Online proceedings - EDA Publishing Association

7-9 October 2009, Leuven, Belgium
VI. APPLICATIONS
Several EEPROM ICs and capacitive pressure sensor
chips have been bonded into a standard DIL housing with
the glass die bond material. Afterwards the chips have been
contacted by standard aluminum bond wires (25 µm) and
encapsulated with glass. Fig. 8 shows an EEPROM chip for
high-temperature operation, encapsulated by glass in order to
protect the chip and the bond wire connections from
mechanical damage, moisture and radiation. In current tests
the chips have shown no failure during storage tests at 250°C
for 800 h.
Fig. 6b. Percentage of wire bond breakage as a function of the number of
cycles between 25°C and 250°C for epoxies and glasses.
V. SENSITIVITY OF TRANSISTOR PARAMETERS TO
PACKAGING
While silicone-, polyimide- and epoxy-based
encapsulation processes employ rather moderate curing
temperatures of about 200°C or less, process temperatures
for glass encapsulants exceed 450°C for up to 1 hours. As
temperatures in this range are also present in the last stages
of CMOS wafer fabrication, there was concern that they
could adversely affect CMOS device parameters.
Therefore test structures were prepared and fabricated in
the IMS H10 High-Temperature SOI process to measure the
effects of the glass encapsulation process on diode and
transistor parameters. The parameters were measured as
soon as possible after each assembly step, and during
subsequent storage at 250°C. Fig. 7 shows the typical
variation of the measured parameters: although there is a
peak deviation associated with the high-temperature filling
step, it is nearly gone after an 816 hour bake. At all times
the parameters were well within the range allowed for
process variations. Except for the time immediately after the
filling step, the deviations are on the same order as those for
standard encapsulation methods.
Fig. 8: Picture of the high-temperature EEPROM IC, before and after
encapsulation.
VII. CONCLUSION
The reliability and performance of different die attach
materials and encapsulants have been examined in storage
and cycling experiments. Although specified to work at
temperatures at least up to 250°C, many encapsulants and
die-bonding materials fail in these tests after short periods or
a small number of temperature cycles.
Glass-based die attach and encapsulation materials
showed the best performance of the tested materials. With
this approach standard aluminum wire bonding techniques
can be used with glass as die attach and packaging material
for high temperature electronic assembly of integrated
circuits. These die attach materials showed the best
performance and no detectable loss in shear strength up to
5000 cycles and for more than 5000 storage hours. It was
also the best packaging material, which showed no bond
wire failure up to 14.000 temperature cycles and storage
hours.
With this glass-based approach SOI chips have been
assembled with glass as die attach and packaging material,
with promising results so far.
Fig. 7. Typical variation of a device parameter (NMOS threshold voltage)
over the stages of glass-based assembly and subsequent high-temperature
bake (square). For comparison, the variation during standard epoxy based
assembly is shown (circle)
©EDA Publishing/THERMINIC 2009 120
ISBN: 978-2-35500-010-2