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Use this section to support the design and
selection of MCMs and Hybrids.
Hybrid designs have encompassed a variety of packaging
techniques, e.g., cordwood, stacked flat packs, compartmentalized modules, multi
stacked substrates, stick modules, and folded modules (flex circuits). The mixture of dice and discretes are mounted
on a substrate PCB contained in a protective case, that might be hermetic
sealed, potted (filled), or plastic encapsulated. These have specific advantages and
disadvantages over conventional devices.
Generally, all have circuit uniqueness, with size reduction as primary
assets. Disadvantages are high
fabrication costs, test and repair difficulty, and difficulty in achieving
cost-effective sparing. Component
obsolescence is also a problem, as Hybrids are usually custom designs of
relatively low volume, often containing discontinued product. Further, reliability uncertainty can be a
concern as internally packaged devices may be an unknown or nonstandard
The two basic packaging classifications for
Hybrids are planar array (two-dimensional) and stacked array
(three-dimensional). There were many
other hybrid packaging designs too numerous to cover but all had a common design
objective of saving space. Early planar arrays were usually contained in
hermetically sealed TO cans and flat-packs.
Stacked arrays usually consisted of cordwood or compartmentalized
construction. Military and NASA products
were predominately hermetically sealed; however, stacked arrays of commercial
design were often plastic encapsulated.
Because of vastly improved encapsulating material compounds and cost
considerations, modern hybrid packages for military applications may be either
hermetic or encapsulated.
The two major categories of MCM packages are
hermetically sealed and encapsulated.
Early designs were simply the packaging of multiple chips in a standard
TO can or flat-pack, interconnected by standard bond wires. Today there are a
variety of package designs within each category. However, the following paragraphs discuss the
most common designs, most of which are considerably larger, with many more
pinouts than the standard TO cans and flat packs used in the past. The
elimination of one tier of bond wires from the die to I/O terminals represents a
probable and significant increase in reliability, as bond wires have always been
a major source of failures. Modern MCM packaging requires interconnecting
circuitry be precisely laid out and controlled as it contributes to both circuit
function and performance. The
interconnecting circuitry, through the deposition technique, provides
controllable parameters directly affecting DC resistance, capacitance,
inductance, and minimizing the problems of crosstalk, propagation time, and
impedance mismatch. The interconnects, by being an integral part of the circuit
design, contribute to the ability of the circuit to function at faster
propagation times and at higher frequencies.
The following examples explain MCM and hybrid device packaging
Hermetically sealed, all ceramic. The
traditional designs consist of a ceramic substrate, which is fired to a glass
frit and is gold plated. The opposite side of the substrate has a metallized
ring, which is soft soldered to a ceramic cover, which also has a metallized
area matching the substrate ring. The
soft solder operation results in a hermetically sealed, all ceramic package. The
I/O terminals are usually fired-on molymanganese and are gold plated for good
Hermetically sealed, metal package.
Similar to the all ceramic package design, except the lid is metal and
soft soldered to the gold plated molymanganese edge of the substrate, producing
an all-metal-to-glass hermetic seal.
package. Essentially uses the same
fabrication techniques and design criteria for the die and substrate as hermetic
designs. However, the protective
enclosure is accomplished by encapsulating the internal elements with a
thermosetting epoxy, containing fillers and additives to enhance its thermal
conductivity, match its TCE to the die and substrate, and provide an effective
barrier to moisture ingression. When required, a heat spreader or heat sink
material (Cu, BeO, Al, AlN, etc.) may be used as an integral part of package
design. A heat spreader would normally be internally attached directly to
affected die, while the heat sink would normally be attached to the entire
substrate on one side with its opposite side exposed. In addition, die
passivation, e.g., Si3N4 or SiC, can be implemented in
non-hermetic packaging to increase resistance to moisture ingression
(atmospheric contamination) and chemical intrusions from any adjacent
Effects of Packaging
Normally the Hybrid and MCM packages have an inherent
space saving advantage, however, there are some potentially adverse effects of
Tight/close physical proximity reduces isolation reduction, resulting in
cross-talk/inter-circuit interference; repair is difficult.
Concentrated areas of power dissipation makes cooling more difficult, effecting
reliability and performance.
Buried/embedded elements limit test points and inspection.
d. High potential
circuits make insulating more difficult, corona, arcing, high impedance leakage,
Dissimilar adjacent materials/metals reduce reliability under temperature
extremes (TCE problems) and increases corrosion possibilities.
f. A unique style may
provide an ideal configuration for a specific application, but it can adversely
effect availability and cost, especially for future buys.
Unwanted signals, electrostatic, electromagnetic or
cosmic/nuclear radiation can adversely affect the performance and reliability of
the Hybrid or MCM In the case of
tantalum capacitors, they can present a health hazard as tantalum is activated
by thermal-neutron bombardment (tantalum has a half life of 111 days). Generally commercial components are more
susceptible to radiated noise, transients, and ground loops when used in
environments containing radiating elements.
Effective preventative techniques are:
Circuit designed for maximum desired signal with minimum spurious
Elements selected that do not radiate unwanted signals and are minimally
affected by radiated signals.
Proximity and layout of elements to minimize coupling of unwanted
Grounding practices that minimize radiated signals and ground
Shielding materials, as required to avert unwanted signals, when above methods
are unattainable or ineffective.
Electrostatic radiation is most likely to be the
concern in modern microelectronic designs and is primarily high frequency (RF)
interference from transmitters, local oscillators, high voltage apparatus, and
fast switching devices. Excessive noise in ground planes can also be a
problem. The design should avoid close
proximity to these sources and provide signal levels well above ground noise.
The use of highly conducive material as a screen or solid shield may be in
order. Even small openings and unsealed
joints may require additional shielding in the form of mesh or conductive
Typically, electromagnetic radiation is not a concern
in modern microelectronic designs except in some special circuits and very high
current applications. Electromagnetic
radiation is most common with low frequency apparatus and can be shielded
effectively only by high permeability materials, by significant separation of
the component from the radiating source(s), and avoidance of ground loop (high
current applications). Aluminum,
however, may be an effective electromagnetic shield in a high frequency
Applications in space, at very high altitudes or near
any nuclear apparatus may require radiation tolerant (RAD-HARD) circuit designs,
or an appropriate shielding technique against high-energy protons, electrons,
neutrons and gamma radiation. This radiation can be hazardous to electronic
components, materials, and a health concern (secondary radiation effects from
activated tantalum). Dense materials (lead and depleted uranium) and very thick
layers of other materials are also effective shields. Commercial devices can be
more susceptible to this type of radiation than military devices. Electronic
components can be designed to be radiation tolerant (RAD-HARD); however, being
produced for the cost-conscious, highly competitive commercial market, this
feature is not a normal consideration in routine product design.
When passive structures are used in microwave device
applications there are specific reliability and application issues to consider
concerning microwave device packaging.
Transmission lines, vias, airbridges and thin-film resistors are
considered a part of overall MMIC device design and must be able to meet all
reliability and performance requirements, from a packaging standpoint, that the
MMIC device itself is required to meet.
Specific packaging issues for microwave components are discussed in the