RF power transistor package

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.

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 package100using a ceramic substrate102mounted on a CuW base104is shown inFIGS. 1A and 1B. The cover106overlies the ceramic substrate104. To ensure an appropriate thermal path between the ceramic substrate102and the CuW base104, the bottom of the ceramic base102must be metallized and then brazed to the CuW base104. This example of an RF package100is an MRF154 RF MOSFET manufactured by M/A-COM.

The package100includes a drain lead112and a gate lead114. The electrically conductive CuW base104also serves as the source lead for the RF package device100. An insulator102separates the drain and gate leads112,114from the CuW base104, a shown in the side elevation view inFIG. 1B.

The CuW base104, however, has a CTE different from that of the heat sink108, which typically comprises Copper (Cu) or Aluminum (Al). The heat sink108expands and contracts more than the CuW base104as the device in package100controls power in cycles giving rise to numerous heat cycles seen in normal operation. As the heat sink108cools and contracts, the fasteners110mounting the CuW base104through holes118to the heat sink108constrain both materials from expanding and contracting freely to their natural extent and will deform the CuW base104, causing the CuW base104to bow up and away from the heat sink108, creating a gap between the CuW base104and the heat sink as time progresses. This bowing decreases the thermal performance of the CuW base104because less surface area of the bottom of the CuW base104is in contact with the heat sink108. The bowing also causes the ceramic substrate102to separate from the CuW base104, further reducing the thermal effectiveness of the CuW base to transfer heat from the ceramic substrate102to the heat sink108, thereby decreasing the effective life of the package100.

An example of another prior art RF power transistor package120is shown inFIG. 1C. In this package120, the CuW base is eliminated, with the ceramic substrate122also serving as the base for the package120. The thermal path from the heat sink128to the ceramic substrate/base122is now direct. The clamp126overlying the cover124clamps the ceramic substrate/base122tightly against the heat sink128with fasteners130. An example of such a package is an ARF 1500 RF Power MOSFET manufactured by Advanced Power Technology.

The clamp126may be made from any suitable strong material such as steel or aluminum. Clamp126holds the substrate/base122in tight contact with the heat sink128from above. In this construction, the substrate/base122is directly fastened together with the heat sink128. The differential expansion rates between the ceramic base122, typically made of BeO, and the heat sink causes the ceramic base122to polish or lap the interface surface and improve the thermal transfer between the base122and heat sink128over 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.

DETAILED DESCRIPTION

FIG. 2is a top plan view of the RF power transistor package40according to an embodiment of the invention andFIG. 3is a top plan view of the RF power transistor package40ofFIG. 2with the cover42removed. Referring toFIGS. 2-3, an array of four dies44are affixed to a top surface of the rectangular shaped ceramic base46. The ceramic base46can be a substrate comprised of Beryllium Oxide. In the example, the package40is arranged rectangularly to support the array of four RF power MOSFET dies44that 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 leads48are attached to the top of the ceramic base46and electrically connected to the dies44via jumper wires45. The source leads48protrude from the ceramic base46on opposite sides of the rectangular base46. Gate lead50and drain lead52are attached to the top surface of the ceramic base46, electrically connected to the dies44and protrude in opposite directions from the other two sides of the rectangular-shaped ceramic base46.

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.

Cover42is rectangularly shaped to cover the base46, die44, source leads48, gate lead50and drain lead52, providing a protective covering for these components. Mounting holes54are arranged at the corners of the cover42to receive screws58to secure the base46and source leads48against a heat sink56, as shown inFIG. 4. The mounting holes54can be arranged in a pattern that matches the mounting pattern of preexisting RF power transistor packages or in entirely new mounting arrangements.

The cover42is made of a material selected to provide high electrical insulation with low dielectric loss since the cover contacts the source leads48and the gate and drain leads50,52. 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. 4is a side elevation view of the RF power transistor package40showing the RF package40mounted on the heat sink56with fasteners58. The cover42includes a recess60, more clearly shown inFIG. 5, which includes a further stair-stepped recessed central die cavity65and is shaped to receive the dies44, base46, source leads48, and gate and drain leads50,52. The recess60has a peripheral portion with depth slightly less than the thickness of the ceramic base46. Typically a ceramic base46can have a thickness of 40 mils (1.016 mm) and the recess60can be shallower by 2-5 mils (0.051-0.127 mm). Doing so ensures a tight fit of the ceramic base46down onto the heat sink56when the fasteners58are tightened to a predetermined torque. The nominal torque value may typically be 10 inch-pounds (113 Newton-centimeters). Also, by making the recess60slightly shorter than the height of the base46and die44, the cover42will remain tightly clamped onto the base46and die44through many heat cycles.

The central die cavity65in recess60is offset from the dies44and any jumper wires45used to make electrical connections from the dies44to the leads48,50,52so that the recess60generally encloses and seals the dies44and any jumper wires45without damaging the same.

The periphery of recess60includes recesses61, also shown inFIG. 5, shaped to allow the gate and drain leads50,52to protrude from the ceramic base46. Such recess61is preferably slightly shallower in depth than the thickness of the gate and drain leads50,52to insure a secure clamping. The gate and drain leads50,52can be 5 mils (0.127 mm) thick with the recess61about 0.5 mils (0.013 mm) shallower than that.

FIG. 5is an exploded cross-sectional view of the RF power transistor package40taken along line5-5inFIG. 2. As shown previously, the cover42is shaped to contain the dies44and clamp the base46down onto a heat sink56. The cover42also clamps the source leads48onto the heat sink56. The source leads48are attached to the top surface of the ceramic base46. The source leads48are then bent down around the edge of the ceramic base46to be in position to contact the upper surface of the heat sink56. The cover42is shaped to bend the source leads48and receive them in outer recesses62, as shown inFIG. 7, between the mounting holes54.

Recess60is shaped to receive and contact the ceramic base46and further shaped to include and added recess or die cavity65shaped to offset the central portion of the underside of the cover from the dies44so as not to contact the dies44, preventing damage to the dies44and jumper wires45.

The cover42may also include corner bosses64protruding from the bottom surface78of the cover42located along an outer edge of the cover42next to and outwardly extending from and adjacent to each of the mounting holes54. When the cover42is secured onto the heat sink56, the combination of the downward bolt force66with the upward and offset supporting force68of the corner boss64creates a downward bending moment70in the cover42inward of the mounting bolts58. The bending moments70on either side of the cover42balance against each other to spread the clamping force72across the peripheral recess60.

In the embodiment that includes corner bosses64, the bending moment70of the cover42will counteract any unwanted upward bowing that may be caused by the expansion or contraction of the heat sink56due to power/heat cycles. When the fasteners58are tightened, the corner bosses64bias the cover42to bow downward toward the ceramic base46. Even when the heat sink56contracts during cooling, the bending moments70caused by the bosses64force the cover down, preventing the cover42from bowing up. Thus, the thermal performance of the RF power transistor40is maintained because the ceramic base46is kept in close contact with the heat sink56over a much greater number of power/heat cycles.

FIG. 6is a detailed side elevation view of a corner of the RF power transistor40showing the corner boss64and recess60relative to the ceramic base46. As described above, the height74of the recess60is slightly less then the height76of the ceramic base46. When the clamping force72shown inFIG. 5is applied to the cover42, this height difference creates a snug interference fit between the cover42and the ceramic base46helping to ensure a proper thermal contact between the ceramic base46and the heat sink56, shown inFIG. 4.

FIG. 7is a detailed bottom plan view of a corner of the RF power transistor package40showing the source lead48extending through a side recess62of the cover42. The source lead48extends from the ceramic base46between the mounting holes54. The depth of the side recess62may be sized to be slightly less than the thickness of the source lead48. The source lead thickness may be around 5 mils (0.127 mm) with the depth of the side recess62sized to be about 0.5 mils (0.0127 mm) shallower. The height difference provides a snug interference fit for the source lead48between the cover42and the heat sink56shown inFIG. 4.

The corner boss64is shown positioned adjacent the mounting hole54and outwardly from the mounting hole54next to an outer edge of the cover42. The tipping edge69of the corner boss64is arranged perpendicular to a diagonal line extending from opposite corners of the cover42. By arranging tipping edges69of the corner bosses64in this manner, the bending moment70shown inFIG. 5will bend the cover42toward the center of the cover42, firmly securing the base46to the heat sink56, as shown inFIG. 5, and firmly securing the source leads48and the gated and drain leads50,52to the ceramic base46.

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.