Microwave component and module manufacture has been well understood and automated for many years. Myriad applications benefit from the capabilities driven by high volumes of microwave point-to-point links. But at frequencies around 60 GHz, the manufacture of millimeter-wave systems turns from a highvolume automated operation into a hand-tuned black art.
Success in building millimeter-wave modules and subsystems by integrating very high-frequency monolithic devices often requires years of hard-won experience and plenty of manual intervention. Industry veterans describe difficulties in designing module packages and how they tune components and add absorbing material in just the right places to make the modules work and optimize performance.
With few exceptions, millimeter-wave companies are small specialists, hand-building products for military, aerospace, and research applications. These packaging and manufacturing difficulties limit the deployment of millimeter-wave systems and the use of these bands because they prevent substantial reductions in cost, size, and weight.
THE DEMAND
Yet with increasing demand for video streaming to mobiles, backhauling high-definition (HD) video in broadcast and CCTV applications, there’s lots of interest in the vast bandwidth available at millimeter-wave frequencies. Uncompressed HD video needs 1.4 Gbits/s. In addition, 3G Long Term Evolution (LTE) service is just starting to be deployed in mobile networks, and each basestation needs 100-Mbit/s backhaul capacity. Up to 7 GHz of bandwidth is available for these applications in various millimeter-wave bands (U.S. band 57 to 64 GHz) (see the table).
The wide bandwidths available at these ultra-high frequencies offer enhanced resolution for radar and imaging systems, too. For example, the range resolution of a frequency-modulated continuous-wave (FMCW) radar is directly proportional to the swept frequency bandwidth. Accurate millimeter-wave radars are now being deployed at 77 and 79 GHz for non-military, highvolume applications such as vehicle adaptive cruise control and lane-change assistance.
The latest screening systems for detecting weapons and explosives under clothing also demand wide bandwidths in the millimeter- wave bands (Fig. 1). Active transmitting systems often rely on a “radar-like” swept frequency approach, and passive, receiveonly systems require good sensitivity and high gain across more than 20 GHz in the W-band around 100 GHz.
PACKAGING ISSUES
Most microwave and millimeter-wave components and subsystems comprise metal, or at least metal-coated, enclosures into which cavities for mounting miniature microwave integratedcircuit (MMIC) chips and other components are milled or formed. Often gold-plated, these enclosures physically protect the MMIC chips, wire-bonds, and other components from damage in manufacture and use and from the external environment. They also protect the components from interference caused by electromagnetic radiation from the electronics in the rest of the system and the operating environment.
Of course, such metal modules are far from ideal in terms of size, weight, and cost for many applications. For example, airborne systems that provide real-time imagery to assist pilots landing helicopters in the dark, in bad weather, and in “brownout” conditions (dust and sand clouds) need small and low-mass millimeter- wave components, as do the imaging systems now being tested on unmanned aerial vehicles (UAVs).
All microwave circuits radiate energy, from interconnect tracks, from bond wires, and from the chips themselves. When the wavelength approaches the dimensions of the MMIC chips, many electromagnetic effects, perhaps negligible at lower frequencies, become much more significant and can even dominate the functionality and destroy performance.
Radiated energy couples into other parts of the circuit and often causes unwanted, sometimes catastrophic, behavior. Examples include resonance in the “cavity” that houses the MMIC chips and non-bulk conduction in filter structures in planar circuit boards. Resonances often render a millimeter-wave module completely non-functional. The ease with which unwanted millimeter-wave radiation “leaks” into and affects all parts of a system makes realworld equipment building a substantial challenge.
FLIP-CHIP SOLUTIONS
One approach that can help mitigate these effects involves the use of flip-chip MMICs, where the chip is mounted face down onto a substrate with interconnections. Example substrates include thin-film or thick-film circuits formed on quartz or ceramic (usually alumina) or on a variety of organic highfrequency circuit board materials.
The chip connections, usually bumps of various types of solder, can provide transitions with low loss at high frequencies. But the proximity of even a nonconductive substrate surface can affect the chip’s high-frequency performance.
Flip-chip assemblies lack a large-area contact to a thermally efficient substrate, though. As a result, the relatively high thermal dissipation of many high-frequency MMIC chips must be managed through the front face of the chip and the mounting bumps. Mismatch between the thermal coefficients of the MMIC chips and the substrate materials can also create reliability problems in operation.
Solutions to this issue, such as inserting non-conductive underfill between the MMIC chip and the substrate, often impact the microwave performance. In addition, the planarity of many MMIC chips may not be sufficient to provide reliable low-loss bumps for all connections. The inability to investigate, adjust, and tune or even rework flip-chipped MMICs also means that flipchip assembly isn’t an approach that’s generally applicable for all millimeter-wave subsystems.
SMALL CAVITIES
It may be possible to make the cavity enclosing the active devices small enough to prevent a resonance mode from being set up at the fundamental operating frequency of the circuit. However, resonance modes at higher frequencies still couple into the devices and structures, so they contiue to seriously impact the circuit performance.
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