Abstract:
Top-side cooling of Radio Frequency (RF) products in air cavity packages is provided. According to one aspect, an air cavity package comprises a substrate, a RF component mounted to the substrate, and a lid structure comprising a first material and being mounted to the substrate that covers the RF component such that a cavity is formed within the lid structure and about the RF component. At least one opening is provided in a top portion of the lid. The air cavity package also comprises a heat transfer structure comprising a second material and comprising a heat path extending from the top surface of the substrate through the opening(s) in the lid to the top outer surface of the air cavity package to provide a top-side thermal interface. In one embodiment, the lid is comprised of a molded material that absorbs RF signals and the heat transfer structure is metal.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of provisional patent application Ser. No. 62/291,664, filed Feb. 5, 2016, the disclosure of which is hereby incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to air cavity composite packages with top-side cooling and low-inductance leads suitable for use by Radio Frequency (RF) products. 
       BACKGROUND 
       [0003]    Air cavity packages are used to minimize dielectric loading effects that limit the bandwidth of high-frequency semiconductor devices. In conventional surface-mount air cavity packages, the semiconductors are mounted on a substrate including metal geometries that conduct heat downward, from the bottom of the Integrated Circuit (IC) to the bottom of the package. High frequency electrical signals are typically distributed to the side of the package. The package is in turn mounted to an application board, which serves the dual purposes of (a) routing Radio Frequency (RF) signals and Direct Current (DC) power to the package and (b) conducting heat away from the package. Traditional laminate air cavity RF packages dissipate heat from the bottom side. The active semiconductor die, typically Microwave Monolithic Integrated Circuits (MMICs), are mounted on a substrate made of fiberglass material. The active side of the MMICs faces the air. The heat from the MMICs is removed downward: from the bottom side of the MMIC, through the die attach material, the top metal layer on the substrate, thermal vias in the substrate, and then to an external heat sink. An example conventional surface-mounted air cavity package is shown in  FIG. 1 . 
         [0004]      FIG. 1  illustrates a conventional multi-channel air cavity laminate module with a lid and a bottom-side heat path. In the embodiment illustrated in  FIG. 1 , a conventional laminate air cavity package  10  is mounted to an application board  12 , typically by soldering. The solder joints joining the conventional air cavity package  10  to the application board  12  are shown as black-filled rectangles in  FIG. 1 . Within the conventional laminate air cavity package  10 , devices  14  are mounted to a substrate  16  that includes both electrical vias  18  and thermal vias  20 . A single via may function both as an electrical via and a thermal via. The electrical vias  18  provide electrical connections between pads on the device  14  and traces on or within the application board  12 . In one embodiment, the device  14  is connected to the electrical vias  18  via wire bonds  22 . The thermal vias  20  provide a conductive heat path between the device  14  so that heat can be transferred away from the device  14  and into the application board  12 , e.g., via a heat transfer path  24 . This approach is referred to as “bottom-side” cooling. For bottom-side cooling, the application board  12  must absorb and dissipate heat as well as route the RF and DC signals. Because the devices  14  are mounted to the inner surface of the air cavity package  10  closest to the application board  12 , this configuration is referred to as having “bottom-mounted” devices. A lid  26  covers the devices  14  and forms one or more air cavities. 
         [0005]    Some application boards, however, are designed for RF signal and DC power routing only and cannot provide a suitable bottom-side heat path. For these application boards, the conventional bottom-side cooling approach shown in  FIG. 1  is unworkable. 
         [0006]    Another conventional approach directs heat flow upward away from the application board  12 , an approach referred to as “top-side cooling.” Applications requiring top-side cooling have made use of specially constructed Ball Grid Array (BGA) packages in which the MMIC or other device is attached to a heat sink on the top of the package. An example of this is shown in  FIG. 2 . 
         [0007]      FIG. 2  illustrates a conventional BGA package with top-side cooling. In  FIG. 2 , an air cavity laminate package  28  contains the device  14  mounted to a metal base  30  that functions to transfer heat away from the devices  14  to the surrounding air instead of going into the application board  12 . A lid  32  covers the devices  14  and forms one or more air cavities. The lid  32  also provides the electrical vias  18  that electrically connect pads to the devices  14  to traces on or within the application board  12 . In the embodiment illustrated in  FIG. 2 , the laminate package  28  is attached to the application board  12  via solder balls, which are shown as black-filled circles in  FIG. 2 . The electrical vias  18  may be connected to the pads of the devices  14  via the wire bonds  22 . Because the devices  14  are mounted to the inner surface of the package that is farthest from the application board  12 , this configuration is referred to as having “top-mounted” devices. 
         [0008]    Due to the position of the devices  14  at some distance away from the application board  12 , however, the electrical vias  18  must be routed horizontally from the pins of the device  14  to the vertical portions of the lid  32 , down through those vertical portions of the lid  32 , and horizontally again to the application board  12 . This creates electrical vias  18  that are relatively long, which results in increased inductance (L), resistance (R), and/or capacitance (C). RF circuits particularly may not be able to tolerate the additional L, R, or C, in which case the BGA package shown in  FIG. 2  is unsuitable for higher frequency RF modules. 
         [0009]    Thus, there is a need for air cavity laminate packages with top-side cooling that are suitable for use by RF devices and circuits. 
       SUMMARY 
       [0010]    Top-side cooling of Radio Frequency (RF) products in air cavity packages is provided. According to one aspect, an air cavity composite package comprises a substrate, a RF component mounted to the substrate, and a lid structure comprising a first material and being mounted to the substrate, that covers the RF component such that a cavity is formed within the lid structure and surrounding (e.g., above and around the sides of, and possibly under) the RF component. At least one opening is provided in a top portion of the lid. The air cavity composite package also comprises a heat transfer structure comprising an extending from the top surface of the substrate through the at least one opening in the lid to the top outer surface of the air cavity composite package to provide a top-side thermal interface, e.g., for an external heatsink. The heat transfer structure comprises a second material different from the first material. 
         [0011]    In one embodiment, the substrate contains signal carrying components and the lid contains no signal carrying components. In one embodiment, the substrate comprises a horizontal heat spreader and both the lid structure and the RF component are mounted to the horizontal heat spreader. In one embodiment, the substrate comprises a thermally insulating layer that prevents or reduces heat flow to the bottom side of the air cavity composite package. In one embodiment, the substrate comprises heat transfer structures that provide or enhance heat flow to the bottom side of the air cavity composite package. 
         [0012]    In one embodiment, the RF component comprises an integrated circuit die. In one embodiment, the RF component operates as a radio frequency transmitter or receiver. 
         [0013]    In one embodiment, the lid comprises a top structure that includes the at least one opening and at least one side wall that extends from the top structure to the substrate. In one embodiment, the lid is comprised of a molded component. In one embodiment, the lid is comprised of insulating or RF absorptive materials to control reflections and transmissions within the cavity. 
         [0014]    In one embodiment, the heat transfer structure is comprised of metal. In one embodiment, the heat path contacts the top surface of the substrate proximate to the RF component. In one embodiment, the heat path forms an internal wall of the package. In one embodiment, the heat transfer structure is a component of an external wall of the package. In one embodiment, the top-side thermal interface comprises fins. 
         [0015]    In one embodiment, the top-side thermal interface resides over a top portion of the lid structure, e.g., forming a “T-shaped” cross-section. In one embodiment, the heat transfer structure comprises a plurality of heat paths, each of the plurality of heat paths extending from the top-side thermal interface through a respective one of the at least one openings in the lid. For example, the heat transfer structure includes two heat paths which extend to the top-side thermal interface such that the heat transfer structure has a Pi-shaped (“Tr-shaped”) cross-section. 
         [0016]    In one embodiment, wherein the lid structure, the heat path, and substrate form a plurality of cavities, each cavity having a respective RF component mounted to the substrate within the cavity. In one embodiment, each RF component operates as a radio frequency transmit or receive channel such that each channel is contained within its separate respective cavity. 
         [0017]    According to another aspect, a method for top-side cooling of RF products in air cavity composite packages comprises providing a substrate, mounting a RF component to the substrate, and mounting to the substrate a lid structure comprising a first material and covering the RF component such that a cavity is formed within the lid structure and about the RF component and where at least one opening is provided in a top portion of the lid. A heat transfer structure is provided, the heat transfer structure comprising a heat path extending from the top surface of the substrate through the at least one opening in the lid to the top outer surface of the air cavity composite package to provide a top-side thermal interface. The heat structure comprises a second material different from the first material. 
         [0018]    Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description in association with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the disclosure and, together with the description, serve to explain the principles of the disclosure. 
           [0020]      FIG. 1  illustrates a conventional multi-channel air cavity package with bottom-side cooling. 
           [0021]      FIG. 2  illustrates a conventional Ball Grid Array (BGA) package with top-side cooling. 
           [0022]      FIG. 3  illustrates an exemplary air cavity composite package with top-side cooling and bottom-mounted devices according to an embodiment of the subject matter described herein, showing both top-side and bottom-side heat paths. 
           [0023]      FIG. 4  illustrates an exemplary air cavity composite package with top-side cooling and bottom-mounted devices according to another embodiment of the subject matter described herein, showing only a top-side heat path. 
           [0024]      FIG. 5  illustrates an exemplary air cavity composite package with top-side cooling and bottom-mounted devices according to another embodiment of the subject matter described herein, showing a “T-shaped” cross-section with cooling fins. 
           [0025]      FIG. 6  illustrates an exemplary air cavity composite package with top-side cooling and bottom-mounted devices according to another embodiment of the subject matter described herein, showing a Pi-shaped (“Tr-shaped”) cross-section without cooling fins. 
           [0026]      FIG. 7  illustrates an exemplary air cavity composite package with top-side cooling according to another embodiment of the subject matter described herein, showing an “m-shaped” cross-section having internal heat paths near the package outer walls. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    Electrical systems require an efficient method to remove the heat generated by the chips (integrated circuits, or “IC”s). These systems will fail when the chips are overheated. Air cavity packages are used to minimize dielectric loading effects that limit the bandwidth of high-frequency semiconductor devices. In conventional surface-mount air cavity packages, the semiconductors are mounted on a substrate including metal geometries that conduct heat downward, from the bottom of the IC to the bottom of the package. The package is in turn mounted to an application board, which serves the dual purposes of (a) routing Radio Frequency (RF) signals and Direct Current (DC) power to the package and (b) conducting heat away from the package. Thus, conventional air cavity packages require heat sinks at the side closest to the semiconductor devices, which are usually located at the bottom of the package, i.e., the side of the package that is mated to the application board. 
         [0028]    Some application boards are designed for RF signal and DC power routing only and cannot provide a suitable heat path. In these environments, the heat path must be through the top of the package, either by convection and radiation, or through a top-side thermal interface designed to conduct heat from the top surfaces of the packaged semiconductor devices. 
         [0029]    The present disclosure relates to methods and systems to remove heat from the substrate, very close to the bottom-mounted devices, to the top-side of the package. This approach is suitable for use by RF devices and circuits and also suitable for use where there is no provision for bottom-side cooling, e.g., for pluggable modules. In a typical application, a “T-shaped” or Pi-shaped (“π-shaped”) thermally conductive element is placed adjacent to a chain of amplifiers (e.g., chips). Examples of thermally conductive material include, but are not limited to, copper, aluminum, other metals, and thermally conductive ceramics, such as Aluminum Nitride (AlN). This element is joined by a conductive material such as epoxy or solder to the substrate on which the chips are mounted. Heat flows from the chip to the substrate to the thermally conductive element and then to an external heat sink. The thermally conductive element can be inserted into a lid and jointed by epoxy as a preassembled component. The lid material can be plastic, fiberglass, or absorptive material. The placement of the thermally conductive structure does not increase the RF trace length, and therefore ensures the maximum operating bandwidth. 
         [0030]      FIG. 3  illustrates an exemplary air cavity composite package with top-side cooling according to an embodiment of the subject matter described herein. In the embodiment illustrated in  FIG. 3 , the air cavity composite package  34  is mounted to the application board  12 , e.g., via solder joints, shown as black-filled rectangles in  FIG. 3 . Within the air cavity composite package  34 , the devices  14  are mounted to the substrate  16  that includes both the electrical vias  18 , which provide electrical connections between pads on the device  14  and traces on or within the application board  12 , and the thermal vias  20 . In one embodiment, the device  12  is connected to the electrical vias  18  via the wire bonds  22 . The thermal vias  20  provide a conductive heat path between the device  14  so that heat can be transferred away from the device  14  and into the application board  12 , e.g., via the heat transfer path  24 . The lid  26  covers the devices  14  and forms one or more air cavities. 
         [0031]    In the embodiment illustrated in  FIG. 3 , the lid  26  creates an air cavity over each of the devices  14 . In one embodiment, the lid  26  is designed to provide an air cavity over a collection of devices, such as an air cavity over each channel of a multichannel transceiver. The air cavities above the semiconductor die  14  serve to minimize dielectric loading on the high-frequency semiconductor die and to provide room for the wire bonds  22  and other components inside the air cavity composite package  34 . The lid  26  material may be made of insulating material or RF absorptive material to minimize reflection and transmission. Many applications require multiple channels, and the lid material can be compartmentalized to provide RF isolation between channels. 
         [0032]    In the embodiment illustrated in  FIG. 3 , the thermal vias  20  extend from a heat spreader plate  36  below the devices  14 . An additional heat transfer structure  38  is present to draw heat from the heat spreader plate  36  up to the top of the air cavity composite package  34 . Thus, in addition to the bottom-side heat transfer path  24 , the air cavity composite package  34  provides a top-side heat transfer path  40 . The heat transfer structure  38  may interface with an active or passive heat exchanger for increased heat transfer capacity. In one embodiment, a conductive epoxy, solder, or other thermally conductive material may be used to provide a good interface between the heat transfer structure  38  and whatever component with which the heat transfer structure  38  is interfacing. 
         [0033]    The 3 dB bandwidth of air cavity composite package  34  is between 0 to 60 GHz. In comparison, the BGA package  28  is typically limited to less than 10 GHz. The improvement in RF performance is a result of the minimal trace length between the IC and the application board. In modern data communication and telecommunication systems, the typical data rate is 20 to 60 GHz per channel. The transceivers are housed in a pluggable housing, without heatsinking on the application board. Unlike the conventional packages shown in  FIGS. 1 and 2 , the air cavity composite packages shown in  FIGS. 3-7  meet the performance requirements for this type of communication systems. 
         [0034]      FIG. 4  illustrates an exemplary air cavity composite package with top-side cooling according to another embodiment of the subject matter described herein. In the embodiment illustrated in  FIG. 4 , the air cavity composite package  34  is soldered to an application board  42  that is designed for RF signal and DC power routing only, and cannot provide a suitable heat path. In the embodiment illustrated in  FIG. 4 , the substrate  16  includes the electrical vias  18  but no thermal vias. In this embodiment, heat from the devices  14  travels through the heat spreader plate  36  and up through the heat transfer structure  38  via the heat transfer path  40 . In this configuration, heat transferred downward into the application board  42  is reduced or prevented entirely. 
         [0035]    In one embodiment, the heat transfer structure  38  comprises a high thermal conductivity material such as the thermally conductive materials listed above. The design retains the flexibility to use a portion of the lid  26  for insulating or RF absorptive material while allowing heat to flow upward. In one embodiment, heat flows horizontally using metal geometries on the package substrate and then upward through one or more of the package walls. The horizontal path in the substrate may consist of either conventional printed circuit board (PCB) traces or conductive material attached to the surface of the PCB for the purpose of lateral heat conduction, such as the heat spreader plate  36 . The top surface of the air cavity composite package  34  may be used to dissipate heat by radiation or convection, or to mate with a heat sink in the application module. This approach frees the application board  42  from any need to conduct heat, so that the board design can focus on RF and DC signal routing only. 
         [0036]      FIG. 5  illustrates a partially-exploded isometric view of an exemplary air cavity composite package with top-side cooling according to another embodiment of the subject matter described herein. In the embodiment illustrated in  FIG. 5 , the heat transfer structure  38 , which is generally “T-shaped”in cross section, includes cooling fins. The lid  26  is transparent to show example locations of the devices  14  and the heat spreader plate  36  atop the substrate  16 . When the heat transfer structure  38  and the remaining portions of the air cavity package illustrated in  FIG. 5  are assembled, the metal protrusion at the bottom of the heat transfer structure  38  would extend through the slot in the top of the lid  26  and contact the top surface of the heat spreader plate  36 , forming the internal heat path to conduct heat upward and away from the devices  14 . 
         [0037]      FIG. 6  illustrates an isometric view of an exemplary air cavity composite package with top-side cooling according to another embodiment of the subject matter described herein. In the embodiment illustrated in  FIG. 6 , the heat transfer structure  38  is generally “π-shaped” in cross section. The lid  26  is transparent to show example locations of the devices  14  and the heat spreader plate  36  atop substrate  16 . 
         [0038]      FIG. 7  illustrates an exemplary air cavity composite package  44  with top-side cooling according to another embodiment of the subject matter described herein. In the embodiment illustrated in  FIG. 7 , the heat transfer structure  38  has three internal vertical heat paths: one in the center of the air cavity package  44 , one on the left side of the air cavity package  44 , and one on the right side of the air cavity package  44 .  FIG. 7  illustrates the point that the internal heat paths may be near the outside walls of the air cavity package  44 , and are not restricted to being just in the center of the package. This provides additional vertical internal heat paths, such as a heat path  46  shown in  FIG. 7 . 
         [0039]    The embodiments described above are intended to be illustrative and not limiting. For example, there are multiple approaches to cool the Microwave Monolithic Integrated Circuits (MMIC) from the top-side without deviating from the concepts of the present disclosure. The heat transfer structure  38  may touch the substrate  16  and/or the heat spreader plate  36  in any number of locations, not just the “T-shaped” and “Tr-shaped” configurations seen in  FIGS. 5 and 6 . The heat transfer structure  38  may be used with or without substrate thermal vias, with or without substrate metal planes, and with or without an added horizontal heat spreader. The exemplary air cavity packages described herein may have multiple heat transfer structures  38 . The concepts described herein may be scaled up—for example, the air cavity composite package  34  may have multiple air cavities, each cavity being for a single channel or multiple channels. Other such variations will be obvious to those skilled in the art, and are considered to be within the scope of the present disclosure. 
         [0040]    The present disclosure provides for, but is not limited to, the following:
       An air cavity package with a vertical heat path incorporated into one or more internal walls and a top surface provided to interface with top-side heat removal in the application, constructed to enable placement of the vertical heat path close to the active MMICs.   An air cavity package with a vertical heat path incorporated into one or more external walls.   A vertical heat path consisting of low-cost thermally conductive material, such as copper, aluminum, other metals, or thermally conductive ceramics.   Vertical heat path and heat spreader in an “I,” “T,” “π,” or finned configuration.   Heat spreader connected to the MMIC using one or more s: (a) the substrate thermal via, (b) the substrate copper or other thermally conductive plane, or (c) the added horizontal thermal spreader.   An air cavity package with a vertical heat path into one or more internal walls and a top surface provided to interface with top-side heat removal in the application, in combination with insulating or RF absorptive materials to control reflections and transmissions within the cavity.   An air cavity package with a vertical heat path incorporated into one or more internal walls, a top surface provided to interface with top-side heat removal in the application, and a thermally insulated layer to reduce heat flow to the bottom side of the package. This insulated layer is formed as part of the substrate by not extending the thermal vias under the chips on the top surface all the way to the bottom surface of the substrate. This insulating layer prevents the chips on the bottom side from overheating and allows the chips on the top-side of the substrate to dissipate the heat generated from operation.       
 
         [0048]    Those skilled in the art will recognize improvements and modifications to the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein.