Abstract:
An RF power transistor package with a rectangular ceramic base can house one or more dies affixed to an upper surface of the ceramic base. Source leads attached to the ceramic base extend from at least opposite sides of the rectangular base beneath a periphery of a non-conductive cover overlying the ceramic base. The cover includes recesses arranged to receive the one or more die, the ceramic base, gate and drain leads and a portion of the source leads. The cover further includes bolt holes arranged to clamp the ceramic base and source leads to a heat sink. Bosses at corners of the cover outward of the bolt holes exert a downward bowing force along the periphery of the cover between the bolt holes.

Description:
TECHNICAL FIELD 
     This disclosure is related to RF power transistors and more specifically to power transistor packages. 
     BACKGROUND 
     Prior art high power RF power transistors utilize a ceramic substrate, typically made from Beryllium Oxide (BeO), onto which a die or an array of dies forming the circuitry of the device is affixed. The BeO substrate is mounted on expensive Copper-Tungsten (CuW) base which can then be mounted onto a heat sink. CuW is used for the base because of its high thermal performance having the ability to efficiently conduct heat from the BeO base to the heat sink and has the same coefficient of thermal expansion (CTE) as the BeO substrate and silicon die. This comparable CTE minimizes fatigue of the joining materials used at the different interfaces. 
     An example of a prior art RF power transistor package  100  using a ceramic substrate  102  mounted on a CuW base  104  is shown in  FIGS. 1A and 1B . The cover  106  overlies the ceramic substrate  104 . To ensure an appropriate thermal path between the ceramic substrate  102  and the CuW base  104 , the bottom of the ceramic base  102  must be metallized and then brazed to the CuW base  104 . This example of an RF package  100  is an MRF154 RF MOSFET manufactured by M/A-COM. 
     The package  100  includes a drain lead  112  and a gate lead  114 . The electrically conductive CuW base  104  also serves as the source lead for the RF package device  100 . An insulator  102  separates the drain and gate leads  112 ,  114  from the CuW base  104 , a shown in the side elevation view in  FIG. 1B . 
     The CuW base  104 , however, has a CTE different from that of the heat sink  108 , which typically comprises Copper (Cu) or Aluminum (Al). The heat sink  108  expands and contracts more than the CuW base  104  as the device in package  100  controls power in cycles giving rise to numerous heat cycles seen in normal operation. As the heat sink  108  cools and contracts, the fasteners  110  mounting the CuW base  104  through holes  118  to the heat sink  108  constrain both materials from expanding and contracting freely to their natural extent and will deform the CuW base  104 , causing the CuW base  104  to bow up and away from the heat sink  108 , creating a gap between the CuW base  104  and the heat sink as time progresses. This bowing decreases the thermal performance of the CuW base  104  because less surface area of the bottom of the CuW base  104  is in contact with the heat sink  108 . The bowing also causes the ceramic substrate  102  to separate from the CuW base  104 , further reducing the thermal effectiveness of the CuW base to transfer heat from the ceramic substrate  102  to the heat sink  108 , thereby decreasing the effective life of the package  100 . 
     An example of another prior art RF power transistor package  120  is shown in  FIG. 1C . In this package  120 , the CuW base is eliminated, with the ceramic substrate  122  also serving as the base for the package  120 . The thermal path from the heat sink  128  to the ceramic substrate/base  122  is now direct. The clamp  126  overlying the cover  124  clamps the ceramic substrate/base 122  tightly against the heat sink  128  with fasteners  130 . An example of such a package is an ARF 1500 RF Power MOSFET manufactured by Advanced Power Technology. 
     The clamp  126  may be made from any suitable strong material such as steel or aluminum. Clamp  126  holds the substrate/base  122  in tight contact with the heat sink  128  from above. In this construction, the substrate/base  122  is directly fastened together with the heat sink  128 . The differential expansion rates between the ceramic base  122 , typically made of BeO, and the heat sink causes the ceramic base  122  to polish or lap the interface surface and improve the thermal transfer between the base  122  and heat sink  128  over continued thermal cycles. 
     What is needed is a power resistor or transistor package that utilizes a ceramic substrate as a base that is kept in proper contact with a heat sink over the normal expected life of the device without using extra, separate clamping devices. 
     SUMMARY OF THE DISCLOSURE 
     One aspect of the disclosure is a power transistor package that includes a rectangular ceramic base, one or more die affixed to an upper surface of the ceramic base with source leads extending from one or two opposing sides of the rectangular base, gate and drain leads extending from the other two opposing sides of the rectangular base and a non-conductive cover that overlies the ceramic base and includes a recess therein to receive the one or more die, the ceramic base and the source leads. 
     The cover includes bolt holes arranged to secure the ceramic base and source leads to a heat sink. The cover can further include bosses protruding from the bottom surface of the cover corresponding to each mounting hole and arranged toward an outer perimeter of the cover, for example at the four corners of the cover positioned outwardly adjacent to and originating from the near the edge of each mounting hole. 
     Another aspect is an RF power transistor packaging system that includes a heat sink, a rectangular ceramic base including one or more die affixed to the top surface of the base with the ceramic base overlying the heat sink, source leads connected to and extending from opposite sides of the ceramic base and a non-conductive cover clamping the source leads to the ceramic base and clamping the ceramic base and portions of the source leads extending from the ceramic base onto the heat sink. 
    
    
     
       The foregoing and other features and advantages will become more apparent from the detailed description of a preferred embodiment, which proceeds with reference to the drawings. 
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a top plan view of a prior art RF power transistor package. 
         FIG. 1B  is a side elevation view of the prior art package of  FIG. 1A  mounted on a heat sink. 
         FIG. 1C  is a side elevation view of another prior art RF power transistor package clamped to a heat sink. 
         FIG. 2  is a top plan view of an RF power transistor package according to an embodiment of the invention. 
         FIG. 3  is a top plan view of the package of  FIG. 2  with the cover removed. 
         FIG. 4  is side elevation view of the package of  FIG. 2  shown mounted on a heat sink. 
         FIG. 5  is an exploded side cross-sectional view of the package of  FIG. 2  taken along line  5 - 5  in  FIG. 2 . 
         FIG. 6  is an enlarged view of a corner of the package as shown in  FIG. 5 . 
         FIG. 7  is a detailed bottom plan view of a corner of the package of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  is a top plan view of the RF power transistor package  40  according to an embodiment of the invention and  FIG. 3  is a top plan view of the RF power transistor package  40  of  FIG. 2  with the cover  42  removed. Referring to  FIGS. 2-3 , an array of four dies  44  are affixed to a top surface of the rectangular shaped ceramic base  46 . The ceramic base  46  can be a substrate comprised of Beryllium Oxide. In the example, the package  40  is arranged rectangularly to support the array of four RF power MOSFET dies  44  that are electrically connected in parallel. More examples of multiple dies affixed to a ceramic substrate can be found in U.S. Pat. No. 6,939,743 to Frey which is incorporated by reference herein. Single die, two-die and other multiples of dies can be similarly packaged. 
     Source leads  48  are attached to the top of the ceramic base  46  and electrically connected to the dies  44  via jumper wires  45 . The source leads  48  protrude from the ceramic base  46  on opposite sides of the rectangular base  46 . Gate lead  50  and drain lead  52  are attached to the top surface of the ceramic base  46 , electrically connected to the dies  44  and protrude in opposite directions from the other two sides of the rectangular-shaped ceramic base  46 . 
     The terminology “gate,” “source” and “drain” leads pertains to MOSFET type devices. It is contemplated that embodiments of the invention can also be used with bipolar type devices and IGBT devices. In the case of bipolar devices, gate corresponds to base, source corresponds to emitter and drain corresponds to collector. In the case of an IGBT device, gate remains gate, source corresponds to emitter and drain corresponds to collector. The terms gate, source and drain will be used throughout but are meant to include base-emitter-collector and gate-emitter-collector leads. 
     Cover  42  is rectangularly shaped to cover the base  46 , die  44 , source leads  48 , gate lead  50  and drain lead  52 , providing a protective covering for these components. Mounting holes  54  are arranged at the corners of the cover  42  to receive screws  58  to secure the base  46  and source leads  48  against a heat sink  56 , as shown in  FIG. 4 . The mounting holes  54  can be arranged in a pattern that matches the mounting pattern of preexisting RF power transistor packages or in entirely new mounting arrangements. 
     The cover  42  is made of a material selected to provide high electrical insulation with low dielectric loss since the cover contacts the source leads  48  and the gate and drain leads  50 ,  52 . The cover material preferably has a high resistance to creep to avoid deformations caused by numerous heat cycles and high yield strength to maintain resistance to mechanical deformations. The cover may be made from a partially glass-filed polyetherimide such as the 30% glass-reinforced ULTEM® 2300 manufactured by GE Plastics. The cover may also be made from a partially glass reinforced liquid crystal polymer such as VECTRA® B130 manufactured by Polyplastics Co., Ltd. 
       FIG. 4  is a side elevation view of the RF power transistor package  40  showing the RF package  40  mounted on the heat sink  56  with fasteners  58 . The cover  42  includes a recess  60 , more clearly shown in  FIG. 5 , which includes a further stair-stepped recessed central die cavity  65  and is shaped to receive the dies  44 , base  46 , source leads  48 , and gate and drain leads  50 ,  52 . The recess  60  has a peripheral portion with depth slightly less than the thickness of the ceramic base  46 . Typically a ceramic base  46  can have a thickness of 40 mils (1.016 mm) and the recess  60  can be shallower by 2-5 mils (0.051-0.127 mm). Doing so ensures a tight fit of the ceramic base  46  down onto the heat sink  56  when the fasteners  58  are tightened to a predetermined torque. The nominal torque value may typically be 10 inch-pounds (113 Newton-centimeters). Also, by making the recess  60  slightly shorter than the height of the base  46  and die  44 , the cover  42  will remain tightly clamped onto the base  46  and die  44  through many heat cycles. 
     The central die cavity  65  in recess  60  is offset from the dies  44  and any jumper wires  45  used to make electrical connections from the dies  44  to the leads  48 ,  50 ,  52  so that the recess  60  generally encloses and seals the dies  44  and any jumper wires  45  without damaging the same. 
     The periphery of recess  60  includes recesses  61 , also shown in  FIG. 5 , shaped to allow the gate and drain leads  50 ,  52  to protrude from the ceramic base  46 . Such recess  61  is preferably slightly shallower in depth than the thickness of the gate and drain leads  50 ,  52  to insure a secure clamping. The gate and drain leads  50 ,  52  can be 5 mils (0.127 mm) thick with the recess  61  about 0.5 mils (0.013 mm) shallower than that. 
       FIG. 5  is an exploded cross-sectional view of the RF power transistor package  40  taken along line  5 - 5  in  FIG. 2 . As shown previously, the cover  42  is shaped to contain the dies  44  and clamp the base  46  down onto a heat sink  56 . The cover  42  also clamps the source leads  48  onto the heat sink  56 . The source leads  48  are attached to the top surface of the ceramic base  46 . The source leads  48  are then bent down around the edge of the ceramic base  46  to be in position to contact the upper surface of the heat sink  56 . The cover  42  is shaped to bend the source leads  48  and receive them in outer recesses  62 , as shown in  FIG. 7 , between the mounting holes  54 . 
     Recess  60  is shaped to receive and contact the ceramic base  46  and further shaped to include and added recess or die cavity  65  shaped to offset the central portion of the underside of the cover from the dies  44  so as not to contact the dies  44 , preventing damage to the dies  44  and jumper wires  45 . 
     The cover  42  may also include corner bosses  64  protruding from the bottom surface  78  of the cover  42  located along an outer edge of the cover  42  next to and outwardly extending from and adjacent to each of the mounting holes  54 . When the cover  42  is secured onto the heat sink  56 , the combination of the downward bolt force  66  with the upward and offset supporting force  68  of the corner boss  64  creates a downward bending moment  70  in the cover  42  inward of the mounting bolts  58 . The bending moments  70  on either side of the cover  42  balance against each other to spread the clamping force  72  across the peripheral recess  60 . 
     In the embodiment that includes corner bosses  64 , the bending moment  70  of the cover  42  will counteract any unwanted upward bowing that may be caused by the expansion or contraction of the heat sink  56  due to power/heat cycles. When the fasteners  58  are tightened, the corner bosses  64  bias the cover  42  to bow downward toward the ceramic base  46 . Even when the heat sink  56  contracts during cooling, the bending moments  70  caused by the bosses  64  force the cover down, preventing the cover  42  from bowing up. Thus, the thermal performance of the RF power transistor  40  is maintained because the ceramic base  46  is kept in close contact with the heat sink  56  over a much greater number of power/heat cycles. 
       FIG. 6  is a detailed side elevation view of a corner of the RF power transistor  40  showing the corner boss  64  and recess  60  relative to the ceramic base  46 . As described above, the height  74  of the recess  60  is slightly less then the height  76  of the ceramic base  46 . When the clamping force  72  shown in  FIG. 5  is applied to the cover  42 , this height difference creates a snug interference fit between the cover  42  and the ceramic base  46  helping to ensure a proper thermal contact between the ceramic base  46  and the heat sink  56 , shown in  FIG. 4 . 
     The bosses  64 , for example, may have a thickness  67  that protrudes 3-5 mils (0.076-0.127 mm) from the bottom surface  78  of the cover  42  with 4.5 mil (0.114 mm) thick bosses  64  shown here. As the bolts  58  in  FIG. 5  are tightened, the bosses  64  force the cover  42  to bend. 
       FIG. 7  is a detailed bottom plan view of a corner of the RF power transistor package  40  showing the source lead  48  extending through a side recess  62  of the cover  42 . The source lead  48  extends from the ceramic base  46  between the mounting holes  54 . The depth of the side recess  62  may be sized to be slightly less than the thickness of the source lead  48 . The source lead thickness may be around 5 mils (0.127 mm) with the depth of the side recess  62  sized to be about 0.5 mils (0.0127 mm) shallower. The height difference provides a snug interference fit for the source lead  48  between the cover  42  and the heat sink  56  shown in  FIG. 4 . 
     The corner boss  64  is shown positioned adjacent the mounting hole  54  and outwardly from the mounting hole  54  next to an outer edge of the cover  42 . The tipping edge  69  of the corner boss  64  is arranged perpendicular to a diagonal line extending from opposite corners of the cover  42 . By arranging tipping edges  69  of the corner bosses  64  in this manner, the bending moment  70  shown in  FIG. 5  will bend the cover  42  toward the center of the cover  42 , firmly securing the base  46  to the heat sink  56 , as shown in  FIG. 5 , and firmly securing the source leads  48  and the gated and drain leads  50 ,  52  to the ceramic base  46 . 
     Having illustrated and described the principles of our invention in a preferred embodiment thereof, it should be readily apparent to those skilled in the art that the invention can be modified in arrangement, detail and application without departing from such principles. While the embodiment described herein is especially useful in packaging RF power device, embodiments of the invention can be configured for use with lower frequency devices. We claim all modifications coming within the spirit and scope of the accompanying claims.