High Current Hermetic Package

The present invention relates generally to a package for an electronic device, and more particularly to a package for hermetically sealing electronic devices and circuits.

It is well known in the art to use packages for enclosing electronic devices, such as semiconductor chips, power semiconductor chips and the like.

Hermetically sealed devices providing connections to the enclosed device are commonly used in the art. A hermetic package arrangement of this type for large scale integrated circuits, or hybrid circuits is disclosed, for example, in U.S. Pat. No. 4,076,955, issued Feb. 28, 1978 to Gates, Jr. A device, such as an integrated circuit wafer, is attached to a base and a sealing ring enclosure surrounding the device is hermetically sealed to the base. The sealing ring has a greater height than the wafer. Above the device, a lid is hermetically sealed to a sealing area provided on the sealing ring. Soldering is used to secure various lid embodiments to areas to be covered and hermetically sealed on the packages. Radial leads with conventional wire bonding are disclosed for providing connections to the device.

A hermetic package for integrated circuits used in high power applications is disclosed in U.S. Pat. No. 4,3398,621, issued Jul. 6, 1982 to Braun. This shows an integrated circuit mounted on a surface of a multilayer ceramic member and wire-bonded to electrically conductive tabs near the periphery of the device. Input/output pins are provided on the exterior surface of a ceramic lid. The package includes a ceramic base member having an interior recess upon which on integrated circuit is mounted. The plurality of electrically conductive tabs is arranged about the circumference of the integrated circuit, for connection via wires bonded thereto at one end and to contact pads on the planer surface of the ceramic base member. The metallized lines are used to connect the pads to electrically conductive bosses on the exterior surface of the ceramic lid for providing input and output pins. Brazing is used to hermetically seal the input and output pins to the bosses, and solder reflow is used for sealing the base and cover members to each other. Also, solder reflow is used for establishing electrical connections between the die and the pins via the corresponding pads.

An important consideration in the design of a package enclosing a power device is the difference in the temperature coefficient of expansion of different materials that may be utilized in adjacent parts in such a package. Problems caused by such differences are alleviated by utilizing materials having temperature coefficients as well matched as possible. Thus, U.S. Pat. No. 4,646,129 issued Feb. 24, 1987 to Yerman et al., for example, discloses a hermetic package including a gasket with a temperature coefficient close to that of a dielectric plate of the hermetic package. The dielectric plate has transistor electrodes on the underside thereof. Electrical access to these electrodes is provided from the top side of the dielectric plate by corresponding leads which are connected via conducting-through holes in the dielectric plate.

U.S. Pat. No. 4,651,192 issued on Mar. 17, 1987, to Matsushita et al. discloses a ceramic package for a semi-conductor integrated circuit device. The package includes an insulating substrate fabricated from a ceramic material, upon which is mounted one or more integrated circuits. An alumina ceramic cap is bonded to the ceramic substrate via a solder glass layer, for forming a hermetically sealed package. Terminals for providing electrical connections to the package are sandwiched between the cap and substrate, whereby interior ends of the terminals are electrically connected to the integrated circuit, with the other ends of the terminals being external to the package for providing signal and power connections. The particular ceramic materials used are specifically disclosed for providing very closely matched coefficients of thermal expansion.

Various other package arrangements are disclosed, for example, in U.S. Pat. No. 2,990,501 issued Jun. 27, 1961 to Cornelison et al.; U.S. Pat. No. 4,514,587 issued Apr. 30, 1985 to Van Dyk Soerewyn; and U.S. Pat. No. 4,560,826 issued Dec. 24, 1985 to Burns et al. Cornelison et al. discloses the use of a housing, including a dish-shaped metal header assembly in a lower portion, electrical leads having portions of the lead outside of the housing, and portions within the interior thereof for making electrical connection to a semiconductor element. Insulating material fills the lower portion of the header assembly for electrically isolating the leads from the metal header. The portions of the leads protruding from the housing are soldered to eyelets, for making electrical connections to printed circuit pads of a substrate. Cornelison et al. also discloses that the lower metal shell can be formed from a ceramic or glass disc with a metallic ring thereabout for bonding the housing to the disc. Van Dyk Soerewyn discloses a power semi-conductor package in a top-hat configuration, that includes an encapsulant within the cylindrical section of the enclosure about the integrated circuit assemblies mounted therein. Burns et al. discloses a hermetically sealed package for an integrated circuit (IC). The package includes the mounting of an integrated circuit on a substrate with all electrical connections to the IC made on top of the substrate to vias. The opposite side of the substrate includes leads electrically connected to leads on the opposite side for providing signal and power connections to the package. In this regard, a pedestal is included for electrically interconnecting the inner and outer leads. A sealing ring is provided about the circumference of the area in which the integrated circuit is mounted, for sealing to a ceramic cap via a sealing skirt about the circumference of the bottom edge of the cap.

It is herein recognized that the use of, for example, aluminum wire bonds for providing connections between a device, such as an IC chip in the package and outside circuitry may require that the metallization on the chip be preferably aluminum and that such a restriction may be undesirable. Furthermore, the use of wire bonds tends to introduce a relatively high parasitic inductance in the connection. It is herein also recognized that connections utilizing straps are preferable in this regard, and that top-side connection to a device is desirable. However, it is also desirable that, in meeting such requirements, the package profile be kept low and that, in so far as is possible, conventional manufacturing processes be utilizable.

It is a goal of the invention to achieve a device package wherein closely matched thermal coefficients of expansion are utilized while providing for good heat dissipation capability. It is another goal of the invention to achieve high current carrying capacity in the connections to a device hermetically contained in the package. It is a further goal of the invention to provide internal electrical isolation within the package. Another goal of the invention is to achieve the foregoing goals in a package that is of light weight and has a small profile

In accordance with one embodiment of the invention, a package for a device, comprises a header having a substantially flat upper surface. An insulating disk is affixed to the upper surface of the header for having the device mounted thereon. A barrel having upper and lower portions of respectively greater and smaller diameters and having a step between the diameters, has a lower edge thereof attached to the upper surface of the header so as to surround the disk. A ceramic lid in the upper portion of the barrel abuts the step and is attached thereat.

In accordance with another embodiment of the invention, a package for hermetically sealing an electronic device, comprises a metal header having an under surface adapted for attachment to a mounting surface and having a substantially flat upper surface. An insulating disk is affixed to the upper surface of the header, the disk being adapted for having the electronic device affixed thereupon. A barrel, having upper and lower generally cylindrical portions of respectively greater and smaller diameters, has a diameter step therebetween, a lower edge of the barrel being attached to the upper surface of the header so as to contain the insulating disk within the lower portion of the barrel. A ceramic lid is attached within the upper portion of the barrel so as to abut the diameter step.

In accordance with another embodiment of the invention, an insulating disk affixed to the upper surface of the header has a conductive pattern formed thereon for mounting lead connections for the electronic device. A ceramic lid is hermetically attached within the upper portion of the barrel so as to abut the diameter step, the lid having a hole pattern therein for passing therethrough the lead connections of the electronic device. In accordance with still another embodiment of the invention, a package for hermetically sealing an electronic device, includes an electronic device mounted on the insulating disk.

The invention will be more fully understood from the following detailed description of the preferred embodiments of the invention, in conjunction with the drawing, in which like items are identified by the same reference designation, whereon:

FIG. 1 shows an exploded assembly view, in isometric projection, of one embodiment of the invention;

FIG. 2 shows a exploded view in a side elevation of the embodiment of the invention of FIG. 1; and

FIG. 3 shows an outside view, in isometric projection of the embodiment of the invention.

As shown in FIG. 1, a metallic header or base portion 2 has a generally flat upper surface 4 and a bottom or under surface adapted for being attached to a mounting surface. Holes 6 are provided for fastening by means of screws, rivets, or other fasteners. A ceramic isolation disk 8 is attached to upper surface 4 of header 2. Ceramic isolation disk 8 has conductive patterns 10, 10', 10" formed thereon for having attached thereto an electronic device or circuit, such as a power device chip, and various leads for providing electrical connections to the device by way of the conductive patterns. In the present exemplary embodiment, the device is a power transistor formed in a silicon semiconductor die 12. Connection to the electrodes of semiconductor die 12 is provided by flat leads for low inductance, high current carrying capacity, and desirable physical ruggedness. Thus, a drain lead 14 is attached directly to conductive pattern 10 for thereby electrically contacting the drain electrode of die 12 A source lead 16 is attached to conductive pattern 10', for thereby contacting the source electrode by way of a source strap 18, which bridges from source lead 16 to the source contact pad on the die. In order to solder strap 18 to the source contact pad, it is desirable to place gold balls on the contact pad on the die to facilitate soldering between nickel plated copper and gold.

In operation, lead 14 and source strap 16 carry the main conduction path current and are therefore of a substantial cross-section for low voltage drop under heavy current conditions and have low inductance. Nevertheless, some voltage drop will inevitably occur, including a voltage drop in metallization pattern 10'. It may sometimes be important to avoid undesirable electrical feedback effects caused by such voltage drops and to that end, a further source lead is provided as a sensing or Kelvin lead 20 which provides a connection to a point in the source circuit which does not include a portion of the voltage drop occurring in the source lead. The gate electrode of semiconductor die 12 is required to carry currents that are very small in comparison with the main source/drain current and consequently, it is adequate to make the connection between a gate lead 22 and the gate electrode contact pad on the die by way of an aluminum bond wire 24.

A metallic barrel portion 26, generally tubular in form, is attached at its lower edge to top surface 4 of header 2. The diameter of the opening at the bottom of barrel portion 26 is such as to fit around ceramic isolation disk 8. Barrel portion 26 has a lower portion 28 that is generally cylindrical and an upper portion that is stepped in its circumference at a shoulder 29 so that the diameter of the opening at the top of barrel 26 is larger than the opening at the bottom. The diameter of the opening at the top of barrel portion 26 is such as to allow a fit around a ceramic lid 30 which abuts shoulder 29 in barrel portion 26. Ceramic lid 30 has therein openings or lead slots 32, 34, 36, and 38 for allowing drain lead 14, source lead 16, Kelvin lead 20, and gate lead 22 to pass through and protrude beyond the upper surface of ceramic lid 30 for allowing connections to be made thereto to external circuitry.

FIG. 2 will be helpful in further clarifying the arrangement of the parts and in illustrating shoulder 29 in barrel portion 26.

While various choices of materials are possible, in the present exemplary embodiment, header 2 is preferably made of a clad system such as a copper-molybdenum-copper clad or copper-invar-copper clad having a total thickness in the order of 0.05 inch. In this manner, high thermal conductivity is obtained together with mechanical strength for mounting. Good matching of thermal coefficients of expansion provides resistance to fatigue related damage as may result from thermal cycling.

Ceramic isolation disk 8 is preferably fabricated from beryllium oxide of about 0.030 inch thickness and is direct bonded with copper on each side of about 0.010 inch thickness. The top copper layer is then etched to form conductive patterns 10, 10', and 10". High thermal conductivity and high breakdown voltage are exhibited by this material combination. The thermal coefficient of expansion is in the order of 8 parts per million per degree Celsius.

Barrel portion 26 is preferably made from 0.030 inch thick 1020 steel using a stamp, coin, and pierce procedure. Such a procedure results in an inexpensive, lightweight structure having a thermal coefficient of expansion of about 12 parts per million per degree Celsius.

Drain lead 14, source lead 16, Kelvin lead 20, gate lead 22, and strap lead 18 are cut and formed from 0.025 inch CDA 102 copper. This material lends itself readily to being formed and provides high current carrying capacity at low cost. The thermal coefficient of expansion is about 16 parts per million per degree Celsius.

Ceramic lid 30 is preferably fabricated from pressed and sintered Forsterite.TM..Only the outer circumference of lid 30 and lead slots 32, 34, 36, and 38 are metallized with lithium molybdate, and thereafter are nickel-plated for solderability. Thus, the non-metallized parts of ceramic lid 30 provide electrical isolation between leads. Forsterite.TM. exhibits a thermal coefficient of expansion of about 10 parts per million per degree Celsius.

In assembling the package, header 2, ceramic isolation disk 8, barrel portion 26, drain lead 14, source lead 16, Kelvin lead 20, and gate lead 22 are mated together, fixtured and brazed using a silver/copper braze alloy. The assembly is then electroless nickel plated for corrosion resistance and solderability.

Silicon die 12 and source strap 18 are then mounted in the package using solder alloy preforms, then reflowed in a hydrogen/nitrogen forming gas atmosphere.

Gate bond wire 24 is preferably a 5 mil (0.005 inch) aluminum wire ultrasonically bonded from gate lead 22 to the gate electrode contact pad on die 12.

Ceramic lid 30 is slid over drain lead 14, source lead 16, Kelvin lead 20, and gate lead 22 and into barrel portion 26. Solder alloy wire preforms are put in place and reflowed in a forming gas atmosphere so as to result in solder joints that form a hermetic seal around lid 30 and drain lead 14, source lead 16, Kelvin lead 20, and gate lead 22.

FIG. 3 shows the exemplatory assembled package. The assembled profile height is typically under 0.25 inch and the weight under 25 grams. Current carrying capacity is in the order of 100 Amperes and the voltage breakdown is in order of 1000 Volts.

While the invention has been described by way of exemplatory embodiments, various changes will be apparent to one skilled in the art which do not depart from the spirit and scope of the invention, and which are meant to be covered by the following claims.