Abstract:
A ceramic semiconductor package provides for being surface mounted on a printed circuit board or other mounting surface. A ceramic frame is directly attached to a lead frame to define a cavity in which the base of a semiconductor device is mounted to the portion of the lead frame exposed at the bottom of the cavity. Interface terminals of the semiconductor device are attached to electrical contacts on the ceramic frame inside the cavity. The ceramic package provides a hermetic insulated path through which the signals can be routed from the device to the external leads. Additionally, because the semiconductor device is directly attached to the lead frame, power dissipation, i.e., heat dissipation, is more effectively provided by this direct connection without intervening layers of ceramic or conductor.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    Priority of U.S. Provisional Patent Application Ser. No. 61/226,413, filed Jul. 17, 2009, for “Lead Frame Based Ceramic Air Cavity Package” is claimed. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    N/A 
       BACKGROUND OF THE INVENTION 
       [0003]    Semiconductor device packages are known and available in different sizes and configurations to address issues of, for example, power dissipation and operating frequency. Ceramic packages are known but the thermal resistance through these types of packages is often too high to conduct heat dissipated by the device in a sufficient manner to avoid excessive temperature rise and a subsequent decrease in the reliability of the semiconductor device. In a conventional ceramic package, a ceramic base is provided having a conductive area on the inside base surface on which a semiconductor device is mounted and having vias therethrough that contact a heat sink on the outer bottom of the base. The thermal paths are provided through the vias and through the ceramic base material itself. 
         [0004]    Mounting the semiconductor device directly to the lead frame provides a low thermal resistance path to remove heat dissipated by the device from the package. Direct mounting of a semiconductor on a lead frame is known in injection molded plastic packages. In such plastic packages, however, the plastic completely fills the space surrounding the semiconductor device. For high frequency applications, the electromagnetic characteristics of the plastic typically detune and degrade the performance of the device embedded in the plastic. In addition, some semiconductor devices have delicate structures on the surface that can be destroyed during the molding process. 
         [0005]    What is needed is a semiconductor package that allows high power and high frequency semiconductor devices to operate effectively. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    One embodiment of a ceramic semiconductor package according to the invention provides for surface mounting on a printed circuit board or other mounting surface. A ceramic frame is directly attached to a lead frame to define a cavity in which the base of a semiconductor device is mounted to the portion of the lead frame exposed at the bottom of the cavity. Terminals of the semiconductor device are attached to electrical contacts on the ceramic frame inside the cavity. The ceramic package thus provides a hermetic insulated path through which the signals can be routed from the device to the external leads. Additionally, because the semiconductor device is directly attached to the lead frame, power dissipation, i.e., heat dissipation, is more effectively provided by this direct connection without intervening layers of ceramic or conductor as in conventional packages. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0007]    Various aspects of at least one embodiment of the present invention are discussed below with reference to the accompanying figures. It will be appreciated that for simplicity and clarity of illustration, elements shown in the drawings have not necessarily been drawn accurately or to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity or several physical components may be included in one functional block or element. Further, where considered appropriate, reference numerals may be repeated among the drawings to indicate corresponding or analogous elements. For purposes of clarity, however, not every component may be labeled in every drawing. These figures are provided for the purposes of illustration and explanation and are not intended as a definition of the limits of the invention. In the figures: 
           [0008]      FIG. 1  is a perspective drawing of a top view of an air cavity package in accordance with one embodiment of the present invention; 
           [0009]      FIG. 2  is a perspective drawing of a bottom view of the air cavity package of  FIG. 1 ; 
           [0010]      FIG. 3  is a perspective drawing of a ceramic frame of the air cavity package of  FIG. 1 ; 
           [0011]      FIG. 4  is a perspective drawing of a lead frame used in making the air cavity package of  FIG. 1 ; 
           [0012]      FIG. 5  is a perspective drawing of the ceramic frame of  FIG. 3  oriented with respect to the lead frame of  FIG. 4  used in making the air cavity package of  FIG. 1 ; 
           [0013]      FIGS. 6A-6C  are perspective drawings representing the trimming of the assembly of the ceramic frame and lead frame into the air cavity package; 
           [0014]      FIG. 7  is a perspective drawing of a top view of an air cavity package in accordance with another embodiment of the present invention; 
           [0015]      FIG. 8  is a perspective drawing of a bottom view of the air cavity package of  FIG. 7 ; 
           [0016]      FIG. 9  is a cross-section of the air cavity package shown in  FIG. 1 ; and 
           [0017]      FIGS. 10A and 10B  are, respectively, a cross-section and detailed cross-section of the air cavity package shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 61/226,413 for “Lead Frame Based Ceramic Air Cavity Package,” filed Jul. 17, 2009, the entire disclosure of which is hereby incorporated by reference herein for all purposes. 
         [0019]    An air cavity package  100  for housing a semiconductor or other electronic or electrical device or circuit comprises a ceramic portion  102  attached to a lead portion  104 , as shown in  FIG. 1 . The lead portion  104 , which is typically copper, but can be made from any one or more other known electrically conductive materials, e.g., copper or copper alloy, is attached to the ceramic portion  102  as will be described below. The ceramic portion  102  comprises an alumina ceramic material. 
         [0020]    The ceramic frame  102  comprises a plurality of conductive traces  106  that are, in one embodiment, screened onto the ceramic frame  102  in accordance with known processes. A cavity or opening  108  is defined in the ceramic frame  102  and in which a semiconductor device or circuit (not shown) may be disposed or positioned. The semiconductor or other device or circuit is mounted in the cavity  108  directly on, i.e., in contact with, a central portion  110  of the lead portion  104 . Wire bonding is used to couple the device to respective conductive traces  106  and each conductive trace  106  is coupled to a lead  112  of the lead portion  104 . After mounting of the semiconductor device in the cavity  108 , the package  100  is sealed by attachment of an appropriate lid (also not shown). The cavity package  100  is particularly suitable for high frequency applications and applications requiring high thermal dissipation, i.e., high-power devices such as power amplifiers, microwave devices, RF power generators, etc., on the order of 8-50 Watts and up to 50 Gigahertz in a package with dimensions of 3 mm on a side. 
         [0021]    As shown in  FIG. 2 , a view of the air cavity package  100  from below, the lead portion  104  includes the central portion  110  and the leads  112 . In one embodiment, as shown, the package is a QFN (Quad Flat No Leads) package for surface mounting on a printed circuit board or other mounting surface. 
         [0022]    To make the air cavity package  100 , the ceramic frame  102 , shown by itself in  FIG. 3 , is directly attached by a brazing operation to a lead frame  400 , refer to  FIG. 4 , that carries the resulting lead portion  104  as will be understood from the description to follow. The ceramic frame  102  is attached to the lead frame  400  such that the cavity  108  is positioned over the central portion  110  of the lead frame  400  as shown in  FIG. 5 . The lead frame  400  may be comprised of Copper or Copper alloy and can be 0.004 inches or thicker depending upon the needs of the intended application or device to be inserted. Further, the lead frame  400  may be gold-nickel plated as known in the art. 
         [0023]    Once the ceramic frame  102  and the lead frame  400  are attached to each other, an intermediate assembly  600 , as shown in  FIG. 6A  results. As shown in  FIG. 6A , the leads  112  of the lead frame  400  are still connected to one another and thus not isolated. A cutting step is implemented to trim the carrier portion of the lead frame  400  in order to isolate the leads  112 . A first cutting step may result in a second intermediate assembly  602 , as in  FIG. 6B , where the lead portion  104  and its leads  112  extend beyond the edges of the ceramic frame  102 . A subsequent trimming operation may result in the lead frame  400  being cut flush with the edge of the ceramic frame  102  as shown in  FIG. 6C  and in  FIG. 1 . 
         [0024]    The size of the air cavity package in accordance with embodiments of the present invention may be altered to accommodate design requirements and includes changing the size of the cavity, the number of traces and the number of leads. Thus, as shown in  FIGS. 7 and 8 , another air cavity package  700  for housing a semiconductor or other electronic or electrical device or circuit comprises a ceramic portion  702  attached to a lead portion  704  with a greater number of leads than that of the air cavity package  100  shown in  FIG. 1 . As above, the lead portion  704  may be copper or copper alloy, but can be made from any one or more other known electrically conductive materials. 
         [0025]    The ceramic frame  702  has a larger number of conductive traces  706  that may be provided by a screening operation. A cavity or opening  708  is defined in the ceramic frame  702  in which a semiconductor device or circuit may be disposed or positioned. The device is mounted in the cavity  708  to be in contact with a central portion  710  of the lead portion  704 . As above, wire bonding or another type of connection technology is used to couple the device to a respective conductive trace  706  where each conductive trace  706  is coupled to a lead  712  of the lead portion  704 . The package  700  would be sealed as described above with respect to the air cavity package  100 . As shown in  FIG. 8 , a view of the air cavity package  700  from below, the lead portion  704  includes the central portion  710  and the leads  712 . The air cavity package  700  would be assembled in a manner similar to that which has been described above and which will be described below in more detail. 
         [0026]    Referring now to  FIG. 9 , a cross-sectional view of the air cavity package  100 , the cavity  108  is shown between conductive traces  106 ,  106 ′. Each conductive trace  106 ,  106 ′ is coupled to a respective lead  112 ,  112 ′ by a corresponding via  902 ,  902 ′ disposed in the ceramic frame  102 . Each via  902 ,  902 ′ is filled with a conductive material such that an electrical signal from the semiconductor device attached to the respective conductive trace  106 ,  106 ′ will be available at the lead  112 ,  112 ′. The conductive traces  106 ,  106 ′ and the vias  902 ,  902 ′ are established by processes such as screening or lithography known to those of skill in the art. 
         [0027]    The ceramic frame  102  may be made from a plurality of layers of ceramic material connected together to arrive at the desired geometry or shape. These process steps are known to those of skill in the art. Further, conductive layers may be disposed between the layers of ceramic in order to direct signals from one area to another or to connect pads or traces to one another. 
         [0028]    Referring now to  FIGS. 10A and 10B , cross-sectional views of the air cavity package  100 , the ceramic frame  102  comprises a plurality of ceramic layers  1002  alternating with thick film co-fired conductor layers  1004 . Each ceramic layer  1002  may be 0.0004 inches to 0.020 inches in thickness and made from alumina but could be any ceramic having suitable or similar mechanical and electrical characteristics, including, but not limited to, Aluminum Nitride, Beryllium Oxide or Low Temperature Co-Fired Ceramic (LTCC). Each conductor layer  1004  may be 0.0002 inches to 0.0008 inches thick. 
         [0029]    A braze material layer  1006 , generally 0.001 inches thick, is positioned between the bottom-most conductor layer  1004  and the lead portion  104 . The braze material layer  1006  is used to mechanically couple the ceramic frame  102  to the lead frame  400  according to known processes. 
         [0030]    In order to accommodate higher frequency devices, the transition from the ceramic frame to the leads is designed to match impedances in accordance with known principles. The number of layers of ceramic material, the thickness of the copper or copper alloy, the dimensions of the traces and the vias, along with grounding considerations, are set depending upon the system requirements. In one non-limiting example, three layers of 0.006 inch tape made from High Temperature Co-Fired Ceramic (HTCC) alumina is used, the copper is 0.010 inches thick, the lines connecting the traces and/or the vias are 0.008 inches on a 0.020 inch pitch and the vias are 0.004 inches in diameter. 
         [0031]    The novel package of the present invention avoids the problems of detuning performance by providing an air filled cavity surrounding the device and structured so as to provide sufficient thermal conductivity. In the novel package, the semiconductor device is mounted directly on the central area of the lead frame which operates as a heat sink. There is thus a relatively large area thermally conductive dissipation path to remove heat from the device. Further, the ceramic frame provides a hermetic insulated path through which signals can be routed to and from the device to the external leads. 
         [0032]    Having thus described several features of at least one embodiment of the present invention, it is to be appreciated that various other alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the invention. Accordingly, the foregoing description and drawings are by way of example only, and the scope of the invention should be determined from proper construction of the appended claims, and their equivalents.