Abstract:
An enclosed electronic component is provided by extruding first and second heatsink members having a plurality of cooling structures disposed on first surfaces thereof and an electronic component board between the first and second heatsinks, with a pair of joining members and conductive gaskets to form an electromagnetically sealed enclosure and a thermal pathway to remove heat from the electronic component.

Description:
BACKGROUND  
         [0001]    This invention relates to packaging of electronic and optical components.  
           [0002]    Modern electronic components have design goals that are conflicting. Generally electronic components emit electromagnetic (EM) radiation and such EM radiation often interferes with the performance of other components. Often in order to assemble electronic components into a system some of the components are shielded from stray EM radiation. Typically such shielding is provided by a conductive enclosure.  
           [0003]    Another design goal relates to the heat produced by electronic components. Often such heat must be removed from the component to avoid excessively high temperatures that can damage or interfere with the component or its operation. Cooling can use a fan to move cooling air past the component to carry off the excess heat. Often heatsink structures are used with or without fans to aid in the conduction of the heat from the electronic components.  
           [0004]    In many instances the enclosure that provides adequate EM shielding leads to difficulties in removing heat efficiently from the component. A solution to this conflict is to die cast an enclosure for an electronic component, which provides both the EM compatibility (EMC) and the thermal coupling to the electronic component.  
         SUMMARY  
         [0005]    In general, in one aspect the invention features a method for producing an enclosed electronic component. The method includes: extruding a first heatsink member having a plurality of cooling structures disposed on a first surface; extruding a second heatsink member having a plurality of cooling structures disposed on a first surface; disposing an electronic component board on a second surface of one of the first and second heatsink members; and joining the first and second heatsink members, with a pair of joining members to form a sealed enclosure for the electronic component board with the sealed enclosure providing electromagnetic isolation of the electronic component and a thermal pathway to remove heat from the electronic component.  
           [0006]    Embodiments of the method may include any one of the following features.  
           [0007]    The extruding the first and second heatsink may further include: forming first ribs on the second surfaces of the first and second heatsinks, with the electronic component board having conductive bands on top and bottom surfaces thereof that are aligned with portions of the first ribs.  
           [0008]    The joining may further include: joining the top and bottom conductive bands of the electronic component board to the first rib of the first and second heatsink members respectively.  
           [0009]    The extruding the first and second heatsink may further include: forming second ribs on the second surfaces of the first and second heatsinks with the electronic component board having a second pair of conductive bands on top and bottom surfaces thereof that are aligned with portions of the second ribs and joining further includes: joining the first and second top conductive bands of the electronic component board to the first and second rib of the first heatsink member and joining the first and second bottom conductive bands of the electronic component board to the first and second rib of the second heatsink member respectively.  
           [0010]    The above method may further include forming positioning pins that align the first heatsink member, the second heatsink member, and the electronic component board.  
           [0011]    A first portion of the positioning pins may provide vertical positioning and a second portion may provide horizontal positioning of the electronic component board.  
           [0012]    The joining may include: securing contacting regions of the first and second heatsinks, the electronic component board, and a pair of joining members with screws, and sealing regions about the contacting regions with conductive gasket material.  
           [0013]    The extruding of the top or bottom heatsink may include forming a substantially cylindrical channel in the top or bottom heatsink wherein the diameter of the channel is suitable for forming threads that engage the screws.  
           [0014]    The joining may include welding contacting regions of the first heatsink, the second heatsink, the electronic component board and a pair of joining members.  
           [0015]    The joining may include securing contacting regions between the first heatsink, the second heatsink, the electronic component board and a pair of joining members by using a combination of screws and sealing regions about the contacting regions with conductive gasket material for some of the contacting regions and welding for other contacting regions.  
           [0016]    The electronic component board may further include heat transfer structures that provide thermal coupling between the electronic component and the top or bottom heatsink.  
           [0017]    In another aspect, the invention features a method for producing an enclosure for an electronic component. The method includes: extruding a first heatsink member having a plurality of cooling structures disposed on a first surface; extruding a second heatsink member having a plurality of cooling structures disposed on a second surface; joining the first and second heatsink members with a pair of joining members to form a sealed enclosure, the sealed enclosure providing electromagnetic isolation of the electronic component and a thermal pathway to remove heat from the electronic component.  
           [0018]    The joining may include: securing contacting regions of the first and second heatsinks, and the pair of joining members with screws and sealing regions about the contacting regions with conductive gasket material.  
           [0019]    The extruding of the top or bottom heatsink may include forming a substantially cylindrical channel in the top or bottom heatsink wherein the diameter of the channel is suitable for forming threads that engage the screws.  
           [0020]    The joining may include: welding contacting regions of the first and second heatsinks, and the pair of joining members.  
           [0021]    The joining may include: securing contacting regions of the first and second heatsink members, and the pair of joining members by using a combination of screws and sealing regions about the contacting regions with conductive gasket material for some contacting regions and welding the contacting regions for other contacting regions.  
           [0022]    In another aspect, the invention features a packaged electronic component. The package includes: an extruded first heatsink member having a plurality of cooling structures disposed on a first surface wherein the cooling structures have smooth surfaces characteristic of an extruded part; an extruded second heatsink member having a plurality of cooling structures disposed on a first surface wherein the cooling structures have smooth surfaces characteristic of an extruded part; an electronic component board disposed within an enclosure provided by the first and second heatsink members, and a pair of joining members secured; and wherein the first and second heatsink, the joining members, and the electronic component board are joined at contacting regions to form a sealed enclosure for the electronic component board with the sealed enclosure providing electromagnetic isolation of the electronic component and a thermal pathway to remove heat from the electronic component.  
           [0023]    The first and second heatsink may include: a first rib on the second surface of the first and second heatsinks and the electronic component board includes conductive bands on a top and a bottom surface of the electronic board and wherein the top and bottom conductive bands of the electronic component board and the first ribs of the first and second heatsink members sealed with conductive gasket material.  
           [0024]    The first and second heatsink may further include: a second rib on the second surface of the first and second heatsinks and the electronic component board includes second conductive bands on the top and the bottom surface of the electronic component board and the second ribs of the first and second heatsink members sealed with conductive gasket material.  
           [0025]    The package may further include positioning pins that align the first heatsink member, the second heatsink member, and the electronic component board.  
           [0026]    The positioning pins provide either vertical or horizontal positioning of the electronic component board.  
           [0027]    The contacting regions may include screws that secure the contacting regions of the first heatsink, the second heatsink, the electronic component board and the pair of joining members and regions of conductive gasket material about the contacting regions.  
           [0028]    The top or bottom heatsink may have a substantially cylindrical extruded channel in the top or bottom heatsink wherein the diameter of the channel is suitable for forming threads that engage the screws.  
           [0029]    The electronic component board may further include a heat transfer structures that provide thermal coupling between the electronic component and the top or bottom heatsink.  
           [0030]    The contacting regions may include welds of the first heatsink, the second heatsink, the electronic component board and a pair of joining members.  
           [0031]    The contacting regions of the first heatsink, the second heatsink, the electronic component board and a pair of joining members may include a combination of screws and conductive gaskets for some of the contacting regions and welds for other contacting regions.  
           [0032]    One or more aspects of the invention may provide one or more of the following advantages.  
           [0033]    The invention provides an enclosure for an electronic component with fine cooling structures. Unlike alloys used for die casting that typically have lower heat conductivities than pure metals the enclosure offers improved conductivity. In addition the fine, high surface area features that provide efficient thermal coupling between the electronic components and the cooling gases are easier to produce. Furthermore, while die-casting is typically relatively expensive and requires a relatively lengthy time from product conception to product manufacture, extrusion is less expensive and has a relatively short time from concept to product.  
           [0034]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0035]    [0035]FIG. 1 is a schematic depiction of an extrusion machine extruding a bottom heat sink portion of an EMC enclosure.  
         [0036]    [0036]FIG. 2 is an isometric plan view of a finished bottom heatsink formed by machining the part depicted in FIG. 1.  
         [0037]    [0037]FIG. 3 is an isometric plan view of an extruded part suitable for machining into the top heatsink of an EMC enclosure.  
         [0038]    [0038]FIG. 4 is an isometric plan view of the finished top heatsink formed by machining the extruded part of FIG. 4.  
         [0039]    [0039]FIG. 5 is an isometric plan view of a machined extruded part suitable for use as the sides of enclosure.  
         [0040]    [0040]FIG. 6 is an exploded isometric plan view of an EMC electronic component enclosure formed from a top heatsink, a bottom heatsink, and a pair of side pieces.  
         [0041]    [0041]FIG. 7 is a side view of the EMC enclosure from FIG. 6.  
         [0042]    [0042]FIG. 8 is a cross sectional view of an EMC enclosure showing positioning pins.  
         [0043]    [0043]FIG. 9 is a cross sectional view of a heatsink member with a hole that is formed by the extrusion die. 
     
    
     DETAILED DESCRIPTION  
       [0044]    Extrusion offers a technique of forming parts for EMC and thermal enclosures of electronic components. The alloys used for extrusion typically have electrical and thermal properties that are at least as desirable as alloys used for die-casting. Furthermore, design features on extruded parts can be finer than the features that are efficiently produced by die-casting. In addition, the initially cost of tooling for new extruded parts and the time to market is much lower than the cost of die-casting. (Typical tooling costs for new extruded parts are about $6000 and the time to market is only 4-6 weeks.)  
         [0045]    The electronic component enclosures are typically formed from five pieces: an extruded top heatsink, an extruded bottom heat sink, a pair of side pieces, and the electronic component itself. The following will detail these parts, how they are made, and how they are put together to form an enclosure.  
         [0046]    Referring to FIG. 1 an extrusion apparatus  10  suitable for forming extruded parts is shown. Molten alloy  13  enters the extrusion machine under pressure along inlet  12  into main chamber  14 . The alloy is forced out of main chamber  14  through die  16 . The features of die  16  determine the shape of the extruded member. As shown in FIG. 1 the die has features that correspond to a negative of a member that can serve as a bottom heatsink  20 . Other dies are used to produce other portions of a package, as depicted in FIG. 5. In general, the cross sectional features of extruded heatsink  20  are formed by the cross sectional features  17   a,    17   b,  and  17   c  of die  16 . Typically die  16  is relatively easy to install and remove from the extrusion apparatus  10 . Furthermore, die  16  is relatively inexpensive to manufacture. Thus, the manufacture of new parts involves manufacture of a new die and installing the new die into the extrusion apparatus. This versatility and relatively low cost compared to die casting allows engineers to readily redesign old parts in order to implement improvements or to readily design new extruded parts customized to a new application. As the alloy is forced into chamber  14 , the alloy is forced through the die to form the member, e.g., bottom heat sink member  20 . Typically, parts used for EMC enclosures and thermal coupling are extruded from aluminum based alloys such as 6063-T6.  
         [0047]    Heatsink  20  is cut to length and its edges are milled. Extruded bottom heatsink  20  has multiple cooling structures, for example fin  26 , on its surface  21 . These fins will provide a thermal coupling mechanism between the interior enclosure and external surroundings. Heatsink  20  has front and back ribs  22  and  24  respectively. Such ribs form part of the front and back seals respectively of the enclosure. Heatsink  20  further includes a latch rib  28  that provides the material that will be machined into part of a latching mechanism. In addition to such features, the heatsink  20  may have features similar to spacers ribs  29   a,    29   b,  and  29   c.  These ribs provided the raw material on the heatsink for features like board spacers.  
         [0048]    Milling machines remove the excess material from extruded bottom heatsink  20  to form the finished bottom heatsink  20 ′ as shown in FIG. 2. Surface features of heatsink  20 ′ that are formed by extrusion, such as the fin  26 , have surfaces that are smooth. In contrast, surface features that are formed by machining have surfaces with distinctive machining marks. Furthermore both of these surfaces are contrasted to the surfaces formed by die-casting. Die-casting often results in surfaces that are rougher than extruded surfaces and are porous and pin-holed. Milling is performed to specifications that are tailored to a particular package. Heatsink  20 ′ includes machined front and back ribs  22 ′ and  24 ′ respectively. Machining removes material from ribs  22  and  24  leaving behind islands  23   a ′- 23   e ′ and  25   a ′- 25   c ′. These islands include drilled holes, which are tapped to receive threaded screws. As the members  22 ′ and  24 ′ form part of the seal for the enclosure, the islands  23 ′ and  25 ′ provide space around the holes tapped to receive screws to allow for adequate gasketing material to form a seal. Rib  28  is machined to form the completed latching mechanism  28   a ′ and  28   b ′ and ribs  29  are machined to form spacers  29   a ′- 25   f ′. The spacers will support the electronic components that will be held inside the enclosure. The spacers provide mechanical support and accurate positioning of the electronic components. Holes  27   a ′ and  27   b ′ are drilled and tapped into the side of heatsink  20 ′. These tapped holes receive screws that connect the heatsink member  20 ′ with side members of the enclosure.  
         [0049]    Using a similar extrusion process as described above a different die produces the extruded top heatsink part  40  shown in FIG. 3. Similar in design to part  20 , part  40  includes multiple cooling structures, for example cooling fin  46 , on its surface  41 . Part  40  further includes front rib  42 , back rib  44  and spacer rib  49 . Additional processing for example milling, modifies the part to produce the finished top heatsink  40 ′ as shown in FIG. 4. Again, in analogy to part  20 ′, the machining of heatsink  40 ′ modifies ribs  42  and  44  to produce ribs  42 ′ and  44 ′ each of which has islands with screw holes  43   a ′- 43   e ′ and  45   a ′- 45   c ′ respectively. Holes  47   a ′- 47   c ′ are drilled and tapped into the side of part  20 ′ for coupling the top heatsink to the enclosure side members.  
         [0050]    Another extrusion process forms extruded side member  60  as shown in FIG. 5. Side member  60  includes chassis guide  62 . Side member  60  forms the sides of the enclosure. Chassis guide  62  forms a channel that mates with guides in the chassis, which guide the finished enclosure into the proper position in the chassis. Extruded member  60  further include ribs  66   a - 66   c  that provide material for screw locations. In some embodiments, side member  60  can include electronic component positioning guide  68 . This structure sits flush on top of the electronic component, and aligns the electronic component with the enclosure. In some embodiments, the side parts can be formed using other manufacturing processing such as die casting or machining raw stock.  
         [0051]    [0051]FIG. 6 shows an exploded view of enclosure  70  formed from finished bottom heatsink  20 ′, finished top heat sink  40 ′, a pair of finished side members  60   a ′ and  60   b ′, and an electronic component board  72 . The board  72  holds the electrical components. The finished side members  60   a ′ and  60   b ′ are created by machining parts similar to member  60 . Clearance holes  67   a ′- 67   e ′ are drilled into the side member  60   a ′. These clearance holes align with the threaded holes  27 ′ and  47 ′ of the bottom and top heatsinks respectively. Top heatsink  40 ′ is positioned onto the electronic component  72 . Side members  60   a ′ and  60   b ′ are attached to the top and bottom heatsinks with screws via clearance holes  67 ′ which engage the tapped holes  27 ′ and  47 ′. Furthermore the screws are passed through clearance holes  43 ′ and  47 ′ and engage threads in holes  25 ′ and  27 ′. In both of these instances where screws are used to connect parts, gaskets  76   a - 76   h  are formed to provide EM compatibility of the electronic component and the enclosure. These gaskets typically use “form in place” EMI gasketing technology. Suitable gasketing materials are Silver/Copper 5513 available from Chomerics, Woburn Mass. or SNN45 M+ available from Laird, Delaware Water Gap, Pa. The applicators and dispensers for the gasketing materials are also available from the same suppliers. In order to ensure proper electrical contact between the parts and the gaskets, the parts are Class III clear chromate prior top application of the gasketing material. Class III MIL-C-5541 chemical conversion coating provides a barrier to corrosion while at the same time providing low electrical resistance required for EMC gaskets to operate effectively.  
         [0052]    [0052]FIG. 7 shows an exploded side view of enclosure  70 . The electronic component board  72  includes conductive bands  74   a  and  74   b  which sit atop ribs  22 ′ and  24 ′ respectively of the bottom heatsink  20 ′ forming a seal with the aid of gaskets  76   a  and  76   d.  Furthermore contact ribs  44 ′ and  42 ′ of the top heatsink  40 ′ form a top seal to bands  74   a  and  74   b  with the aid of gasket  76   b  and  76   c.  Typically bands  74   a - b  are copper bands on the electronic component board  72 . These structures form the front, back, top, and bottom of the sealing enclosure. With the aid of the gasket  76   e - 76   h,  side pieces  60 ′ form the sides of the sealing enclosure. Referring to FIG. 7, conductive band  74   b  is continuous from top surface of board  72  over the edge of board  72  all the way to the bottom surface of  72 . This band prevents emissions through any internal layers in the electronic component board. Conductive band  74   a  (near the backplane connector) may or may not have a continuous edge plating (from top to side to bottom) since a gasket on the backplane makes continuous contact with the rectangular box formed by top heatsink  20 ′, bottom heatsink  40 ′, and two side pieces  60   a ′ and  60   b ′ thus reducing emissions from the left (backplane connector side) side of this box.  
         [0053]    In another embodiment, the enclosure enhances heat conduction from the electronic component board to the heat sink members  40 ′ and  20 ′ as shown in cross section in FIG. 8. In such embodiments, the enclosure  70  further includes top positioning pins, for example pin  90 , bottom positioning pins, for example pin  92 , and screws, for example screw  94 . Such positioning pins control the distance and, from the electronic component board  72  and top and bottom heatsinks respectively. In addition to vertical positioning the pins provide horizontal positioning. For example, in FIG. 8 the pins control the positioning of the electronic board  72  with respect to side pieces  60   a ′ and  60   b ′ such that there is adequate clearance so that the board does not interfere with the fit of the side pieces  60   a ′ and  60   b ′ with heatsink  40 ′ and  20 ′. The positioning pins themselves provide a thermal pathway between the board  72  and the top or bottom heatsinks. In addition, often electronic components or their corresponding electronic component boards are designed with a pad  74  and heat transfer structure  75 . The heat transfer structure is specifically placed to remove heat from board  72 . For example, it may be the heat transfer portion of an application specific integrated circuit (ASIC). Typically the height of such a structure is well known. Thus, positioning pins are designed to control distances such that after assembling the enclosure about the electronic board, the top of heat transfer structure  75  is close to the bottom of heatsink  40 ′. Typically an overfill compound is applied. Overfill is a two part paste composed of conductive particles suspended in a silicone type substrate. After mixing the 2 parts with a special syringe the overfill is injected through a hole (not shown) in the heatsink or conductive pedestal, which provides a access for the paste to flow through and spread over the heat transfer structure and thus voiding the gap between the heat transfer structure and heatsink surface of any air pockets. The overfill paste cures to a flexible solid in a matter of hours. Overfill can be purchased from Chomerics (T644 or T645), or Dow Corning in many different forms depending on the applications requirements. The overfill fills this small gap between the heat transfer structure and the heatsink  40 ′ and provides a thermal bridge between the two parts. The overfill provides thermal contact between heat transfer structure  75  and the top heatsink  40 ′. Furthermore in some embodiments electronic board  72  is designed with backplane connectors that electrically connect the components of electronic board  72  with other electronic components. For example, the enclosure  70  is often placed into a chassis box. Guides  62  on the side pieces  60   a ′ and  60   b ′ guide the enclosure into the chassis box and the connectors on the backplane of the chassis box connect to the backplane connectors of the enclosure. The vertical position of the board  72  is determined by the height of pins  90  and  92  and thus these pins control the height of the backplane connectors of board  72 , and therefore ensure proper alignment of the enclosures backplane connectors with the connectors on the chassis box backplane. In addition to providing positioning, these pins also provide mechanical stiffening to correct for any twist, bow, or warp that may be present in extruded parts.  
         [0054]    In another embodiment, the drilled and holes  27 ′ in the bottom heatsink, are formed directly by the extrusion process as shown in FIG. 9. FIG. 9 shows a side view cross section of part of the extruded top heatsink  40 ′. In addition to extruding the fins  26 , the die is shaped to form cylindrical channels  110 . Once formed, these channels are tapped at least along some portion. This additional feature in the extrusion process avoids the need to drill holes separately.  
         [0055]    In other embodiments, the enclosure can be formed by welding the seams between the top and bottom heatsink, and the side pieces. In other embodiments, the side pieces can be welded to either the top heatsink member or the bottom heatsink member, and the two resulting pieces can be joined using screws and gasketing. In some embodiments a combination of welding, mechanically fastening and gasketing is employed to produce the enclosure.  
         [0056]    In some embodiments, the enclosure forms a substantially hermetic seal such that any contaminating particulates in the exterior of the enclosure cannot enter the enclosure. This protects the electronic components in the enclosure and it also makes the cooling mechanism easier. For example, in such embodiments, the cooling air does not need to be filtered.