1. Field of the Invention
The present invention relates to mechanical structures for mounting a mcirowave device such as a diode in a microstrip line circuit.
2. Description of the Prior Art
The generation of high power microwave output signals by microwave devices such as diodes requires adequate dissipation of heat generated by such devices at, for instance, the diode junction. Thus, in order to increase the power handling capability of a diode, the diode is mounted in intimate thermal contact with a heat sink. Adequate dissipation of heat, achieved as indicated above by "heat sinking," is especially important in those applications where the diode is in high power, long pulse and high duty cycle operation.
A generally applied technique for heat dissipation consists of "flip-chip" bonding, in which one surface of a diode is brought in contact with a heat sink through a process known as "thermal compression." While such a technique is effective for diodes having very small diameters, surface finishing (smoothing) techniques for larger areas (500 micrometers) are generally so inadequate as to result in poor thermal contact, which consequently renders flip-chip bonding inadequate for larger (500 micrometers) diameter diodes such as power diodes. Such large diameter diodes utilize an integral copper heat spreader which is created by electroplating a heavy copper layer to a given surface of the diode. The electroplated diode side may typically serve also as an electrical terminal. A detailed description of the formation of the integral heat spreaders may be found in an RCA Laboratories Report PRRL-72-CR-37 entitled "S-Band avalanche Diode Amplifiers," by H. Kawamoto, H. J. Prager, et al. published July 31, 1972.
Microwave stripline (microstrip) circuits are typically constructed from commercially available laminates consisting of a sheet of insulation initially provided on both sides with layers of a suitable conductive material such as copper. Portions of one of the conductive layers are removed, suitably by etching, to form a stripline conductor. The second conductive layer, on the other side of the sheet of insulation, is suitably left intact and forms the ground plane (second conductor) of the microstrip circuit. The microstrip circuit is typically mounted on a metallic, conductive base or chassis, with the ground plane of the circuit in contiguous contact with the conductive base.
The use of integral heat spreaders, and the finite life of diodes require that the diodes by removably connected in the microstrip circuit. Typically, the diode is mounted in a conductive package fabricated separately from the conductive base, prior to insertion of the diode into the microstrip circuit, and the package subsequently mounted in the base. The diode is then electrically connected to the strip line circuit. The package is suitably formed of a material having good thermal and electrical conductivity such as copper. Generally, such mounting is achieved by machining in the base a recess of a given shape, typically a pyramid of two rectilinear parallelpipeds, diminishing in size, such that the smaller one is exposed to the ground plane of the microstrip circuit. An aperture in the microstrip circuit is made in alignment with the opening in the base facing the recess. The diode and integral heat spreader are mounted, suitably by soldering, on the package of conductive material, which serves, in conjunction with the base, as a heat sink. The package is of such a shape as to fit in, mate with, and extend through the recess in the baseplate. The diode is admitted through the aperture in the circuit board and is electrically connected to the microstrip line by a metallic, suitably gold, foil. Electrical connection between the diode and the ground plane is established by the contact of the package with the base.
The package may be flanged and fastened to the base by screws passing through the flange and into the base. Electrical and thermal contact between the base and package is enhanced by torqueing the screws, thereby creating a force on the package in a vertical direction. The horizontal surfaces of the package are thus pressed against the respectively adjacent surface portions of the base.
The above described mounting structures are generally unsatisfactory in that machine tolerances generally do not provide reliable mating of the package to the base, and, consequently gaps occur between the vertical sidewalls of the package an the base. Such gaps, at microwave frequencies often form resonant cavities and create parasitic capacitance and inductance, having the effect of detuning the circuit. The RF power output, DC to RF efficiency, and bandwidth of the circuit are thus adversely affected.
The present invention overcomes the disadvantages of the prior art by providing a microwave diode package and mounting structure which serves to provide reliable mating of the package to the recess whereby electrical and intimate thermal contact is established between the package and the base, substantially eliminating gaps, between adjacent vertical surfaces. Thus parasitic capacitance and inductance are substantially reduced, causing a substantial increase in bandwidth, and RF power output of the circuit.