METHOD AND APPARATUS FOR HEAT SINK MOUNTING

Abstract

A method is disclosed of mounting a heat sink through a printed circuit board to reach a component on the opposite side of a board. The heat sink is passed through a window in the board to contact a component at a predetermined pressure optimized for thermal performance at minimum stress. The heat sink is affixed in place on the printed circuit board using through-hole pins which can be soldered to maintain the heat sink's position and the predetermined pressure.

Claims

1. A method of mounting to a substrate a heat sink member in contact with a first component residing on the substrate, the method comprising: inserting mounting pins of the heat sink member coaxially into pin receiving openings of the substrate, the axes of the mounting pins and receiving openings oriented in a first direction; bringing a contact surface of the heat sink member into abutment with the first component at a first pressure, wherein displacement of the heat sink member in the first direction is limited solely by contact with the first component; and rigidly affixing the mounting pins inside the receiving openings such that contact between the first component and heat sink member at the first pressure is maintained.

2. The method of claim 1 wherein the first pressure is a predetermined pressure selected to enhance heat transfer characteristics of a thermal interface material interposed between the first component and heat sink member.

3. The method of claim 1 wherein the heat sink member is configured to exert zero force directly on the substrate in the direction of insertion while exerting the first pressure on the first component.

4. The method of claim 1 wherein the heat sink member is a first heat sink comprising one contiguous metal body.

5. The method of claim 2 wherein the thermal interface material is preapplied in a sheet to the heat sink member before abutting the first component.

6. The method of claim 2 wherein the thermal interface material is less than 50 microns in thickness.

7. The method of claim 2 wherein the thermal interface material is a thermal grease, phase change material, thermal gel, or thermal epoxy.

8. The method of claim 5 wherein the thermal interface material is a gap filling thermal pad, and the first pressure causes a compression of the gap filling thermal pad.

9. The method of claim 4 wherein the contiguous metal body includes copper or aluminum.

10. The method of claim 4 wherein the contiguous metal body is formed from stamped sheet metal.

11. The method of claim 4 wherein the contiguous metal body is formed by casting, cold forging, and/or machining.

12. The method of claim 4 wherein the contiguous metal body is configured to be fastened to a second bulk heat sink.

13. The method of claim 4 wherein the mounting pins are integral to the contiguous metal body.

14. The method of claim 12 wherein the fastening to the second bulk heat sink comprises fastening using screws or a barb coupling.

15. The method of claim 1 wherein the rigidly affixing the mounting pins inside the pin receiving openings is by soldering.

16. The method of claim 1 wherein bringing a contact surface of the heat sink member into abutment with the first component is through an access opening provided in the substrate.

17. The method of claim 16 wherein contacting the heat sink member to the first component through the access opening in the substrate prevents the heat sink member from shielding RF transmissions in or out of the first component.

18. The method of claim 1 wherein the first component is joined to the substrate via a Ball Grid Array, and the rigidly affixing the mounting pins inside the pin receiving openings creates a load-bearing path which redirects loads placed on the contiguous metal body away from the Ball Grid Array and into the substrate.

19. A circuit board comprising: a substrate comprising a first surface, and a second surface substantially parallel to the first surface, the substrate having an access opening spanning in a first direction from the second surface to the first surface of the substrate; an integrated circuit, aligned with and covering at least a portion of the access opening, the integrated circuit joined electrically to the first surface of the substrate; a contiguous metal body, a contact interface surface of the contiguous metal body passing in the first direction through the access opening to abut the integrated circuit via a thermal interface material substantially along the contact interface surface; pins, comprising a first pin oriented along the first direction, disposed inside pin receiving openings in the second surface of the substrate, the pins configured to hold the contiguous metal body in position relative to the substrate, at least one of the pin receiving openings located adjacent an opposing side of the access opening from at least one other of the pin receiving openings; wherein the extent of an incursion of the contiguous metal body in the first direction is bounded by its abutment to the integrated circuit; and wherein the pins are rigidly affixed into the pin receiving openings to maintain the contiguous metal body in contact with the integrated circuit via the thermal interface material.

20. A circuit board comprising: a substrate comprising a first surface and a second surface substantially parallel to the first surface; an integrated circuit joined electrically to the first surface of the substrate; a contiguous metal body, a contact interface surface of the contiguous metal body abutting the integrated circuit via a thermal interface material substantially along the contact interface surface; pins, comprising a first pin oriented along a first direction, disposed inside pin receiving openings through the first surface of the substrate, the pins configured to hold the contiguous metal body in position relative to the substrate; wherein the extent of an incursion of the contiguous metal body in the first direction is bounded by its abutment to the integrated circuit; and wherein the pins are rigidly affixed into the pin receiving openings to maintain the contiguous metal body in contact with the integrated circuit via the thermal interface material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] For a more complete understanding of the present embodiments, and the advantages thereof, reference is now made, by way of example, to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0035] FIG. 1A is a cross-sectional view of a prior art heat sink which is hard-stopped on a PCB substrate.

[0036] FIG. 1B is an inset view of the mechanical manufacturing tolerances that influence the gap variability between the heat sink and integrated circuit.

[0037] FIG. 2 is a cross-sectional view of a stamped metal heat sink with through-hole mounting pins, and a bulk finned heat sink attached using screws, according to an embodiment of the present invention.

[0038] FIG. 3 is a cross-sectional view of a forged or cast heat sink with mounting pins integrally formed and a bulk finned heat sink attached using a barb coupling, according to an embodiment of the present invention.

[0039] FIG. 4 is a cross-sectional view of a one-piece bulk finned heat sink, attached to the PCB substrate using soldered integral mounting pins, and hard-stopped to the integrated circuit without an intermediate metal body, according to an embodiment of the present invention.

[0040] FIG. 5 is a cross-sectional view of a one-piece bulk finned heat sink, attached to the PCB substrate topside using soldered integral mounting pins, and hard-stopped to the integrated circuit without an intermediate metal body, according to an embodiment of the present invention. This heat sink is configured to transfer heat out of the top side of a TSOP or other through-hole package.

[0041] FIG. 6 is a cross-sectional view of a stamped sheet metal heat sink applied to the topside of the PCB substrate with through-hole mounting pins, and a bulk finned heat sink attached using screws, according to an embodiment of the present invention. This heat sink is configured to transfer heat out of the top side of a BGA or other surface mount package.

[0042] FIG. 7 is a flowchart describing the procedure of mounting a contiguous metal body to a substrate in contact with an IC according to an embodiment of the invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0043] For illustrative purposes, specific example embodiments will now be explained in greater detail below in conjunction with the figures.

[0044] The embodiments set forth herein represent information sufficient to practice the claimed subject matter and illustrate ways of practicing such subject matter. Upon reading the following description in light of the accompanying figures, those of skill in the art will understand the concepts of the claimed subject matter and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.

[0045] Various embodiments disclosed herein may contain features described individually or in combination. It will be appreciated that features disclosed in one embodiment may be combined with features disclosed in a further embodiment without departing from the scope of the disclosure.

[0046] With reference to FIG. 2, an assembly 200 is pictured comprising an integrated circuit 103 mounted to a PCB substrate 101 via connection posts 104. PCB substrate 101 may be constructed of, for example, an epoxy-impregnated fiber weave such as FR-4 and be laminated with various layers including copper traces, interlevel dielectrics, solder mask, and silkscreen. The PCB provides mechanical rigidity to an assembly and pathways to interconnect terminals of components attached thereto. When manufactured, its thickness, may vary +/−10% from the nominal value.

[0047] An access opening 102 is cut or otherwise formed in the PCB so that a heat sink member comprising a contiguous metal body 207 may contact the IC from the opposite side of the PCB to which the IC is mounted. In one embodiment, the heat sink is not mounted above IC 103 if it would interfere with RF transmissions in or out of IC 103. The opening size can allow for sufficient overlap between the IC 103 and the substrate 101 so that all required electrical contacts 104 may be made between IC 103 and substrate 101.

[0048] Further referring to FIG. 2, connection posts 104 may comprise, by way of non-limiting examples, solder balls, leadframe leads, surface mount lands, pins, or copper pillars to provide a mechanical and electrical connection between IC 103 and PCB 101. Many of these surface-mount or through-hole technologies require an amount of solder to make connection to a PCB. Solder mounting involves heating the solder above its solidus temperature and deforming it so that it flows and makes intimate contact with mating surfaces of the IC and PCB before cooling below the solidus temperature. The final distance 114 between PCB and IC is subject to variation based on the repeatability of the heating and deforming process.

[0049] Contiguous metal body 207 forms the interface of a heat sink with PCB 101 and IC 103. In some embodiments, contiguous metal body 207 may be formed from sheet metal comprising a copper or aluminum alloy and shaped by a stamping process. Die stamping forms in the sheet metal a step height 215 of at least the PCB thickness 111 plus the connection post height 114 plus a margin, preferably significantly more than the sums of the tolerances in PCB thickness 111, connection post thickness 114, and heat sink step height 215. Step height 215 serves to create a portion of contiguous metal body 207 which may be passed through access opening 102 to make thermal contact with IC 103. In this or a subsequent stamping operation, some of the sheet metal may be bent to form mounting pins 208 which are placed to mate with receiving openings 209 drilled or otherwise formed in PCB 101. Such receiving openings may also be known in the art as plated through-holes.

[0050] The condition of the thermal contact interface between contiguous metal body 207 and IC 103 is of prime importance. If placed in direct contact, contiguous metal body 207 and IC 103 would have an irregular air gap present between them, owing to microscopic surface imperfections preventing full planar contact. Thus a thermal interface material 206 is interposed between them to fill the gap and eliminate the thermal resistance caused by air in the contact. The thermal interface material 206 in some embodiments may be preapplied to the heat sink (e.g. in a sheet), and in others may be applied to the IC package first.

[0051] In the prior art, the remaining gap thickness (see FIG. 1B) 116 of gap fill 106 had to be made sufficiently thick to allow for varying stress, under compression, confined by dimensions which vary according to manufacturing tolerances of PCB substrates 111, solder reflow 114, and even the thickness tolerance of the gap fill pad itself. This was because the heat sink was hard stopped against the PCB, forcing the gap to vary with PCB and connection post dimensions. However, inserting pins 208 into openings 209 provides alignment without creating a hard stop against the PCB. The contiguous metal body 207 can be abutted against IC 103 at a predetermined pressure regardless of board and solder manufacturing tolerances because the pins are free to slide into the openings whatever distance necessary to compensate for the tolerances. The metal body thus exerts zero force on the PCB while being brought into contact with the IC. It is a technical advantage of the present invention that hard-stopping the insertion of contiguous metal body 207 through window 102 where it abuts IC 103 allows for precise control of the abutting pressure as well as use of an arbitrarily thin thermal interface material 206. Advantageously, thermal interface materials of under 50 microns in thickness are possible due to eliminating the gap dependence on manufacturing tolerances.

[0052] The abutting pressure is preferably chosen according to a minimum value needed for satisfactory thermal contact when using a given thermal interface 206. Choosing the minimum pressure affords the technical advantage of improving the reliability of the assembly, as it reduces stress on IC 103 and its connection posts 104. In the case of compressible gap-filling thermal pads comprising an elastomer (e.g. silicone) and a heat conductive filler powder (e.g. graphite, silver), the pressure is selected to produce a predetermined deflection or compression in the thermal pad. Compression of approximately 10% is considered sufficient for thermal conductivity when using most of these materials. Alternatively, thermal interface 206 could comprise a thin layer of thermal grease, phase change material, thermally conductive powder filled epoxy, thermal gel, a thermal tape, or any other known material which can gap fill the surface irregularities of contiguous metal body 207 and IC 103 while minimally adding to the thermal resistance between IC 103 and contiguous metal body 207.

[0053] As referred to in FIG. 2, mounting pins 208 serve to insert into receiving openings 209 in PCB 101 to maintain alignment of contiguous metal body 207 in the plane of the PCB and IC. The position of contiguous metal body 207 along an axis normal to this plane, in a first direction normal to the IC and parallel to the mounting pins and axes of the receiving openings, is set by abutting it to IC 103 at a predetermined pressure and is held constant by affixing mounting pins 208 inside receiving openings 209. In the embodiment of FIG. 2, solder 210 is employed to fix the pins 208 in place. Once cooled and solidified, solder 210 provides the vast majority of mechanical stability of contiguous metal body 207 relative to PCB 101, so that IC 103 and connection posts 104 do not bear loads put on the heat sink. As an option, the solder composition can be chosen according to the metal composition of the mounting pins. Solders used for aluminum generally contain zinc with some lead, cadmium, tin, copper, or aluminum. However, any solder that contains tin may cause an electrochemical corrosion problem due to its galvanic potential. For affixing copper pins, the range of useful solders is much wider including most combinations of In, Sn, Pb, or Ag.

[0054] Stamping operations may also form portions of contiguous metal body 207 into a receiving cavity for a bulk metal heat sink 205 composed, in some embodiments, of aluminum or copper. Advantageously, bulk metal heat sink 205 may be cast from a die similar in shape to the die that forms the heat sink-facing surface of contiguous metal body 207 so that there is an intimate fit between contiguous metal body 207 and bulk heat sink 205. In some embodiments, bulk metal heat sink 205 may be affixed to contiguous metal body 207 using screws 211 passing through the sidewalls of the receiving cavity into threaded openings in bulk heat sink 205. The bulk heat sink 205 is optional for many implementations, as the heat transfer out through contiguous metal body 207 may in many cases be sufficient for the application due to the superior thermal conductivity afforded by the thin thermal contact 206. In those cases, stamping operations may be used to form heat radiating fins rather than a receiving cavity with sidewalls designed for screw stability.

[0055] FIG. 3 illustrates an alternate embodiment of the present invention. Printed circuit board 101, die 103, access opening 102, and die connection posts 104 function as discussed previously, so their functions need not be discussed further. In this embodiment, the contiguous metal body 307 is still inserted through access opening 102 to abut IC 103 via a thermal interface material 206 at a predetermined pressure. Mounting pins 308 are still integral to the contiguous metal body 307 and provide alignment of metal body 307 to access opening 102. Mounting pins 308 are, in similar fashion to FIG. 2, affixed in place by soldering into receiving openings 309.

[0056] In the example embodiment of FIG. 3, contiguous metal body 307 may in some embodiments be extruded and machined, or may be formed from a casting or cold forging operation which simultaneously shapes mounting pins 308, heat sink step height 315, and the receiving cavity for holding bulk metal heat sink 305 using barb couplings 311, or thermal fins in embodiments in which bulk metal heat sink 305 is not used.

[0057] In a barb coupling, one member takes the role of an elastic deformer. The portion of contiguous metal body 307 which extends to form the sidewall of the receiving cavity is capable of elastic deformation upon insertion of a bulk metal heat sink 305. A barb 311 on the interior face of the receiving cavity wall is shaped like a wedge to cause elastic spreading of the wall as heat sink 305 is inserted. The elastic strain on the sidewall is released when barb 311 lines up with the notched edge in heat sink 305. An audible “click” alerts the user that the heat sink is secured into place without the need for a tool such as a screwdriver. It will be appreciated that the barbed sidewall of the receiving cavity can function as a thermal fin in lower heat dissipation applications when bulk metal heat sink 305 is not required. The insertion process is also reversible if means (not shown) are provided to elastically stretch contiguous metal body 307 sufficiently for barbs 311 to be clear of the edge notch.

[0058] FIG. 4 illustrates, according to a further embodiment, further simplification of the heat sink construction. A heat sink, PCB, and IC assembly 400 is provided. For applications for which the contiguous metal body alone is never sufficient to remove all the heat from the IC, a single piece heat sink 405 takes on the role of both contiguous metal body contact and bulk finned heat sink. Single piece heat sink 405, which can, by way of non-limiting example be cast, cold forged, extruded, and/or machined, can be affixed to PCB 101 using integral mounting pins 408 inserted into receiving openings 609. Alternatively, machine pins (not shown) can be attached to heat sink 405 in place of integrally formed pins 408 and be affixed to receiving openings 409 using solder. As in previously mentioned embodiments, no matter the composition or manufacturing method of the heat sink, the critical part of the assembly method is abutting the heat sink 405 to the IC 103 via thermal interface material 206 using only enough force to form thermal contact, and subsequently soldering the mounting pins in place, thus providing rigid support for the heat sink/PCB joint and minimizing stress load from heat sink 405 on IC 103 or the members 104 used to connect it to the PCB 101.

[0059] The embodiment pictured in FIG. 5 is another embodiment directed to ICs which may have heat sinks mounted on the top side of the package in addition to those which must mount the heat sink underneath. While MMICs and other radio-frequency ICs with copackaged antennas must allow for unshielded transmission, any other IC requiring a heat sink can also benefit from the technical advantages of the present invention. Top side in this application refers to the side of a circuit board to which the first component of interest is attached.

[0060] With reference to FIG. 5, board assembly 500 comprises general IC 503 which does not necessarily contain a transmitting/receiving antenna. It is joined to general circuit board 501 which is not equipped with any access opening to reach the backside of IC package 503. IC 503 is mounted using a general board population technology, which may or may not be surface mount. The non-limiting example of a lead frame TSOP is illustrated, with leads 504 soldered into plated through-holes in circuit board 501, leaving a standoff distance 514 between the IC 503 and PCB 501. Variations in die encapsulation mold size, lead frame bend angle, and performance of IC insertion equipment may give rise to variations in standoff distance 514, IC encapsulation thickness 513. These unknown dimensions cannot be adequately predicted and compensated by the heat sink step height 515 during the manufacture of bulk metal heat sink 505, which may be manufactured by any of the aforementioned metalworking methods.

[0061] In contrast to previously mentioned embodiments, heat sink 505 may be provided with a cavity to fit over IC 503 rather than a protrusion to insert into an access opening. However, even from the topside of the PCB 501, heat sink 505 can be brought into abutment with IC 503 at a preselected pressure suitable to create an optimal thermal interface using thermal interface material 206. The designed thickness of thermal interface material 206 need not take into account any of the dimensions 513, 514, or 515 because pins 508 can insert into receiving openings 509 any distance which is required to achieve a hard stop of heat sink 505 on IC 503. Solder 510 is then applied to affix heat sink 505 in position over and in contact with IC 503 via interface material 206 permanently at the preselected pressure.

[0062] A further example of an embodiment utilizing topside heat sinking is described in FIG. 6. Integrated circuit 603 is in a BGA package, joined to substrate 601 via solder ball grid array 604. As BGAs are particularly sensitive to both shear stresses and stresses normal to the solder balls, the present invention is particularly beneficial to assemblies involving BGA packages. Contiguous metal body 607 is brought into contact with BGA IC 603, the thermal interface mitigated, as in other embodiments, by thermal interface material 206. A light pressure, just enough to fully enable heat transfer through interface material 206, is used to contact the components, which increases reliability by lowering stress on the solder ball array. Simultaneously, mounting pins 608 enter pin receiving openings 609 in substrate 601 from the top side. With the contiguous metal body held in place, solder 610 is applied to create a strong reinforcement, in other words a load bearing path, relieving ball grid array 604 of external stresses caused by the weight and inertia of the heat sink or mechanical shocks that may strike the assembly during handling and use. Optional bulk heat sink 605 is shown joined to receiving cavity sidewalls by screws 611, though in this and other embodiments a barb coupling or other fastener known to those skilled in the art may be used as an alternative means of joining bulk heat sink 605 to contiguous metal body 607.

[0063] With reference to FIG. 7, a flowchart 700 describes the method used to mount a heat sink member comprising a contiguous metal body to a circuit board substrate, the contiguous metal body in contact with an IC already attached to the substrate. In step 701, a component placing apparatus may bring the contiguous metal body into alignment with a circuit board substrate, the alignment being a coaxial alignment of pins on the contiguous metal body with pin receiving openings (plated through-holes) on the substrate. The through-holes are machined in locations that match up with the locations of pins on a heat sink design. In step 702, the component placing apparatus translates the contiguous metal body along the pin and through-hole axial direction. The pins enter the pin receiving openings at a point in time before the contiguous metal body makes contact with the IC via a thermal interface material. The contiguous metal body ceases moving relative to the IC when it abuts against the IC, but there is no force exerted on the contiguous metal body by the substrate itself. Stated another way, the contiguous metal body hard stops on the IC. In step 703 a solder, glue, epoxy, or other filling and joining material is introduced into the through-holes to form a strong mechanical bond between the contiguous metal body and the substrate. In step 704, the component placing apparatus withdraws from the contiguous metal body. The mechanical bond is capable of maintaining the position of the heat sink as well as its pressure on the IC. A key function of this mechanical bond is to bear shear and compressive/tensile stresses exerted between the heat sink and substrate, not allowing destructive levels of strain to damage the electrical attachments between the IC and substrate.

[0064] Although a combination of features is shown in the illustrated embodiments, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system or method designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments.

[0065] While this disclosure has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the disclosure, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.