Extreme Environment Electronics Assembly and Packaging
Packaging of SOI, SiC
All compositions are
and GaN components and
in weight percent
circuits
for
High
Temperature:
Silicon
97%Au, 3%Cu
Carbide
and
Gallium
72%Au, 26%Cu, 2%Ni
Nitride based electronics
can operate at temperatures
of
500-600oC,
while
53%Sn, 35%Ni, 12%Au
silicon-on-insulator (SOI)
55%Ni, 41%Sn, 4%Cu
technology can be used up
32%Cu, 28%Ni, 26%Au,
14%Sn
to 300oC, providing high
levels of integration for
analog, digital control and
power
electronics.
AuSn - 2,000hrs@400C
However,
to
build
Ti/TiW/Au– Au(20 m)/Ni/Cu
functional
electronic
products, the devices must be packaged and interconnected. This research area is
evaluating materials and processes for interconnect substrate fabrication, die attach, wire
bonding and packaging. Liquid phase transient and thermocompression bonding are
being explored for traditional die attach as well as for a sandwich approach where
substrates are bonded to both sides of the device, eliminating the need for wire bonding.
Substrates – For power modules, direct bond copper on Al2O3 and AlN and active metal
braze copper on Si3N4 substrates are being evaluated. A key issue is the thermal cycle
performance over a wide temperature range due to the coefficient of thermal expansion
(CTE) mismatch between the copper and the ceramics. Surface finishes for the copper
compatible with high temperatures in air are also an issue. For low power applications
(digital, analog, control), thick film on AlN, Al2O3 and SiN4 is being investigated.
Die attach – AuGe and AuSi are being studied for SOI die attach, while liquid phase
transient (LPT) bonding using Au-Sn and Au-In is being developed for applications
above 300oC. No degradation in die shear strength after 2000 hours at 400oC has been
achieved with Au-Sn LTP bonding. Thermocompression bonding of patterned bumps is
also being used to achieve even higher temperature capability.
Wire bonding – Au and Pt wire (1 mil for SOI and 10 mil for SiC power die) are being
investigated. The normal Al bond pads on the SOI are electroless Ni/electroless Au
plated, providing a monometallic interface for Au wire bonding. SiC and GaN are
typically fabricated with Au wire bond pads. Au wire has better electrical conductivity
than Pt, but Pt has better high temperature mechanical strength.
Packaging – Individual and multichip packaging is being developed for high temperature
applications. These packages are ceramic based to avoid issues with glass-to-metal seals
in metal packages.
Packaging of Si and SiGe components and circuits
for Low Temperature: SiGe devices have been
demonstrated to operate at 10oK. The current
activities center around developing single and
multichip packages that operate over the temperature
range from -230oC to +120oC for lunar applications.
Substrates – Multilayer copper/polyimide on Si, AlN
and Si3N4 substrates are being fabricated and tested
for use down to -230oC. A key issue is the CTE
match of the material set over the wide temperature
range required for the lunar environment.
Die attach, wire bonding and flip chip – Indium based
die attach is being used with thermosonic gold wire
bonding for chip & wire assembly. Indium based
solders are also being used for flip chip assembly.
Indium remains malleable down to cryogenic
temperatures. With a CTE match between the die and
the substrate, underfill will not be required.
Packaging – Commercially available Al2O3 ceramic
packages are being used. AlN packages would
provide a better CTE match, but only a very limited
number of packages are available as open tooling. A
metallized Al2O3 lid has been developed to matcht he
CTE of the package. The lid is solder sealed with
Pb/In.