Download the PDF file
Designers are continually requesting smaller and more
complex circuits with less real estate to support increasingly complex
systems. The result has been smaller
packaged microcircuits. The need for
unique circuit requirements, often unavailable as a standard microcircuit, has
become a problem as real estate restrictions prohibit the use of multiple
discrete microcircuits. The solution in
many cases is the integral use of chips and discrete parts on a common substrate
in a single package, a design known as the
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 demand for higher density circuits, with lower
real estate requirements, necessitates the need for smaller, more complex
devices, i.e., the MCM. Unlike the
Hybrid, the MCM is a chip-only technology with the substrate being an integral
part of the product design, not simply a component carrier. The dice are mounted on, or embedded in, a
multilayered substrate that is contained in a protective case that may be
hermetic sealed or plastic encapsulated.
Typically, the substrate consists of several layers, each containing
circuitry or providing power and ground planes with vias interconnecting the
layers. For the highest density designs,
the dice are embedded in the substrate structure. Disadvantages for limited usage designs can
be initial fabrication costs.
Disadvantages for routine designs are test and repair difficulty and
finding cost-effective spares. However,
since only bare dies are involved and no individual discretes, obsolescence is
less of a problem. MCMs usually have
significantly fewer series connections from die to pinout, giving the potential
for a more reliable product.
Military specifications and
a. QML-PRF-38534 Hybrid/MCM performance specification.
b. QML-38534 Hybrid/MCM QML qualified
Other support sites:
Institute for Interconnecting and Packaging Electronics Circuits.
b. http://www.jedec.org/ JEDEC
Solid State Technology Division.
c. http://www.eia.org/ Electronic