Microwave circuits are comparable in size to their operating wavelength, so all interconnections, components layout and enclosure walls must be weighed in circuit design. In addition, electrical, mechanical, environmental and reliability specifications must be considered.
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 multidisciplinary 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. They have become more reliable, smaller and lighter, as well as less expensive and with higher density circuit integration. There are a number of developments that have evolved that illustrate ways in which new packaging technologies are being used. These developments and technical advances, by companies such as Merrimac Industries, have further improved the packaging capabilities that should provide a whole new series of subsystems for the microwave industry.
Microwave devices and circuits are packaged at several levels of complexity that depend in large measure on the applications being considered. At the most basic, Level 1, 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 monolithic microwave ICs (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 front end, tens of millions of which have been made 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 multithrow 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-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 fanout of conductors deposited on the substrate. An example of this would be a multistage amplifier with integrated voltage regulator.
At the next tier of the packaging integration hierarchy, Level 3, several Level 1 or 2 enclosures may be mounted on a board that is placed into a larger module housing. 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 packaging has been the dominant product design approach for microwave subsystem and system integration in military applications. That's partly because 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 outsourcing to qualified vendors that supply components.
Level 3 approaches that have appeared within the past 15 years include wafer-scale integration, by Westinghouse (now Northrop Grumman); microwave high-density interconnect, by General Electric (now Lockheed-Martin); flip-chip mounting, by Hughes Aircraft (now Raytheon Defense Systems); glass microwave ICs, by M/A-Com (now part of AMP Industries); compliant interconnect, by TRW; waffleline high-density packaging, by Harris; and microwave common modules, by a U.K. consortium.
The final level, Level 4, has evolved within the past five to seven years. This 3-D-subsystem level of integration has been spurred by the desire to realize even higher-density microwave packaging, driven by cost considerations and by the successful achievements of the digital circuit design community. For monolithic microwave ICs, higher-density integration results in fewer square millimeters of gallium arsenide substrate material and hence lower cost.
ATR Laboratories in Japan was one of the earliest to propose the multilayer MMIC. For the U.S. Department of Defense, the motive for focusing and strengthening the nation's 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 of great importance for both the defense and commercial worlds. It is needed in 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 and intelligent transportation systems.
Recent advances in both hard- and soft-lamination technologies (low-temperature co-fired ceramic, or LTCC); high-temperature co-fired ceramic, or HTCC; and polymer materials) have demonstrated the potential capability of doing 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. Multilayer substrates enable dense packaging of components and modules.
HTCC technology initially was used in VHF-UHF-RF transistor-chip packages that were subsequently expanded to include matching networks and cascaded stages. Attempts to extend the enclosures for packaging multiple MMIC chips have been fairly successful as long as refractory metal conductors could be gold- plated to minimize transmission line losses-buried traces in multilayer assemblies have been found to cause significant and unacceptable RF and dc attenuation. The ability to braze the ceramic to high-conductivity (both elec trical and thermal) metal baseplates that match GaAs' coefficient of thermal expansion have made the HTCC packages attractive for high-power, high-dissipation chip designs. But post-firing shrinkage factors have often caused uncontrollable misalignment in multilayer-circuit registration, inhibiting practical use at the higher microwave frequencies. And for experimental purposes the front-end tooling costs tend to be uneconomical.
Copper, silver and gold conductors can be used with LTCC technology because of its lower firing temperature, but tooling costs are comparable to those of HTCC. Maximum operating frequency is limited to about 10 GHz because of dielectric losses, but emerging developments promise to reduce those losses and improve performance. Multilayer 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.
The Defense Advanced Projects Agency (Darpa) invested $30 million in a five-year program labeled "High Density Microwave Packaging (HDMP) for Next-Generation Aircraft and Space-Based Phased Array Radar." Contractor teams led by Hughes, Texas Instruments and Westinghouse initiated different approaches to reduce the manufacturing costs of transmit/receive (T/R) modules.
The Hughes team's approach focused on development of multilayer tiles fabricated from aluminum nitride, which exhibits a good thermal match to both silicon and gallium arsenide. The TI team's approach was based on substrate layers fabricated from the metal matrix composite aluminum-silicon carbide and a microwave high-density interconnect technology from GE. And Westinghouse, teamed with IBM and TRW, used MMIC and digital ICs in a flip-chip configuration. RF circuits were fabricated using LTCC multilayers, and the dc and low-frequency sections were constructed from multilayer HTCC. The two sets are sandwiched together with a "button board" interconnect system. The 3-D tile T/R module architecture is representative of the packaging for military hardware that has resulted from this program.
2 x 2 array
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. This module architecture lends itself to semiautomated batch fabrication and assembly for low-cost subarray manufacture. The production methodology, together with the Merrimac Multi-Mix approach, points the way that microwave packaging technology appears to be heading.
The Multi-Mix process for microwave multilayer ICs and micro-multifunction modules was developed at Merrimac Industries based on fluoropolymer composite substrates. The fusion bonding of multilayer 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 module structure is the package. The unit is rugged and lightweight, and its format-external interface and surface mount-is compatible with microstrip or coplanar waveguide planar transmission lines.
The platform strategy of Multi-Mix module modeling and simulation cuts engineering cycle time and yields an economical solution for new circuit designs. The benefits include high-density circuit integration; improved performance, reliability and quality; reduced size, weight and cost; improved yield; and the potential for millimeter-wave apps.