Multi-Mix® Whitepaper

The primary purpose of packaging any electrical or electronic equipment is to provide protection from physical damage, mechanical forces, and atmospheric or chemical contamination that may exist in a typical operating environment [1]. Packaging has been defined in a broad sense to not only include the materials and technologies required to provide electronic components with physical protection and electronic connections, but also to include system architecture and partitioning approaches, power management, thermal management, data flow and timing, and input/output interfaces [2]. In view of the fact that microwave circuits are comparable in size to their operating wavelength, all interconnections, components layout and enclosure walls must be addressed in the design of the circuit to insure that the desired results are achieved. Electrical, mechanical, environmental, and reliability specifications must be simultaneously considered to optimize the overall circuit performance. Maintaining proper electromagnetic impedance levels and ensuring an appropriate thermal, vibration, shock, and hermetic environment become an integral part of the packaging design challenge. It is not surprising then, to find that the engineering of good microwave and millimeter-wave packages is a multi-disciplinary process that requires additional expertise in materials technology, computer-aided modeling and simulation, and familiarity with manufacturing processes.

Many advances have been made in recent years in microwave packaging, in the directions of greater reliability, smaller size and lighter weight, combined with lower cost and higher-density circuit integration. The objective of this memorandum is to provide a brief historical introduction to some of these advances, and to illustrate the ways in which the new packaging technology introduced and developed by Merrimac Industries provides further state-of- the-art improvements. These new improved packaging capabilities should permit the availability of a whole new series of subsystems for the microwave industry.

2.0 Early Developments

One of the earliest descriptions of the packaging of RF components and associated hardware may be found in Volume 1 of the MIT Radiation Laboratory Series [3]. In the development of radar in the early 1940's, it was demonstrated that "good system performance depended not only on a well-designed set of microwave components, but also on a properly coordinated mounting of them, relative to each other and relative to the other parts of the radar." At the time a considerable body of theory-and-techniques knowledge had been accumulated on coaxial and waveguide transmission lines, from which virtually all the available components were fabricated. The key microwave components of radar included the transmitter, receiver, and the antenna. The 'RF head' or transmit-receive unit consisted of the following:

Magnetron Tube, Pulse Transformer, Duplexer TR and ATR Tubes, Local Oscillator, Radar Front-End including Receiver and AFC Mixers, AFC Control Circuits and IF Preamplifier

Figure 1 Radar Transmit-Receive Package of World War II Vintage.

These components were mounted as a group in a closed container. This unit is illustrated in Figure 1.

It was located as near to the antenna as was practical to minimize the long line effect that could lead to frequency instabilities in the magnetron. In addition to the obvious electrical advantages that were found from such a grouping, the container could be carried by one or two men. If a failure occurred, the entire unit could be replaced with a spare in a very short time, allowing the faulty unit to be diagnosed off-line.

The evolution of radar in World War II was closely followed by a similar military technology race in the development of unmanned aircraft, missiles and ultimately spaceborne weapons. Application of radio frequencies to the guidance and control [4] of these platforms proceeded at a rapid pace in the decades that followed. The need to cram bulky waveguide and coaxial circuits into deployable packages for these airborne and space-based applications required a great deal of ingenuity with the existing resources. There was tremendous motivation to reduce the size and weight of guidance system hardware using the vacuum tubes and crystal rectifiers that were the only active components available. In the civilian market, commercial and consumer applications of microwave links for telecommunications and cable television (CATV) networks were evolving at a much different pace. Historically, the commercial, industrial, and consumer markets have concentrated on producing functional equipment at the lowest possible cost, while military requirements were more concerned with high reliability.

The development of planar transmission media in the early 1950's had a major impact on RF circuit and component packaging technology. The engineering of the microwave printed circuit [5] and the supporting analytical theories for stripline and microstrip occurred at a rapid pace. These early years were devoted almost entirely to the design of passive circuits such as directional couplers, power dividers, filters, antenna feed networks, and attenuators. Augmenting the refinements to dielectric materials and the microwave-circuit fabrication process itself, were significant engineering improvements to coaxial connectors, such as their miniaturization and the establishment of interface mating standards.

Paralleling these new microwave transmission-line accomplishments was an equally impressive series of inventions of new microwave semiconductor devices, each of which carried with it a packaging challenge to enable efficient coupling to an external circuit. The invention in the early 1950's of the low- frequency germanium (Ge) and silicon (Si) transistor led to a rapid extension of the technology to the VHF and UHF bands. Later refinements in gallium arsenide (GaAs) material technology enabled the microwave field-effect transistor to become a viable device. As the theory of semiconductor physics became more widely disseminated, solid-state device technology literally exploded, opening up an entirely new era of processing technology that spun off in several directions simultaneously in the years that followed. In 1958, the 'integrated circuit' concept was introduced. This concept considered the processing of active and passive devices on a semiconductor substrate with their in-situ interconnections [6] which not only had a profound impact on digital circuit design, but also led to the evolution of the monolithic microwave integrated circuit or MMIC that we know today. A historical perspective on the development and applications of microwave integrated circuits, including stripline, microstrip, coplanar waveguide, slotline and finline, has been given by Howe [7].

3.0 Contemporary Packaging Approaches

The packaging of microwave devices and circuits may be viewed as occurring at several levels of complexity [1] that depend in large measure on the applications being considered. At the most basic level, Level 1, illustrated in Figure 2, an individual transistor chip is shown bonded into a package that provides input and output RF leads that may include separate source/gate/drain (or emitter/base/collector) bias pin connections.

Further, the device may also contain input and output RF matching networks or simply consist of the semiconductor die by itself. Millions of such discrete microwave transistors have been manufactured in packages of this type and used in a variety of amplifier and control system applications during the past 25 years. More recently, with the advent of MMICs, a tiny chip may be mounted into a similar package that performs a single function, such as a switch, attenuator, mixer, or amplifier. Or the chip may include more than one circuit function such as the mixer/preamplifier stage for a direct broadcast satellite (DBS) front end, which has been manufactured in quantities of tens of millions in the past five years. Increasingly with MMICs, we are seeing higher levels of integration in simple, low-cost housings. Nearly every current trade journal carries advertisements offering a variety of multi-throw switches with integral TTL drivers mounted in plastic surface-mount packages.

If the individual packaged chip is placed on a substrate or carrier plate that contains additional chips, discrete devices, passive circuit elements, and interconnecting transmission lines, a second Level 2 of packaging complexity is reached. The entire component assembly fits in a frame with coaxial connectors, feed-through pins or tabs for external RF contact. Additional feed-through pins may be required for external control signal or bias connections that are distributed internally by an appropriate fan-out of conductors deposited on the substrate. An example of this would be a multi-stage amplifier with integrated voltage regulator.

At the next tier of the packaging integration hierarchy, Level 3, several level 1 or level 2 enclosures may be mounted on a board which is placed into a larger modulehousing. The degree of complexity at each level is dependent upon the circuit partitioning methodology and the system architecture. Assembly and testing considerations may be important factors as well. For more than 20 years, Level 3 type packaging has been the dominant product design approach for microwave subsystem and system integration in military applications. It has been aided, in part, by the fact that the large system contractors responsible for the functional partitioning of complex microwave hardware generally specify the piece parts that make up the system by adapting to internal manufacturing capabilities as well as out-sourcing to qualified vendors who supply components. Examples of Level 3 packaging approaches that have appeared within the past fifteen years include

Wafer Scale Integration by Westinghouse (now Northrop Grumman)
Microwave High Density Interconnects by GE (now Lockheed-Martin)
Flip Chip Mounting by Hughes Aircraft Co. (now Raytheon Defense Systems)
Glass Microwave Integrated Circuits by M/A-Com (now part of AMP Industries)
Compliant Interconnect Concept by TRW
Waffleline High Density Packaging by Harris Corp.
Microwave Common Modules by a consortium in the UK

The fourth and final packaging, Level 4, evolving within the last 5 - 7 years, has been prompted by the desire to realize even higher density microwave packaging, driven largely by cost considerations and, in part, by the successful achievements of the digital circuit design community. This 3-D subsystem level of integration has manifested itself in several embodiments. For monolithic microwave integrated circuits, higher density integration results in fewer square millimeters of GaAs substrate material, and hence lower cost. ATR Laboratories in Japan was one of the earliest to propose the Multi-layer MMIC [8]. The motivation of the Department of Defense for focussing and strengthening the US investment in electronic packaging technology is the potential for significant advancements in system performance and affordability. Microwave and millimeter-wave multichip module (MCM) packaging is an emerging technology that is of great importance for both defense and commercial applications. Military applications such as radar, electronic warfare, communications and smart munitions, and commercial applications such as telecommunications, direct broadcast satellite, cellular radio, personal communication systems (PCS) and intelligent transportation systems (ITS), all require microwave and millimeter-wave electronics.

Recent advances in both hard and soft lamination technologies (low temperature co-fire ceramic, LTCC, high temperature co-fire ceramic, HTCC, and polymer materials) have demonstrated the potential capability of accomplishing high-density routing and interconnections for microwave circuits. These technologies offer the reduction or elimination of wirebonds, increased reliability, improved yield, and lowered fabrication costs. Multi-layer substrates enable dense packaging of components and modules.

The HTCC technology found initial application in VHF/UHF/RF transistor-chip packages that were subsequently expanded to include matching networks and cascaded stages. Circuit designers invariably have had to depend on a very limited number of domestic ceramic processing vendors. Attempts to extend the enclosures for packaging multiple MMIC chips has been fairly successful, as long as refractory metal conductors could be gold plated to minimize transmission line losses. Buried traces in multi-layer assemblies have been found to cause significant and unacceptable RF and DC attenuation. The ability to braze the ceramic to high-conductivity (both electrical and thermal) metal base plates that match GaAs's CTE have made the HTCC packages attractive for high-power, high-dissipation chip designs. Post-firing shrinkage factors, however, have often caused uncontrollable misalignment in multi-layer circuit registration, inhibiting their practical use at the higher microwave frequencies. For experimental purposes or limited production applications, the front-end tooling costs tend to be high and uneconomical.

The LTCC technology, by virtue of its lower firing temperature, enables the use of copper, silver and gold conductors, but tooling costs are comparable to HTCC. Maximum operating frequency is limited to about 10 GHz due to dielectric losses, but emerging developments promise to reduce these losses and improve performance. Multi-layer substrates have been developed for a variety of complex microwave circuit designs and interconnection techniques, but attachment of active devices or chips must be surface mounted or inserted in cavity cutouts with exposed conductors within the sublayers.

Developing "High Density Microwave Packaging (HDMP) for Next Generation Aircraft and Space-Based Phased Array Radar" [9] has been a five-year, $30M DARPA-funded, three-contractor-team undertaking that will be completed in June 1998. The teams led by Hughes, Texas Instruments, and Westinghouse initiated different packaging approaches, with a common goal of reducing the manufacturing costs of transmit/receive (T/R) modules. The Hughes approach has focussed on development of multi-layer tiles fabricated from aluminum nitride, which exhibits a good thermal match to both Si and GaAs. The TI team approach is based on substrate layers fabricated from the metal matrix composite aluminum-silicon carbide, and a GE proprietary, Microwave High Density Interconnect (MHDI) technology. Westinghouse, teamed with IBM and TRW, is using MMIC and digital ICs in a flip chip configuration. RF circuits are fabricated using LTCC multi-layers, and the DC and low frequency sections are constructed from multi-layers of HTCC. The two sets are sandwiched together with a "button board" interconnect system.

The 3-D tile T/R module architecture [10] illustrated in Figure 3 is representative of the packaging for military hardware that has resulted from this program.

The stacked tile concept consists of a 2 x 2 array of identical modules with multiple interconnection layers containing MMICs and other RF components mounted near the top surface for coupling to the antenna. Control-function and power-conditioning circuits are disposed on additional layers with the RF and DC manifolds near the bottom, as shown. This type of module architecture lends itself to semi-automated batch fabrication and assembly for low-cost subarray manufacture. This production methodology, together with the Merrimac Multi-Mix^TM approach to be described next, point in the direction that microwave packaging technology appears to be heading in the next millennium.

4.0 The Multi-Mix^TM Microwave Packaging Approach

4.1 Overview

The Multi-Mix^TM process for microwave, multi-layer integrated circuits and micro-multifunction modules (MMFM^TM) is a patent-pending method developed at Merrimac Industries based on fluoropolymer composite substrates. The fusion bonding of multi-layer structures provides a homogeneous dielectric medium for superior electrical performance at microwave frequencies. The bonded layers may incorporate embedded semiconductor devices, etched resistors, passive circuit elements and plated-through via holes, to form a 3-D subsystem Level 4 enclosure that requires no further packaging. In fact, the MMFM^TM structure is the package. The small-footprint, low-profile unit is of rugged, lightweight construction, and the external-interface, surface-mount format is compatible with microstrip or coplanar waveguide planar transmission lines. The process controls for the Multi-Mix^TM method allow a low-cost manufacturing approach that is suitable for high or low volume production. The platform strategy of MMFM^TM modeling and simulation reduces engineering cycle time and enables the Multi-Mix^TM product to be an economical solution for new circuit designs. Some of the perceived benefits of the technology process include:

High-Density Circuit Integration
Improved Performance
Improved Quality
Size and Weight Reduction
Reduced Cost
Improved Yield
Increased Reliability
Potential for Millimeter-Wave Applications.

An example of Merrimac's new MMFM^TM process is the S/C-band multi-channel filter bank. In this filter, six multi-layer boards that comprise the assembly. There are fourteen contiguous filter channels distributed over three circuit substrates separated by three dielectric and ground plane spacers. After fusion bonding and edge plating, the finished assembly shown on the right measures approximately 3" x 4" x 3/8" and weighs less than 2 ounces. The MMFM^TM multi-layer package replaces a 1 cubic foot block weighing 13 pounds.

4.2 The Multi-Mix^TM Process

The Multi-Mix^TM process begins with commercially available polytetrafluoroethylene (PTFE) composite, copper-clad laminate material having inherently low dielectric loss and stable microwave properties. A low Z-axis coefficient of thermal expansion (CTE), close to that of copper and aluminum, ensures excellent reliability of plated-through holes. Controlled XY-plane thermal expansion, together with a low modulus, affords excellent reliability of surface-mounted devices in the most severe thermal cycling and thermal shock environments. Further, a low and uniform thermal coefficient of dielectric constant, coupled with the low CTE, result in consistent electrical performance over a wide operating temperature range.

Complex microwave circuit patterns and transmission-line geometries may be chemically photoetched on the copper, maintaining dimensional tolerances of 0.0025 +/- 0.0005 inch. The incorporation of thin metal-film etched resistors can be routinely accomplished with accuracies of 5% that allow power dissipation greater than that achievable with discrete chips. Indexing holes for layer-to-layer alignment in assembly are precision machined or drilled into each board. Layer-to-layer plated-through holes may be realized with a minimum diameter of 0.005 inch and typical aspect ratios of 20:1. The process capability will be certifiable to ISO-9001, MIL-P-55110 and IPC-HF-318A.

Active device attachments are embedded in cover layer cavities to provide environmental protection and allow pre-cap inspection and test. The pick and place of discrete components can be demonstrated on array panels up to 18 x 24 inches, with the potential to accommodate flip chip packages and ball grid arrays. Operating-frequency capability of current circuit developments has concentrated on L-band through X-band in the near term, with extension to Ka-band in progress.

In fabricating a bonded multi-layer assembly, the stacked layers are placed in a fixture to which carefully controlled, uniform pressure and temperature is applied to meet the substrate fusion bonding requirements. After cooling and removal from the fixture, edge plating for EMI shielding and ground-plane integrity is performed. Finish platings for environmental protection include immersion tin, fused tin/lead and nickel/gold.

4.3 Platform Strategy of Module Architecture

Whether it be for military or commercial applications, improved electronic design automation (EDA) modeling/simulation tools have been developed for microwave and millimeter-wave circuits and packages. Integrated mathematical, electromagnetic, thermal and mechanical modeling capabilities on a modern PC type platform allow total package analysis to be performed prior to actual hardware fabrication, resulting in rapid turn-around and lower-cost prototypes.

In addition, the Multi-Mix^TM module architecture utilizes a structural template which overlays outlines and commonizes interconnection paths. Process steps are programmed parametrically for each layer of a multi-layer microwave assembly. Actual test data for functional layer blocks are stored in a pre-designed library. Merrimac has adopted the HP Eesof Series IV/PC software toolset to provide customer access to MMFM^TM S-parameter data for the purpose of predicting system performance, thus permitting On-line Co-design^TM database availability to customers. Our experience with this approach thus far has been met with a high level of positive feedback.

Complete design concepts from schematic to circuit layout may be performed within a single operating environment. Functional components may be inserted utilizing 'drag-and-drop' techniques from graphical palettes for insertion into system designs which can be tailored for a specific application. Merrimac's library of validated functional modules is augmented by model data for over 90,000 active and passive devices in the HP Series IV/PC library.

4.4 Product Descriptions

The Multi-Mix^TM process lends itself to an almost unlimited number of microwave component and subsystem product designs. As noted earlier, the ability to maintain tight dimensional tolerances in the fabrication process enables low-pass, high-pass and band-pass filter and multiplexer circuits to be realized with reproducible characteristics that meet rigid system specifications. Other simple passive circuits such as directional couplers, quadrature hybrids and multiport power dividers may be packaged individually or cascaded into supercomponents to create complex antenna feed-distribution networks, array beamformers, or comparators for monopulse receiving systems. Integration of beam-lead receiver or detector diodes in singles, pairs, quads or multi-ring configurations enable the manufacture of mixers and modulators to satisfy a broad range of application requirements in radar, electronic warfare, and commercial communications systems.

Additional signal control and power conditioning devices such as digital attenuators, limiters, and switch matrices may be prepackaged with their driver circuitry and embedded in cover layer cavities within the Multi-Mix^TM multi-layer assembly. Interfacing each device with its necessary RF transmission lines and video/bias traces to external connectors is accomplished during the product design and layout phase.

A view of a typical multifunction module is illustrated in the Figure 4 below.

Active devices including amplifiers, oscillators, frequency multipliers, and dividers can also be integrated in this manner. High isolation is maintained between parallel conductors and between adjacent multi-layers by following established design guidelines developed as part of the software platform strategy.

Figure 4. Multifunction Module Example

4.5 Applications

The initial and most dramatic improvements benefiting from the Multi-Mix^TM packaging approach have been existing applications that have used the older coaxial and waveguide microwave component technologies. However, even more recent system designs using conventional stripline and microstrip hardware are realizing the advantage of combining functions to reduce overall size using Multi-Mix^TM. Increasing circuit density can improve electrical performance and lead to reduced system cost by eliminating parts, especially connectors, and shrinking board size.

The key military applications that will benefit from Multi-Mix^TM innovations are the microwave systems on airborne and space-based radar and electronic warfare platforms. Reduced size and weight translates directly into increased payload and performance. Many millimeter-wave smart munition sensors should be candidates for this packaging approach, as well.

Even more exciting possibilities are the commercial applications where RF technology has begun to penetrate. The potential of current telecommunication and wireless system installations including cellular radio, personal communication (PCS), and local area networks (LANs) are only beginning to be realized. Cable television companies are competing with communication system providers for market share in offering virtually unlimited bandwidth in the transmission of data, voice, and video information over RF carriers. Base station and mobile receiver terminals are candidates for Multi-Mix^TM packaging technology.

The following block diagram shows a typical arrangement of signal processing components which may be integrated.

The number of satellite constellations in orbit or proposed for deployment within the next decade are almost too numerous to count. Direct broadcast satellite transmission of television signals is already a major market around the world, and the potential number of consumer receiver terminals in the undeveloped countries of the world is enormous.

A Japanese consortium of Toshiba Corp., Toyota Motor Corp., and Fujitsu [11] recently announced a joint digital-satellite-broadcast project targeting 70 million automobiles in this island nation for personal mobile receivers. Using the 2.6 GHz S-band frequency allocation, the system will deliver TV-quality video and CD-quality audio in addition to various types of data. The development of low-cost planar, steerable phased arrays for vehicle rooftop installation has been underway in Japan for almost a decade. The microstrip patch antenna, feed-distribution network, and receiver front end are proposed as a multi-layer board implementation. This is one of several planned intelligent transportation system applications (ITS) where Multi-Mix^TM should have a role.

Diagram 1 Functional Diagram For Millimeter-Wave Transceiver

The application of Multi-Mix^TM technology to the computer industry is perhaps the most exciting and difficult market for which to define requirements. Clock speeds are already approaching gigabit data rates, and a satisfactory solution to the high-frequency packaging problem has yet to be addressed. Current approaches have utilized existing multipin hybrid and multichip modules, which must incorporate signal integrity circuitry to compensate for delays, impedance mismatches, cross-talk, and multiple reflections attributable to interface problems. Achieving high performance and low error-rate, gigabit performance will require a radical departure from present motherboard designs and pin grid array packages, especially if computers are expected to communicate using the wireless and photonic technologies of the future.

5.0 References

[1] "MMIC Packaging," B. Berson, F. Rosenbaum and R. Sparks, Chap. 10, ofMonolithic Microwave Integrated Circuits, Edited by R. Goyal, Artech House, Norwood, MA; 1989.

[2] "Special Technology Area Review on Microwave Packaging Technology," p. 3,Report of DoD Advisory Group on Electron Devices; February 1993.

[3] Radar System Engineering, Edited by L. N. Ridenour, Vol. 1,MIT Rad. Lab. Series, pp. 419-432, McGraw-Hill Book Co. Inc.; 1947.

[4] Principles of Guided Missile Design: Guidance, A. S. Locke, D. Van Nostrand Co. Inc., New York; 1955.

[5] "Microwave Printed Circuits - The Early Years", R. M. Barrett,IEEETrans. MTT-S, Vol. MTT-32, pp. 983-990; September 1984.

[6] "Monolithic Microwave Integrated Circuits: A Historical Perspective", D. N McQuiddy, Jr. et. al.,IEEE Trans. MTT-S, Vol. MTT- 32, pp.997-1007; September 1984.

[7] "Microwave Integrated Circuits - An Historical Perspective", H. Howe, Jr.,IEEE Trans. MTT-S, Vol. MTT-32, pp.991-996; September 1984.

[8] "MMIC Transmission Lines for Multi-Layered MMIC's", H. Ogawa, et. al.,IEEE MTT-S International Microwave Symposium Digest, pp. 1067-1070; June 1991.

[9] "High Density Microwave Packaging Program", E. D. Cohen,IEEE MTT-S International Microwave Symposium Digest, pp. 169-172; May 1995.

[10] "High Density Microwave Packaging Program Phase 1 - Texas Instruments/Martin Marietta Team", J. A. Reddick, III, et. al,IEEE MTT-S International Microwave Symposium Digest, pp. 173-176; May 1995.

[11] Electronic Engineering Times, p. 24; 4 May 1998.