a distal finger portion, wherein the distal finger portion includes a concave finger tip extending in a direction substantially parallel to the base portion of the grounding spring, and wherein the interposing step includes a step of engaging each concave finger tip with one of the conductive pads on the surface of the printed circuit board.