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MMIC packaging with Waffleline.

MMIC Packaging with Waffleline(*)

Introduction

Monolithic microwave and mm-wave integrated circuits (MMIC) represents the state of the art in manufacturing technology in terms of performance, size, weight and cost. However, to reap the benefits of this technology to its full extent, packaging and interconnect methods must be developed that will facilitate the insertion of MMIC technology into various electronic systems and not compromise the performance, size, weight and cost benefits accrued by MMIC techniques. These packaging and interconnect methods must offer low loss, high isolation and good impedance match; complete environmental protection; high density interconnects; minimum size and weight; high producibility; high reliability; and minimum cost.

Packaging Approach

MMIC packaging falls into two distinct areas, generic packaging and specific application packaging. In working together to develop MMIC modules, Harris and ITT have taken the approach of studying and evaluating various generic packaging techniques that are applicable to virtually any MMIC system and then reducing these techniques to practice in specific systems.

Typical tradeoffs to be made are the study and evaluation of die attach techniques, carrier materials, single and multiple chip packages and multilayer interconnect techniques. Specific application packaging takes generic packaging concepts and, through a series of tradeoffs, selects the optimum solution for the system under consideration.

For each system, the parameters themselves, as well as their relative weighting, may be different. In some instances, multiple packaging approaches may be found that meet all the system criteria. This will allow both optimized solutions and back-up approaches. In other instances, however, no viable packaging solutions will be found with existing packages and a packaging development effort must be undertaken. In many cases, existing packaging technologies can meet a majority of today's microwave system needs.

Many hermetic packages have been developed in various formats, such as surface mount, flatpack and microstrip, that can accommodate single or multiple chips. Surface mount packages are available that operate at frequencies up to 7 [GHz,.sup.1], and custom designs are in process that will extend that range to 10 GHz. Hermetic packages intended for microstrip mounting have been developed that operate at frequencies to 20 [GHz.sup.2].

As for second level package-to-package interconnections, conventional interconnect techniques, such as microstrip and stripline, are viable through 20 GHz provided the loss, isolation and/or fabrication costs are acceptable. A relatively new packaging technology, or more appropriately interconnection technology, called Waffleline, has been shown to be a viable and cost effective second level package [format.sup.3,4,5].

Waffleline

Waffleline is a patented packaging technique that was designed and developed to meet the stringent demands placed on today's monolithic assemblies. The structure is shown in Figure 1. The body of the structure is described best as a waffle iron grid. Dielectric coated signal and/or power wires lie in the channels formed by the grid. A top metal foil completes the ground plane, forming a coaxial-like structure. Interconnections to first level packages and other components are made either by right angle connections or through an in-line launcher to areas in the Waffleline grid structure that have been hollowed out to accept packaged chips or chip carriers.

The resulting transmission line is periodic with repeating segments of closed and open transmission-line cross sections as shown in Figure 2. In this configuration, the relative dimensions of the inner conductor to the grid dimension determine the characteristic impedance, and the absolute dimensions determine the frequency of operation. Approximate solutions of the characteristic impedance were obtained and then experimentally optimized to provide a characteristic impedance of 50 [OMEGA]. The characteristic impedance of Waffleline is not limited to 50 [OMEGA]. Geometries with impedances of 25 to 100 [OMEGA] are practical within the same assembly. The use of enameled magnet wire allows DC and control lines to share a common channel.

Waffleline Performance

Standard Waffleline has an X-Y grid size of 50 mils, center conductor to center conductor. Although this grid spacing will function at frequencies to 40 GHz, it is used most commonly at frequencies of 20 GHz and below. At higher frequencies, a scaled version of Waffleline with a grid spacing of 25 mils is used. This smaller grid spacing has been shown to operate at frequencies to above 60 GHz. Figures 3 and 4 show the performance of a 50 mil Waffleline measured with K-type coaxial connectors for the input and output. Figure 3 shows measured insertion loss per inch vs. frequency from 2 to 20 GHz. Shown for reference on the same curve is a coaxial transmission line with dimensions equal to the Waffleline transmission line. The losses of the two transmission lines are essentially equal. The measured isolation of two adjacent transmission lines two inches in lenght is greater than 30 dB through 20 GHz. Leaving an open channel between lines increases the isolation between the same two inch lines to 80 dB at 20 GHz. Even greater isolation can be achieved by altering the geometry of the channel. One such alteration is the reduction of the Waffleline grid spacing to 25 mils. Test results on that configuration have shown an insertion loss of 0.3 dB/wavelength and a channel-to-channel isolation of typically 60 dB or better.

One of the key features of Waffleline package is its ability to provide high isolation and controlled impedance RF crossovers. Crossovers are formed by crossing one wire over another at right angles. In order to allow crossovers, the channel depth is actually greater than the diameter of the outer conductor. Figure 4 is a graph of measured isolation between two such lines. It can be seen from the figure that crossover isolation is greater than 30 dB at 20 GHz. Should greater isolation be required, the insertion of foil shielding between the two crossed lines increases the isolation to greater than 70 dB at 20 GHz.

A Waffleline Assembly

A typical Waffleline assembly is shown in Figure 5. At microwave frequencies, RF inputs and outputs to the Waffleline assembly typically are made through SMA type connectors mounted orthogonally to the Waffleline plate or in line with the propagation direction of the Waffleline transmission line. Transitions of both types have been developed for use through 20 GHz. This figure shows the diversity in the types of modules or subassemblies that can be interconnected using Waffleline.

Waffleline Packaging Example

Figure 6 shows the front view of a Waffleline assembly recently developed and qualified by Harris. The package shown is one of 48 identical sub-column assemblies (SCAs) required for the Navy's airborne telemetry systems (ATS). The system is an S-band phased array that provides 10 independent beams. It utilizes over 6000 packaged MMIC chips integrated using Waffleline. The wire side of the SCA assembly is shown in Figure 7. There are over 300 interconnects on this assembly alone. The qualification requirements met by the ATS SCA are shown in Table 1.

SHF Satcom Waffleline Subassembly

Through the next decade in order to satisfy government requirements for low cost, lightweight, man-transportable satellite terminals, the development of an SHF man transportable satellite communications terminal has been focused on. This terminal, designated a man transportable terminal (MTT), will communicate through the defense satellite communication system (DSCS). The DSCS consists of a family of geosynchronous SHF satellites that serve tactical military users, strategic military users, the diplomatic telecommunications system and the White House Communications Agency. When fully operational, the system will have five satellites in equatorial, geosynchronous orbit. The users of the system will be served by DSCS II and DSCS III satellites through the year 2000. The current system provides downlinks in the 7.25 to 7.75 GHz frequency band, and receives transmissions in the 7.9 to 8.4 GHz frequency band.

Without the enabling technology of MMIC circuitry, the stringent MTT performance, size and weight requirements simply could not be met. The SHF Satcom MMIC converter subassembly has been partitioned to enable insertion of MMIC multi-application modules and chips, combined with phase-locking circuitry for transponder selection. The MMIC converter subassembly combines five MMIC TO-8 modules, five common dielectric resonator oscillator (DRO) modules and a printed circuit board that contains an internally derived frequency reference and the phase-locking networks for the DROs, all integrated in a Waffleline framework. Each of a the TO-8 modules consists of one or two GaAs chips mounted on a microwave header. Figure 8 shows a typical TO-8 module. The DRO modules are contained in a custom package to accommodate an oscillator chip, a dielectric resonator and a tuner. All modules utilize thick-film substrates mounted on a copper-tungsten heatspreader.

Figure 9 shows a schematic diagram of the converter subassembly. The converter subassembly consists of a transmit and a receive signal path. On the receive side, the signal is accepted from a low noise amplifier (LNA) located in the feed at the antenna, and then is filtered, amplified and downconverted from SHF to 700 MHz on the converter subassembly. The LO is provided from the MMIC SHF oscillator contained in the converter subassembly. The downconverted signal then is filtered again on the converter subassembly and fed to the second downconverter module that provides a 70 MHz signal. The LO for this module is provided by an external frequency synthesizer to the converter subassembly. Final amplification of the 70 MHz signal is provided by a commercial silicon MMIC amplifier in a TO-8 package.

Waffleline Application: EW Waffleline Subassembly

Many GaAs monolithic subassemblies using MMICs are under development to replace existing units built with conventional MIC technology. Currently, ITT Avionics is working with Harris to evaluate the potential advantages of Waffleline technology to MMIC modules in several EW systems.

A schematic diagram of one such module is shown in Figure 10. This module serves as a transmitter driver for a high power TWT. It consists of ten chips, two MIC filters and associated biasing and control circuitry. In order to serve as a form, fit and function replacement for two components in an existing subassembly, the overall size and the location of RF and DC inputs and outputs must match those of the existing subassembly. A mock-up of the form, fit and function Waffleline module is shown in Figure 11. It consists of seven microwave TO-8 headers and one custom package, containing the filters, interconnected by a Waffleline assembly.

mm-Wave Waffleline Packaging

As GaAs MMIC technology matures, the frequency coverage provided by the chips produced extends into the mm-wave range. Through advances in devices, materials and processing, GaAs MMIC technology has progressed through 60 GHz and is approaching 100 GHz. To meet the overall system's needs, mm-wave packaging must advance as well. In general, although the end item performance requirements remain the same, the realization of the packages that meet these requirements at mm-wave frequencies becomes an order of magnitude more difficult. Several old packaging standards, such as microstrip and stripline, are replaced by slotline and finline and connectorized interfaces are replaced by waveguide flanges. Waffleline has its own mm-wave interface. It has been designated EHF Waffleline.

Millimeter-wave energy is propagated in a quasi-coaxial medium. The grid and wire dimensions of the basic EHF Waffleline structure have been reduced, as compared to the microwave version, and a unique mm-wave chip carrier was required to allow the interconnection of chips in Waffleline.

For EHF Waffleline, the dimensions of the basic grid structure essentially are reduced by one half. For EHF Waffleline, a grid spacing of 0.025" is used, with the wire diameter and grid dimensions scaled as well. As in the microwave version, the channel depth is made greater than the outer diameter of the wire to allow for crossovers.

The chip carrier is an integral part of the EHF Waffleline technique. This carrier is a disk with channels cut in it orthogonally to one another. The channel dimensions are chosen to maximize the available area for die attachment while limiting undesired waveguide modes to above the frequency of operation. Figure 12 shows this carrier with a typical integrated circuit layout. The interfaces to the GaAs chips are made through short lenghts of interconnect substrates that are typically microstrip on duroid, quartz or alumina. The round chip carrier usually is fabricated from either kovar or copper-tungsten. Copper-tungsten has excellent heat transfer characteristics while having a thermal expansion rate very close to GaAs. The thermal expansion coefficient of the Waffleline body, typically an aluminum alloy, is much higher than that of the carrier. Thus, when the Waffleline body is heated, the cavity in which the carrier is placed expands and allows insertion of the chip carrier. As the body cools to normal operating temperatures, the cavity shrinks, thus trapping the carrier. This technique results in a well-grounded assembly that also provides good heat sinking.

mm-Wave Waffleline Applications

A representative example of a subsystem using EHF Waffleline is shown in Figure 13. The waveguide inputs are connected to the various mm-wave carriers through a perpendicular waveguide transition to the Waffleline wires that travel through the grid structure.

An actual example of an EHF Waffleline assembly is the 60 GHz receiver front end shown in Figure 14. This subsystem consists of a monolithic microstrip antenna array directly integrated to the RF components. The circuitry is mounted behind the antenna substrate and consists of three chip carriers containing a low noise amplifier, a mixer and an IF amplifier. The receiver exhibited less than 10 dB DSB noise figure and gain of 20 dB. The whole assembly is only 1.6" in diameter by 0.25" thick, and weighs less than 35 g. More complex subsystems, such as transceivers, multichannel receivers and circuitry for phased arrays, also can be packaged in EHF Waffleline. The high package density capability allows the system designer to locate the RF components at the front end of the subsystem input. Thus, overall system size can be reduced and system performance can be increased.

Conclusion

In summary, a new packaging and interconnection technology, Waffleline, has been developed that meets the demands for future low cost GaAs MMIC subsystem. This packaging technique offers a unique, high density, low cost solution to packaging mm-wave circuits, and has been demonstrated to 60 GHz exhibiting excellent electrical performance.

Acknowledgment

Some of the effort described in this paper has been sponsored by DARPA as part of the Phase I MIMIC program, with program management provided. As part of the MIMIC program, Harris Corp. is working with ITT Avionics on microwave EW module development, and with Martin Marietta on the mm-wave module development. The authors would like to acknowledge the support of Mr. T. Burke (LABCOM), Technical Program Manager for the ITT/Martin Marietta Phase I MIMIC contract. [Figure 1 to 14 Omitted] (*)Invited paper.

References

[1]L. Seieroe, et al., "Surface Mount Products Match TO-8 Performance and Availability," RF Design, Oct. 1989, pp. 22-30. [2]D. Koopman-Larson, et al., "Microwave Chip Carrier of Monolithic Integrated Circuits," 1985 IEEE GaAs IC Symposium Technical Digest, pp. 155-158. [3]D. Heckaman, et al., "Waffleline - A Packaging Technique for Monolithic Micro-wave Integrated Circuits," 1984 GaAs IC Symposium Technical Digest, pp. 59-62. [4]D. Larson, et al., "Low Cost TO Packages for High Speed/Microwave Applications," 1986 IEEE MTT Technical Digest, pp. 437-440. [5]G. Reider, et al., "Waffleline Packaging Techniques for mm-Wave Integrated Circuits," 1988 IEEE GaAs IC Symposium Technical Digest, p. 217-220.

Robert W. Perry received his BSc and MSc degrees from Northeastern University. From 1967 to 1978, he was a microwave design engineer with Raytheon Missile Systems Division's Antenna and Microwave Department. In 1978, he joined Harris Government Systems, where he has held various technical and engineering management positions primarily in microwave circuit and subsystems design. Since 1980, he has been involved directly with Harris' GaAs MMIC technology development. Currently, he is a senior principal engineer with the Aerospace Systems Division, where his prime responsibility is as technical manager of Harris' MIMIC Phase 1 program.

E. Ronald Schineller received his BSEE degree from Manhattan College in 1960 and his MSEE degree from the Polytechnic Institute of Brooklyn in 1964. From 1960 to 1970, he was employed by Wheeler Laboratories, where he was engaged in microwave, antenna and electric-optic laser work. From 1972 to 1978, he was worked on various antenna subsystems including the microwave landing system. Since joining ITT in 1978, Schineller has been engaged in the development of MIC and MMIC circuitry. Currently, he is manager of the Design Technology Department and is responsible for planning and directing advanced technology programs, both internally funded and government sponsored. Since 1986, he has served as engineering manager for the MIMIC program at ITT, with direct responsibility for the EW system brassboard development.

Tim Ellis received his BSEE degree from Penn State University in 1988 and currently is enrolled in the MBA program at the University of Central Florida. Since 1988, he has worked in the RF and antennas section of Harris' Government Aerospace Systems Division, where he recently was promoted to senior engineer. He has been involved in the research and development of RF packaging technology for microwave and mm-wave systems. Currently, he is responsible for the development of microwave EW module packaging techniques on Harris' Phase 1 program. Biography photograph unvailable at press time.
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Title Annotation:monolithic microwave integrated circuit
Author:Perry, Robert W.; Ellis, Tim.T.; Schineller, E.Ronald
Publication:Microwave Journal
Date:Jun 1, 1990
Words:2855
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