View - Micro Systems Engineering

film alumina have a better performance regarding
thermal conductivity (λ ≈ 24 W/m*K). Their weakness
is, however, the hermeticity of vias through the
substrate.
Thermal vias in LTCC are able to improve
the integral value of the thermal conductivity.
Depending on the array formation (diameter of vias
and pitch), values comparable to alumina or even
better can be achieved [1][2].
Fig. 1 shows the build-up structure of a
MMIC-package mounted on a FR-4 board with
thermal vias and an embedded metal core. Thermal
vias in the LTCC conduct the heat to the package
heatsink which act as a heat spreader. The latter is
glued or soldered to the copper pad on the FR-4. It
should be mentioned that the bonding methods for
die, heat sink and package is important for the
thermal management as well [3].
Fig. 1: Build-up structure for a MMIC in a LTCC package assembled on a FR-4 board with metal core
Hermeticity Test Coupon Design and Procedure
Since hermeticity is a required function of
the package, the thermal options need to be evaluated
in more detail. The material system selected was
DuPont 951AX. Due to the space application
requirements, the Au conductor system was selected
for the test. Vias in LTCC have a high metal content
(for good conductivity) and a very low glass content
as bonding agent. Therefore, bonding to the glassceramic
walls is not fully achieved. Increasing the via
diameter (typical for thermal vias) increases the risk
for a potential leakage. For a single layer with thermal
vias it is doubtful whether full hermeticity can be
realized.
In the study, six different design cases were
considered (Fig. 2). Packages might be required with
(case a and d) or without heat spreader attached. The
other options are two versus four layer designs and
thermal vias with or without diameter variation
between layers.
Fig. 2: Design options considered for the
hermeticity test pattern
The via diameter used are 250 µm and
130 µm after firing. Furthermore, two ways of
feeding signals into the package were considered.
Fig. 3 shows a schematic cross section with RF
feeding lines from package pads to the inner cavity.
The cavity wall is used for the cover assembly which
can be realized by a metal lid, kovar frame + lid,
metal cover or metallized plastic cover. The reasons
to investigate the two configurations were to verify
the hermeticity (leakage across the line type b) as
well as the electrical performance.