Online proceedings - EDA Publishing Association

TABLE I
COEFFICIENTS OF THERMAL EXPANSION AND SHORE HARDNESS
Material CTE [ppm/K] Shore Hardness
Silicon 2 - 4
Al203 (Substrate) 6,50
Glass 4 - 7 >90
Epoxy 19 - 65 60-80
Polyimide 20 - 120 60-80
Silicone 200 - 300 25-30
7-9 October 2009, Leuven, Belgium
electrical connections after more than 14.000 temperature
cycles and 14.000 hours of storage.
IV. ENCAPSULATION / GLOB-TOP
After die attach and wire bonding the integrated circuits
have to be protected from mechanical damage, moisture and
radiation by an encapsulant. Thirteen different encapsulants
have been tested. Besides epoxies and polyimides, silicones
and glasses have been tested. The materials were selected
according to the following considerations:
• The low hardness of the silicones can compensate
different thermal expansion coefficients between the
substrate, the die and the bond wires (low hardness,
high CTE mismatch to the die).
• As the thermal expansion coefficient of the polyimides
and the epoxies is in the range of the bond wires, the
expansion of the encapsulant should not tear up the
bond wires (medial hardness, medial CTE mismatch to
the die).
• As glasses are very hard and their thermal expansion
coefficient is lower than the CTE of the bond wire,
glasses will force the bond wire to their expansion
(high hardness, low CTE mismatch to the die).
Fig. 5a and 5b show the results of the storage test at 250°C
for different time stamps. Most of the encapsulated chips
show a steadily increasing amount of defect wire bonds with
the passing of storage time. All encapsulants made of
silicone, epoxy or polyimide show first wire bond breakage
after 25 to 50 storage hours. In contrast to the epoxy
encapsulants, which destroyed all bond wires after 1000 h,
not all bond wires had been destroyed in systems
encapsulated with polyimide or silicone. Due to their elastic
behavior silicones sometimes reconnect broken bond wires.
The surface of the silicones do not show any change,
whereas flaws and fractures can be found on the surface of
the polyimide and epoxy encapsulants. Only the glasses
show no wire bond breakage for up to 14.000 h.
Fig. 6a and 6b demonstrates the results of the cycle test
between room-temperature and 250°C. Most of the
encapsulants destroy all bond wire connections after 50
cycles. After 500 temperature cycles all bond wires
encapsulated with silicone or epoxy and also one of the
polyimides have been destroyed completely. The second
polyimide encapsulant shows a low amount of destroyed
bond wires (