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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