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Aluminum Nitride Cap Design Obviating Problems of Thermal Conductivity and TEC Associated With Polymer Ceramic Substrate

IP.com Disclosure Number: IPCOM000122653D
Original Publication Date: 1991-Dec-01
Included in the Prior Art Database: 2005-Apr-04
Document File: 2 page(s) / 98K

Publishing Venue

IBM

Related People

Bross, A: AUTHOR [+3]

Abstract

The disclosure is a module design whereby a substrate of high thermal expansion coefficient (TEC) and low elastic constant (i.e., liquid crystal polymer or polymer-ceramic substrate) is constrained by a ceramic cap having a low TEC, a high elastic constant and a high conductivity such that the substrate will (1) exhibit a TEC close to that of the chip thus minimizing any fatigue reliability problem for the chip-substrate interconnections, and (2) will dissipate sufficient heat through the cap or cap/heat-sink assembly to accommodate high power chips.

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Aluminum Nitride Cap Design Obviating Problems of Thermal Conductivity
and TEC Associated With Polymer Ceramic Substrate

      The disclosure is a module design whereby a substrate of
high thermal expansion coefficient (TEC) and low elastic constant
(i.e., liquid crystal polymer or polymer-ceramic substrate) is
constrained by a ceramic cap having a low TEC, a high elastic
constant and a high conductivity such that the substrate will (1)
exhibit a TEC close to that of the chip thus minimizing any fatigue
reliability problem for the chip-substrate interconnections, and (2)
will dissipate sufficient heat through the cap or cap/heat-sink
assembly to accommodate high power chips.  A specific application is
the liquid crystal polymer (LCP) with an aluminum nitride cap sealed
by a thermosetting polymer, as shown in the figure, which also
illustrates how direct chip contact to the cap (which further
enhances dissipation) is achieved through the application of a force
to the substrate periphery during the sealing operation, thus causing
the flexible substrate to store the required elastic energy assuring
long-term contact.

      Polymer-ceramic substrates can be made at extremely low cost;
however, the composite has a high and anisotropic TEC, low
conductivity, and low elastic modulus, which limit the fatigue life
of chip interconnections and the maximum power level that can be
used.  An aluminum nitride (AlN) cap provides the needed rigidity to
constrain the composite and the right thermal conductivity such that
high-power chips can be used to meet the thermal and
thermo-mechanical requirements.

      The polymer-ceramic composite can be made by injection-molding
at a fraction of the cost of the commonly used ceramics.  The
composite also contains glass-fibers and spheres to control the
mechanical properties.  The major drawbacks are the high TEC (>30
ppm/C) and the low thermal conductivity (about .005 W/C.cm) which are
respectively about seven times and 500 times lower than the values
for aluminum nitride.  Therefore, the concerns are solder joint
fatigue (large thermal mismatch with the chip) and poor thermal
performance of power dissipation, thus restricting the application of
t...