Abstract:
There is disclosed a printed circuit board assembly mounting tray combined with a hinged thermal tray. The thermal tray, having a plurality of heat pipes and a thermal hinge, transfers heat gathered from heat sources scattered on the printed circuit board assembly and conducted to the mounting tray. The thermal hinge provides a heat transfer connection between the thermal tray and an external heat dissipation apparatus. Additionally, the thermal tray and thermal hinge combination provides easy inspection, board installation and serviceability.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to heat transfer and, more specifically, to a system for extracting heat from electrical devices on a printed circuit board assembly. 
     BACKGROUND OF THE INVENTION 
     In general, most printed circuit board assemblies (PBAs) contain one or two heat generating sources that necessitate heat removal devices. In many cases a PBA is assembled with many low heat generating components such as capacitors, diodes and transistors. Although each device generates very little heat so that individually the heat can be ignored, the cumulative heat from the combined devices can be substantial. Furthermore, it is more difficult to extract heat from the scattered heat sources (devices) than it would be to extract heat from a single high heat source. 
     Generally, heat is usually extracted from a concentrated heat source by either heat sink or fan or both. Integrating fans into the PBA to remove heat has typically proven to produce a relatively unreliable PBA due to interference by the fan motor. Most heat producing devices are mounted directly to a heat sink, but only if there is room for the heat sink and if there are only one or two heat generating devices on the PBA. In most cases, PBAs are assembled with surface mount devices that require components to be soldered on the bottom of the printed circuit board assembly. This creates a mounting problem because of the potential short circuits between solder connections that could occur because of a metal mounting tray. 
     There is therefore a need in the art for providing a system and method that will transfer heat from heat producing devices that are scattered over a printed circuit board assembly. It would further be desirable to provide a system and method for passive heat transfer that did not interfere with individual devices on board the printed circuit board assembly. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, a passive heat transfer system for use in a printed circuit board assembly. 
     It is a further object of the present invention to provide a system for transferring heat away from multiple, scattered components on a printed circuit board assembly. 
     It is still another object of the present invention to provide a system for transferring heat from multiple scattered components on the printed circuit board assembly and allow easy access for installation and service to the PCB. 
     The foregoing objects are achieved as is now described. A printed circuit board assembly mounting tray combined with a hinged thermal tray. The thermal tray, having a plurality of heat pipes and a thermal hinge, transfers heat gathered from heat sources scattered on the printed circuit board assembly and conducted to the mounting tray. The thermal hinge provides a heat transfer connection between the thermal tray and an external heat dissipation apparatus. Additionally, the thermal tray and thermal hinge combination provides easy inspection, board installation and serviceability. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
     Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or and the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like numbers designate like objects, and in which: 
     FIGS. 1A-B depicts different views of a high-level diagram of a system for extracting heat from a printed circuit board assembly (PBA) according to an embodiment of the present invention; 
     FIG. 2 illustrates a cross-section of a PBA installed in a system for extracting heat according to an embodiment of the present invention; 
     FIG. 3 depicts a perspective of an exemplary assembly of a system for extracting heat from a PBA in accordance with an embodiment of the present invention; and 
     FIG. 4 is a high-level flow diagram illustrating the operation of the exemplary system for extracting heat from a PBA according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 through 4, discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged heat transfer device for a printed circuit board assembly. 
     FIGS. 1A-1B depict different views of a high-level diagram of a system for extracting heat from a printed circuit board assembly (PBA) according to an embodiment of the present invention. The system may be utilized in any installation that contains multiple PBAs. In FIG. 1A, the present invention is shown installed in radio unit housing (RU)  102  of a wireless Base Station (not shown). RU  102  contains multiple printed circuit board assemblies, all capable of generating heat. Shown is PBA hinged mounting assembly  100  comprising PBA mounting tray  104 , thermal tray  106 , heat pipes  112 , and thermal hinges  108  of the radio unit housing  102 . In FIG. 1B, a top view displays thermal tray  106 , PBA mounting tray  104 , PBA  110 , thermal hinges  108 , interior  114  of RU housing  102 , cooling fins  116  and cross section  118  of RU housing  102 . 
     During operation, PBA hinged mounting assembly  100  is rotated to a closed position with A on mounting assembly  100  joining B on RU housing  102 . Forced air passing over mounting assembly  100  may provide some heat transfer but the most efficient, and majority of the total, heat transfer takes place by conduction via mounting tray  104 , thermal tray  106  and thermal hinge  108 . PBA  110  is enclosed, along with many other PBAs in a power amplifier housing. Individual heat producing devices that are installed on mounting assembly  100  may not produce very much heat individually, but when combined with all the others on the board in an enclosed space, the cumulative effect is significant. Forced air moving across the PBAs has been the only fairly effective method for removing heat buildup in an enclosure. The present invention transfers heat, by conduction, directly from thermal vias installed in PBA  110  into mounting tray  104 . The heat transfer continues from mounting tray  104  onto thermal tray  106  and to thermal hinge  108  via heat pipes  112 . In this embodiment, heat pipes  112  contain a liquid for transporting the heat to thermal hinge  108 . However, heat pipes do not necessarily require liquid in order to function efficiently. 
     After the heat has transferred through heat pipes  112  to thermal hinges  108 , the heat is then transferred to the radio unit housing  102 . Heat is then, radiated to the ambient surroundings from housing  102  and heat transfer fins  116 . All heat transfer, from PBA  110 , through mounting tray  104 , thermal tray  106 , heat pipes  112 , thermal hinges  108  and external housing  102 , takes place by conduction. There are no moving parts and no heat sinks to take up room on the PBA. PBA  110  is easily available to repair and inspect and there are no moving parts which means low maintenance. Thermal hinge  108  allows efficient heat transfer to housing  102  and provides easy access to PBA  110 . 
     Referring to FIG. 2, a cross-section of a PBA installed in a system for extracting heat according to an embodiment of the present invention, is illustrated. PBA hinged mounting assembly  100  is shown in cross section. An enlarged view of thermal via  206  is also shown. PBA hinged mounting assembly  100  comprises thermal hinges  108  to which thermal tray  106  is mechanically and thermally connected. Heat pipes  112  are embedded in thermal tray  106 . PBA hinged mounting assembly  100  also comprises mounting tray  104 , lands  200  (i.e., raised portions of mounting tray), printed circuit board assembly (PBA)  110 , and PCB devices  202 . Also shown are thermal vias  206  that penetrate PBA  110  for conducting heat to mounting tray  104 . 
     An enlarged view of thermal via  206  is shown wherein PCB  110  is mounted on mounting tray  104  and thermal via  206  is in thermal contact with mounting tray land  200 . Also shown is PCB device  202  with solder connection  208 . Solder connection  208  extends into space  210  which is an open area between lands to provide clearance for solder connections. Thermal via  206 , in the enlarged view, is shown with arrows indicating the direction of heat transfer, “Q”, from devices on PCB  110 . There are multiple thermal vias incorporated into PCB  110  and all connect to mounting tray lands. Heat is transferred into the mounting tray land  200  and further, into mounting tray  104  itself. Referring to the cross-sectional view, heat is transferred, via the metal to metal contact between mounting tray  104  and thermal tray  106 . The heat is transferred, via heat pipes  112  and thermal tray  106 , to thermal hinge  108 . The heat is then transferred to the metal enclosure of the radio unit and dissipated to the surrounding atmosphere. 
     Referring now to FIG. 3, a perspective of an exemplary system for extracting heat from a PBA in accordance with an embodiment of the present invention is depicted. Heat transfer system  300  includes: thermal hinge  302 , heat transfer tray  304 , heat pipes  306 , PBA mounting tray  308  and mounting hardware  310 . Not shown in this figure is a printed circuit board assembly which would mount on top of PBA mounting tray  308 , and “thermal vias” which mount between the printed circuit board assembly and PBA mounting tray  306 . Thermal vias, as noted above, are devices that transfer heat from the printed circuit board assembly to mounting tray  308 . Mounting tray lands  312  provide thermal contact between the thermal vias on the printed circuit board assembly and mounting tray  308 . Open spaces  314  are provided to prevent solder connections on the bottom of the printed circuit board assembly from shorting to the metal mounting tray. 
     As illustrated, mounting tray  308  is shown with an exploded view of one example of mounting hardware  310 . There are multiple mechanical connections between mounting tray  308  and thermal tray  304 , so to simplify the figure, only one set of hardware is illustrated. Hardware for connecting the two trays is installed in open spaces  314  to prevent contact with exposed solder connections on the bottom of the printed circuit board assembly. 
     Thermal tray  304  comprises thermal hinge  302  and heat pipes  306 . Heat pipes  306  in this embodiment are liquid filled but depending on configuration and heat transfer needs, may be filled with any fluid, including air. Heat is transferred through the thermal vias that are connected to mounting tray  308 , and then to thermal tray  304 . Even though contact between thermal tray  304  and mounting tray  308  is mechanical, both are metal trays and the hardware connections are at multiple points. This multiple point and metal to metal contact enhances the transfer of heat from mounting tray  308 . Heat transferred to thermal tray  304  is then conducted to thermal hinge  302  via heat pipes  306 . In turn, thermal hinge  302 , mechanically connected to a metal housing (not shown) for the PBAs, transfers heat received from heat pipes  306  to the metal housing. 
     Referring to FIG. 4, a high-level flow diagram of the operation of the exemplary system for extracting heat from a PBA according to one embodiment of the present invention, is illustrated. The process begins with step  400 , which depicts heat generated from individual devices on a printed circuit board assembly. The PBA is installed in a preferred embodiment of the present invention, including a mounting tray, thermal tray with heat pipes and thermal hinge for mounting to an enclosure. The process continues to step  402 , which illustrates the generated heat being transferred from the devices on the PCB, utilizing thermal vias, to the mounting tray. The process then passes to step  404 , which depicts the heat being transferred from the mounting tray, via mechanical and thermal contact, to the thermal tray. 
     The process then proceeds to step  406 , which illustrates heat being transferred via embedded heat pipes in the thermal tray to a thermal hinge. The heat may be transferred due to the mechanical and thermal connection between the thermal tray and thermal hinge in addition to the heat transferred by the heat pipes. The process then passes to step  408 , which depicts the heat being transferred to a heat dissipation apparatus (housing) external to the PCB, via the thermal hinge. Finally, the process proceeds to step  410 , which illustrates the heat being rejected from the housing to the surrounding air. All heat transfer in the present invention is passively conducted away from the PCB assembly. 
     Previous methods of heat transfer from heat generating devices are usually confined to utilizing heat sinks and forced air. Heat generating devices generally require substantial surface areas. Utilizing heat sinks when there are many heat generating devices required on a single board, becomes cumbersome and sometimes impossible. The present invention utilizes a design that allows for a metal mounting tray to be thermally and mechanically joined. The metal mounting tray is designed to provide open spaces beneath solder connections and metal/thermal lands for connecting to thermal vias that are mounted on the underside of printed circuit boards. The metal mounting tray is used for mounting PBAs in an enclosure (metal) and for transferring heat to a more efficient heat transfer device. The heat transfer device—thermal tray and thermal hinge combination—removes heat from the metal tray and interior of the enclosure by conduction. The heat is then radiated by the external panels of the enclosure itself. Heat is transferred efficiently from devices that may be scattered across the printed circuit board assembly without the use for large, heat spreading devices or forced air cooling. 
     Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.