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274 MEMS and Microstructures in Aerospace Applications

Second, because ceramic-to-metal adhesion is not as strong as ceramic-to-ceramic
adhesion, sufficient ceramic surface area must be available to assure a good bond
between layers. This eliminates the possibility of continuous ground planes for
power distribution and shielding. Instead, metal grids are used for these purposes.
Third, the processing temperature and ceramic properties limit the choice of
metal lines. To eliminate warping, the shrinkage rate of the metal and ceramic
must be matched. Also, the metal must not react chemically with the ceramic
during the firing process. The metals most frequently used are W and Mo. There
is a class of low temperature co-fired ceramic (LTCC) packages. The conductors
that are generally used are Ag, AgPd, Au, and AuPt. Ag migration has been reported
to occur at high temperatures, high humidity, and along faults in the ceramic
of LTCC.

12.2.3 THIN-FILM MULTILAYER PACKAGES
Within the broad subject of thin-film multilayer packages, two general technologies
are used. One uses sheets of polyimide laminated together in a way similar to that
used for the LTCC packages described above, except that a final firing is not
required. Each individual sheet is typically 25 mm and is processed separately
using thin-film metal processing. The second technique also uses polyimide, but
each layer is spun onto and baked on the carrier or substrate to form 1 to 20 mm-
thick layers. In this method, via holes are either wet etched or reactive ion etched
(RIE). The polyimide for both methods has a relative permittivity of 2.8 to 3.2.
Since the permittivity is low and the layers are thin, the same characteristic
impedance lines can be fabricated with less line-to-line coupling; therefore, closer
spacing of lines is possible. In addition, the low permittivity results in low line
capacitance and therefore faster circuits.

12.2.4 PLASTIC PACKAGES
Plastic packages have been widely used by the electronics industry for many years
and for almost every application because of their low manufacturing cost. High-
reliability applications are an exception because serious reliability questions have
been raised. Plastic packages are not hermetic, and hermetic seals are generally
required for high-reliability applications. The packages are also susceptible to
cracking in humid environments during temperature cycling of the surface mount
assembly of the package to the motherboard. For these reasons, plastic packages
have not gained wide acceptance in the field of space applications. However, there
are notable semiconductor designs that are beginning to be flown in space applica-
tions. Programs such as commercial off-the-shelf (COTS), which include plastic
encapsulated microelectronics (PEMs) are gaining acceptance. For example, suit-
able PEMs were used for the Applied Physics Laboratory Thermosphere–Iono-
sphere–Mesosphere Energetics and Dynamics (TIMED) program. The size, cost,
and weight constraints of the TIMED mission were achieved only through the use of
commercially available devices. 2

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