Abstract:
In one embodiment, an electronic system includes a printed circuit board, one or more packaged semiconductor devices, and a vapor chamber having a top and a bottom and enclosing a sealed cavity that is partially filled with a coolant. The vapor chamber comprises a thermo-conductive and electro-conductive material. The top of the vapor chamber has one or more depressions formed therein, each depression receiving and thermo-conductively connected to at least part of a bottom of a corresponding packaged semiconductor device, which is mounted through a corresponding aperture in the PCB. A heat sink may be thermo-conductively attached to the bottom of the vapor chamber.

Description:
[0001]    This application claims the benefit of the filing date of U.S. Provisional Application No. 62/001,773 filed on May 22, 2014, the teachings of which are incorporated herein by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    The current disclosure relates to electronics, and more specifically but not exclusively, to heat dissipation for active semiconductor devices. 
         [0003]    Wireless communication systems may include base stations that transmit electromagnetic (EM) signals. The base station transmitters employ radio-frequency (RF) power amplifiers to amplify RF EM signals for transmission. RF power amplifiers are active semiconductor-based devices that may employ any of various different technologies to achieve amplification, e.g., LDMOS (laterally diffused metal oxide semiconductor), GaN, or GaAs. Typically, a base-station transmitter comprises a series of amplification stages—which may be referred to as a lineup—that increases in power level from lower-level amplification to higher-level amplification. A final-stage RF power amplifier typically generates more heat than an earlier-stage RF power amplifier. A lineup of RF power amplifiers comprising final-stage and/or pre-final-stage power amplifiers may be mounted onto a printed circuit board (PCB) that comprises additional electronic components. 
         [0004]      FIGS. 1A-1D  show various views of exemplary conventional packaged RF power amplifier device  100 .  FIG. 1A  is a first orthogonal view of amplifier device  100 .  FIG. 1B  is a second orthogonal view of amplifier device  100  of  FIG. 1A .  FIG. 1C  is a third orthogonal view of amplifier device  100  of  FIG. 1A .  FIG. 1D  is a perspective view of amplifier device  100  of  FIG. 1A . Amplifier device  100  is packaged in an earless flanged LDMOS package. 
         [0005]    Specifically, amplifier device  100  comprises drain lead  101 , gate lead  102 , source lead  103 , and encapsulant  104 . Drain lead  101  and gate lead  102  are in the form of fins. Source lead  103  forms the earless flange of amplifier device  100  and may be referred to as flange  103 . Note that LDMOS packages with eared flanges (not shown) have flanges that extend further out to the sides, where the extensions may have slots or holes for screws or similar attachment means. Also note that LDMOS packages with earless flanges are sometimes referred to elsewhere as flangeless packages. Encapsulant  104  may comprise, for example, ceramic and/or epoxy. Amplifier device  100  also comprises a semiconductor die that is encapsulated by encapsulant  104  and not visible in  FIGS. 1A-1D . 
         [0006]    The semiconductor die comprises a power transistor whose terminals are conductively connected to the corresponding external leads. In other words, the power transistor&#39;s drain, source, and gate terminals are conductively connected to drain lead  101 , source lead  103 , and gate lead  102 , respectively. The transistor may also have a bulk-semiconductor terminal that is conductively connected to source lead  103 . Note that the transistor may be a compound transistor where a plurality of smaller individual transistors are connected together so as to function like a single larger transistor. The leads  101 ,  102 , and  103  are metallic—e.g., copper. Most of the heat generated by amplifier device  100  is dissipated through flange  103 , which has relatively large surface area. 
         [0007]    RF power amplifiers tend to generate a considerable amount of heat, where a higher power level generally correlates with more heat generated. Heat generated by RF power amplifiers needs to be dissipated to prevent device failure and in order to extend the operational life of the RF power amplifiers and/or nearby components. Conventional means of heat dissipation include the attachment of a finned heat sink to the RF power amplifier. 
         [0008]      FIG. 2  is a simplified exploded perspective view of exemplary conventional RF power amplifier system  200 . System  200  includes PCB  201 , metal pallet  202 , and two RF power amplifier devices  203 , which each may be substantially similar to amplifier device  100  of  FIGS. 1A-1D . PCB  201  has two apertures  204  for the two corresponding amplifier devices  203  and holes  205  for corresponding screws (not shown). Metal pallet  202  is a bulk metal plate comprising, for example, copper or aluminum. Pallet  202  has two depressions  206  for the bottom surfaces of the flanges of the two amplifier devices  203  and holes  207  for the above-mentioned corresponding screws. 
         [0009]    Amplifier devices  203  are mounted onto PCB  201 , where the drain, gate, and source leads of the amplifier devices  203  are electrically connected to corresponding contacts (not shown) on PCB  201 . The flanges of the amplifier devices  203 , which correspond to the source leads, are inserted through corresponding apertures  204  in the PCB  201  and into corresponding depressions  206  on pallet  202 . PCB  201  also has mounted thereon additional components  208 . Components  208  and amplifier devices  203  are electrically interconnected via traces (not shown) on PCB  201 . 
         [0010]    System  200  may further include a heat sink (not shown) whose top attaches to the bottom of pallet  202 . The heat sink comprises, on the top, a bulk metal plate and, on the bottom, an array of metallic fins extending out from the bulk metal plate. The top of the heat sink may include screw holes for the above-mentioned corresponding screws for attachment to pallet  202  and PCB  201 . A solid thermal medium or thermal grease (not shown) may also be applied between the pallet  202  and the heat sink to help facilitate proper thermal transfer between the mating surfaces. 
         [0011]    Metals are relatively efficient heat conductors and a conventional heat sink conducts heat from the heat-generating device to the medium surrounding the fins—which is typically air—thereby effectively increasing the heat-dissipating surface area of the heat-generating device. However, the heat-dissipating capabilities of the pallet and heat sink combination are limited and, as result, the amplifier devices  100  may need to be spaced relatively far apart from each other so that they are not damaged by excessive heat from neighboring amplifier devices  100 . More-efficient means of dissipating heat from an active device would lower the device&#39;s temperature, help extend its life, and provide additional benefits. 
       SUMMARY 
       [0012]    One embodiment of the disclosure can be an apparatus comprising a vapor chamber having a top and a bottom and enclosing a sealed cavity that is partially filled with a coolant. The vapor chamber comprises a thermo-conductive material. The top of the vapor chamber has at least one depression formed therein. The depression is adapted to receive and thermo-conductively connect to at least part of a bottom of a corresponding packaged semiconductor device mounted through a corresponding aperture in a corresponding printed circuit board (PCB). 
         [0013]    Another embodiment of the disclosure can be a method for assembling an apparatus. The method comprises mounting a set of one or more packaged semiconductor devices onto a printed circuit board (PCB) and attaching the PCB to a vapor chamber. Each packaged semiconductor device of the set of one or more packaged semiconductor devices has a bottom and is mounted through a corresponding aperture in the PCB. The vapor chamber has a top and a bottom and encloses a sealed cavity that is partially filled with a coolant. The vapor chamber comprises a thermo-conductive material. The top has a set of one or more depressions formed therein corresponding to the set of one or more packaged semiconductor devices. Each depression is adapted to receive and thermo-conductively connect to at least part of the bottom of a corresponding packaged semiconductor device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Other embodiments of the invention will become apparent. In the accompanying drawings, like reference numerals identify similar or identical elements. 
           [0015]      FIG. 1A  is a first orthogonal view of an amplifier device. 
           [0016]      FIG. 1B  is a second orthogonal view of the amplifier device of  FIG. 1A . 
           [0017]      FIG. 1C  is a third orthogonal view of the amplifier device of  FIG. 1A . 
           [0018]      FIG. 1D  is a perspective view of the amplifier device of  FIG. 1A . 
           [0019]      FIG. 2  is a simplified exploded perspective view of an exemplary conventional RF power amplifier system. 
           [0020]      FIG. 3  is a simplified exploded perspective view of an exemplary vapor-chamber amplifier (VCA) system in accordance with one embodiment of the present invention. 
           [0021]      FIG. 4  is a simplified exploded perspective view of an exemplary vapor-chamber amplifier (VCA) system in accordance with another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    A vapor chamber is a sealed metal container that is partially filled with a coolant—which may also be referred to as a working fluid—and from which most air is removed. As a result, the pressure inside the vapor chamber may be significantly lower than atmospheric pressure. A vapor chamber functions like a heat pipe and may be considered to be a planar heat pipe. A vapor chamber is typically shaped substantially like a flat box on the outside. The interior surface of a vapor chamber includes a liquid-wicking structure such as, for example, sintered metal. A vapor chamber may additionally include internal support structures—e.g., columns—to prevent collapse of the vapor chamber. 
         [0023]    In operation, a heat source on one side of a vapor chamber heats the coolant, which is initially liquid, so that the liquid evaporates. The coolant gas then travels to another side of the vapor chamber where it cools and condenses back to the liquid phase and, in the process, releases heat to that side. The liquid coolant then returns to the hot side via the wicking action of the wicking inner surface of the vapor chamber and the cycle repeats. Vapor chambers generally provide superior heat dissipation—and lighter weight—compared to bulk metal—or other solid—slugs of similar dimensions since vapor chambers dissipate more heat more uniformly than comparable slugs. Additional product weight, size, and cost reduction may be realized by using smaller and/or simpler heat sinks in conjunction with the more-efficient vapor chambers. 
         [0024]    A vapor chamber is typically assembled from a top piece and a bottom piece that are brought together, whereupon the resulting chamber is both (i) filled with the coolant liquid and (ii) evacuated. The particular order and details of the steps may vary. For example, the top and bottom pieces may be brought together and mostly sealed, then—through the unsealed segment—filled with the coolant, then evacuated, and then fully sealed. Or the chamber may be evacuated before filling with the coolant. Or one or more of the steps—e.g., filling with coolant and/or attachment of top and bottom pieces—may be performed under vacuum, or near-vacuum, conditions. The top and bottom may be attached together by, for example, welding, pressing, and/or soldering. 
         [0025]      FIG. 3  is a simplified exploded perspective view of exemplary vapor-chamber amplifier (VCA) system  300  in accordance with one embodiment of the present invention. VCA system  300  comprises two final-stage RF power amplifier devices  301 , pre-final-stage RF power amplifier device  302 , PCB  303 , vapor chamber  304 , heat sink  305 , and screws  306 . PCB  303  has two apertures  307  for the two corresponding final-stage RF power amplifier devices  301  and holes  308  for corresponding screws  306 . 
         [0026]    In VCA system  300 , vapor chamber  304  is used instead of a conventional metal pallet. Vapor chamber  304  includes top piece  309  and bottom piece  310 . Top piece  309  of vapor chamber  304  includes two depressions  311  for the two corresponding final-stage RF power amplifier devices  301  and holes  312  for corresponding screws  306 . It should be noted that holes  312  do not provide an opening in or out of the coolant-holding cavity (not shown) of vapor chamber  304 . In other words, holes  312  are walled cylinders akin to tunnels running the height of vapor chamber  304 . Depressions  311  are shaped to receive the bottoms of flanges  313  of the corresponding amplifier devices  301 . Note that depressions  311  may also be referred to as recesses. Heat sink  305  includes tapped screw holes  314  for corresponding screws  306 . 
         [0027]    Pre-final-stage RF power amplifier device  302  may be in any suitable package—such as, for example, a dual in-line package (DIP)—and is mounted on the top side of PCB  303  using any suitable mounting means, such as, for example, soldering or solder reflow. The two final-stage RF power amplifier devices  301  are packaged in LDMOS packages similar to amplifier device  100  of  FIGS. 1A-1D  and are mounted on the top side of PCB  303  using any suitable mounting means, such as, for example, soldering or solder reflow. The three RF power amplifier devices  302  and  301 , as well as additional electrical components (not shown) mounted on PCB  303 , may be electrically interconnected via conductive traces on or in PCB  303 . 
         [0028]    The gate and drain leads of power amplifier devices  301  are electrically connected to corresponding contacts (not shown) on the top surface of PCB  303 . The apertures  307  of PCB  303  are shaped to admit the flanges  313  of the corresponding power amplifier devices  301 . The flange  313  of each power amplifier device  301  is inserted through the corresponding aperture  307  in the PCB  303  and into the corresponding depression  311  of the vapor chamber  304  so that the bottom of the flange  313  is in thermo-conductive and electro-conductive contact with at least the bottom of the depression  311 . A thermo-conductive contact is a contact adapted to conduct heat and may be direct or through a conductive material. 
         [0029]    Preferably, in addition, a portion of the sides of the flange  313  is in conductive contact with a portion of the side walls of the recess  311  for increased contact area—and, consequently, increased thermal and electrical conductance—between the flange  313  and the corresponding recess  311 . The flange  313  may be directly connected to a common—e.g., ground—terminal on the PCB  303  or may be conductively connected to a common terminal on the PCB  303  via the vapor chamber  304 . Notably, the vapor chamber  304  may serve as a common—e.g., ground—path for all of the RF power amplifier devices  301  and the PCB  303 . In other words, the vapor chamber  304  may provide a uniform path for ground currents to flow among the flanges  313  and the PCB  303 . 
         [0030]    PCB  303 , vapor chamber  304 , and heat sink  305  may be attached together using screws  306 . Note that, in order to improve thermal conductance and/or improve cohesion, solder and/or thermal paste may be used between various components of system  300 . For example, solder and/or thermal paste (not shown) may be used between flanges  313  and corresponding recesses  311 . Solder and/or thermal paste (not shown) may be used between the bottom surface (as shown in  FIG. 3 ) of PCB  303  and the top surface of top piece  309  of vapor chamber  304 . Similarly, solder and/or thermal paste (not shown) may be used between the bottom surface of bottom piece  310  of vapor chamber  304  and the top surface of heat sink  305 . 
         [0031]    In one implementation, the three amplifier devices  302  and  301  are soldered (e.g., directly or reflowed) to the PCB  303 , then the resulting assemblage is soldered to already assembled vapor chamber  304 , and then that assemblage is screwed onto heat sink  305 . If the already assembled vapor chamber  304  is attached to the first assemblage—i.e., PCB  303  and amplifier devices  302  and  301 —using a solder reflow process, then special fixtures may be used in the reflow oven to ensure that the top piece  309  does not separate from the bottom piece  310  during the reflow process. 
         [0032]    In another implementation, the vapor chamber  304  is attached to the PCB  303  with a first solder—e.g., in a first solder reflow process. Note that vapor chamber  304  may be assembled during this first solder reflow process or may be assembled beforehand. After the first solder reflow process, amplifier devices  301 , amplifier device  302 , and/or other components (not shown) are mounted onto PCB  303  of that assemblage using a second, low-temperature solder reflow process with a second, different, low-temperature—e.g., bismuth-based—solder. Note that the second solder reflow process includes soldering flanges  313  of amplifier devices  301  to the corresponding depressions  311  of vapor chamber  304 . The use of the low-temperature solder helps protect the assembled vapor chamber  304  as it flows through the reflow process by reducing thermal stress on the vapor chamber  304  and helping prevent separation and/or leaks. This lower reflow processing temperature also helps protect the integrity of the solder and/or thermal paste connection that may be used between the bottom surface of PCB  303  and top piece  309 , which may be at higher risk of damage due to increased thermal conductivity of vapor chamber  304 . 
         [0033]    In another implementation, the amplifier devices  302  and  301  are soldered to the PCB  303 , then the resulting assemblage is soldered to unattached top piece  309 , and then that assemblage is soldered to unattached bottom piece  310  to form an assemblage including assembled vapor chamber  304 . 
         [0034]    In yet another implementation, amplifier devices  301 , amplifier device  302 , PCB  303 , and vapor chamber  304  are all substantially simultaneously attached together—possibly together with additional components (not shown)—in one solder reflow step using only one type of solder. Any of the above implementations may be combined with any suitable process—such as, for example, those described above—for evacuating the vapor chamber  304  of air and providing the vapor chamber  304  with its coolant. After the attachment of vapor chamber  304  to PCB  303 , amplifier devices  301  and  302 , and any other PCB-mounted components—as, for example, in any of the above-described implementations—that assemblage is attached to heat sink  305  using screws  306 . 
         [0035]    The localized heat generated inside RF power amplifier devices  301  gets quickly distributed over the volume of the vapor chamber  304  and spread over a larger surface. This rapid heat distribution reduces the overall flange temperature of the RF power amplifier devices  301  while improving the mean time before failure (MTBF) of the amplifier devices  301  and, consequently, the MTBF of the corresponding system  300  that includes the amplifiers  301 . In general, improved heat dissipation also allows placement of multiple RF power amplifier devices closer to each other, thereby reducing the overall size, weight, and cost of the corresponding product. 
         [0036]      FIG. 4  is a simplified exploded perspective view of exemplary vapor-chamber amplifier (VCA) system  400  in accordance with another embodiment of the invention. Elements of VCA system  400  that are substantially similar to corresponding elements of VCA system  300  of  FIG. 3  are similarly labeled, but with a different prefix. 
         [0037]    VCA system  400  comprises four pre-final-stage RF power amplifier devices  402  and four final-stage RF power amplifier devices  401 . Each final-stage RF power amplifier device  401  comprises one flange  413 , two gate leads  420 , and two drain leads  421  and functions substantially the same as RF power amplifier device  100  of  FIGS. 1A-1D . PCB  403  has four corresponding apertures  407  for the four amplifier devices  401 . Vapor chamber  404  has four corresponding depressions  411  for the four amplifier devices  401 . The various elements of VCA system  400  may be attached together in any suitable manner, such as, for example, described above for the corresponding elements of VCA system  300 . The power amplifier devices  401  are spaced relatively close together, where the distance separating adjacent amplifier devices  401  is less than the length of an amplifier device  401 . This relatively close spacing is possible thanks to the improved heat dissipation from the use of vapor chamber  404 . 
         [0038]    Embodiments of the invention have been described where the fins of the power amplifier devices correspond to the gate and drain nodes of the contained transistor and the flange corresponds to the source of the transistor. The invention is not, however, so limited. In alternative embodiments, the external leads of the device may be organized in different ways. In other words, the nodes of the transistor within an amplifier device may be connected in a different manner to the external leads of the device. 
         [0039]    Embodiments of the invention have been described where the final-stage RF power amplifier devices are packaged in LDMOS packages. The invention is not, however, so limited. In alternative embodiments different package types are used for the final-stage RF power amplifier devices. It should be noted that, in general, embodiments of the invention may have one or more power amplifiers devices—final-stage or not—where at least one power amplifier device is configured through an aperture in a PCB and into a recess in a vapor chamber. 
         [0040]    Embodiments of the invention have been shown where VCA systems use screws to attach and hold together various components. The invention is not, however, so limited. Alternative embodiments attach and hold together a PCB, a vapor chamber, and a heat sink without the use of any screws for the attachment. Any suitable alternative attachment means may be used to hold together the various elements of a VCA system. For example, in one alternative embodiments, the various elements, including the heat sink, are held together with solder but without screws. 
         [0041]    Embodiments of the invention have been described where a VCA system includes a conventional heat sink attached to the vapor chamber. The invention is not, however, so limited. In some alternative embodiments, the vapor chamber is attached to an element other than a conventional heat sink. In some other alternative embodiments, the bottom piece of the vapor chamber is not attached to anything but instead itself functions as a heat sink, radiating heat to its environment. 
         [0042]    Embodiments of the invention have been described where the vapor chamber is formed by attaching together a top piece and a bottom piece. The invention is not, however, so limited. In alternative embodiments, the vapor chamber may be formed by any suitable process. For example, a vapor chamber may be assembled from more than two pieces. A vapor chamber may be assembled from pieces other than a top and bottom piece. For example, a vapor chamber may be assembled from a left and a right piece. A vapor chamber may be formed from a single piece that is molded, milled, or otherwise excavated to form the inner cavity, where the cavity is sealed after partial filling with the coolant and/or de-pressurization. In these alternative embodiments, the vapor chamber&#39;s top and bottom correspond, respectively, to the above-described top piece and bottom piece. 
         [0043]    It should be noted that not every instance of plural elements—such as, for example, RF power amplifier devices, screws, and screw holes—is necessarily labeled in the figures; rather, in some cases, only exemplary instances are labeled. It should also be noted that, as used herein, the term apparatus may refer to a particular element—such as, for example, a vapor chamber—as well as a larger system incorporating that element—such as, for example, a VCA system. 
         [0044]    Exemplary embodiments have been described wherein particular entities (a.k.a. modules) perform particular functions. However, the particular functions may be performed by any suitable entity and are not restricted to being performed by the particular entities named in the exemplary embodiments. 
         [0045]    It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this invention may be made by those skilled in the art without departing from the scope of the invention as expressed in the following claims. 
         [0046]    Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation.” 
         [0047]    Unless explicitly stated otherwise, each numerical value and range should be interpreted as being approximate as if the word “about” or “approximately” preceded the value of the value or range. As used in this application, unless otherwise explicitly indicated, the term “connected” is intended to cover both direct and indirect connections between elements. 
         [0048]    For purposes of this description, the terms “couple,” “coupling,” “coupled,” “connect,” “connecting,” or “connected” refer to any manner known in the art or later developed in which energy is allowed to be transferred between two or more elements, and the interposition of one or more additional elements is contemplated, although not required. The terms “directly coupled,” “directly connected,” etc., imply that the connected elements are either contiguous or connected via a conductor for the transferred energy. 
         [0049]    The use of figure numbers and/or figure reference labels in the claims is intended to identify one or more possible embodiments of the claimed subject matter in order to facilitate the interpretation of the claims. Such use is not to be construed as limiting the scope of those claims to the embodiments shown in the corresponding figures. 
         [0050]    The embodiments covered by the claims in this application are limited to embodiments that (1) are enabled by this specification and (2) correspond to statutory subject matter. Non-enabled embodiments and embodiments that correspond to non-statutory subject matter are explicitly disclaimed even if they fall within the scope of the claims. 
         [0051]    Although the steps in the following method claims are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those steps, those steps are not necessarily intended to be limited to being implemented in that particular sequence.