This invention relates generally to microwave circuits and more particularly to microwave circuit packaging techniques.
As is known in the art, microwave systems have made extensive use of discrete resistors, capacitors, inductors, and active devices, as well as, integrated circuits such as monolithic microwave integrated circuits (MMIC) to provide microwave sub-assemblies for both low power and high power applications.
In low power microwave circuits, an aluminum oxide (Al.sub.2 O.sub.3) carrier is provided to carry all the individual microwave components. Such components include low power MMIC components, as well as, discrete capacitors and resistors which may be used to provide bias to the circuits. The Al.sub.2 O.sub.3 carrier is generally also mounted to a base portion of a microwave package. At low power levels, typically less than 0.3 watts power dissipation, the relatively poor thermal conductivity of aluminum oxide does not significantly degrade the operation of the microwave circuit so that the aluminum oxide substrate may be used to carry the circuit.
With high power circuits, however, (i.e. those circuits having a power dissipation greater than 0.3 watts), the relatively poor thermal conductivity of aluminum oxide makes it unacceptable as a carrier for the circuit. The approach in the prior art, therefore, is to use a precision machined slab of a thermally conductive metal as the carrier for the high power circuit.
A typical metal carrier 12 for use with a MMIC circuit 10 is shown in FIG. 1. The metal carrier 12 is machined from a slab to here provide support region 12a over which is soldered an MMIC 14, as well as, thick regions 12b and 12c which are used to receive screws or studs for subsequent mounting of the metal carrier 12 into a microwave package 24 as will be described. Bias bonding regions are provided by Al.sub.2 O.sub.3 dielectric carriers 20 and 22. Input and output transmission lines are also provided by separate dielectric carriers 16 and 18, with each carrier 16, 18 comprised of a layer of here gold which acts as a ground plane conductor 16a, 18a which is spaced from a patterned strip conductor 16b, 18b by a dielectric provided by the aluminum oxide carrier 16c, 18c. Individual chip capacitors 23a, 23b are also mounted on carrier 12. Thus, here the carrier 12 supports six individual components which must be integrated with the MMIC 14 to provide a practical circuit.
There are several problems with the approach shown in FIG. 1. The principal problems concern the relative cost involved with machining the metal carrier 12, as well as, the cost involved in mounting the individual circuit elements 14, 16, 18, 20, 22, 23a, and 23b. Integrated circuit technology is being developed inter alia to reduce the many hand bonding operations commonly employed in hybrid circuit technology. But, with high power devices, due to thermal conductivity considerations, it is necessary to provide the discrete circuit elements as shown on the common metal carrier 12.
There are also several electrical problems associated with this approach. For example, ground planes 14a, 16a, and 18a often are non-planar. This arrangement introduces unwanted parasitics which makes modeling of the circuit more difficult and which can degrade circuit performance. Furthermore, the use of the individual circuit elements typically increases the size of the metal carrier 12.
With the approach shown in FIG. 1, many components in addition to the monolithic microwave integrated circuit 14 are supported on the metal carrier 12. Each of these components must be interconnected by wire bonds to form a practical circuit. The use of additional wire bonds increases the cost of the circuit since wire bonding is a labor intensive operation. Further, a large number of such wire bonds also increases the unpredictability of microwave circuit parasitics.
Problems also arise with packaging of the carrier 12. Typically, the circuit 10 is bonded into the microwave package 24 as also shown in FIG. 1. Although not shown in FIG. 1, many such carriers 12 of the type shown would be disposed in package 24 to provide a microwave assembly. The package 24 shown in FIG. 1 also presents problems. The first problem is that since many of the carriers 12 are used in the package 24, the problem of non-planar ground planes mentioned above is compounded. This increases the complexity of modeling such an arrangement so that parasitics can be reduced or compensated for. Further, the size of the individual carrier 12 is generally large due to mounting of individual circuit components. Mounting of many of such carriers 12 within an individual package will unavoidably increase the size of the package 24. At microwave frequencies, it is undesirable to have the size of the package exceed the waveguide cut-off wavelength of the highest operating frequency of the circuit. If such a threshold is reached, undesired internal wave propagation modes will be created which will interfere with proper microwave performance. Thus, package size also becomes a limiting factor for microwave performance.
Moreover, the carriers 12 are generally bolted into the package 24 by studs 26a, 26b which are impressed into the package base, as is generally known. This arrangement provides the possibility for poor electrical and thermal contact between the bottom portion of the carrier 12 and the base portion 24a of package 24. Further, if excessive, torque is applied to the studs 26a, 26b, the carrier may slightly flex or bend, leaving voids between the surface of the carrier 12 and the base. With sufficient flexing and bending such excessive torque can crack the integrated circuit 14 mounted on the carrier 12 either during assembly or during subsequent thermal cycling or use of the circuit 10.