Freescale Semiconductor, Inc.
Interfacial Permeability
Lead leakage is
mechanism, like corrosion, to occur more readily. It also
causes a systematic pressure measurement error. Figure 8
shows the result of lead leakage measurements as a function
of temperature cycling. The polymer housing material (and its
CTE as a function of temperature), the leadframe material
(and its CTE), surface preparation and contamination, the
polymer matrix composition, and polymer processing all
contribute to this effect. It is accelerated by media,
temperature cycling, and applied pressure.
a
specific example of interfacial
permeability. It is pressure leakage through the polymer
housing material/metallic leadframe material interface from
the inside of the pressure sensor package to the outside of the
pressure sensor package or vice versa [22]. In addition, other
material interfaces can result in leakage. We describe another
specific example of this in the next section. Lead leakage is
like polymer swelling in that it may allow another failure
2.0
1.5
EPOXY
PPS GRADE 1
PPS GRADE 2
PBT
LCP
1.0
0.5
0.0
0
200
400
600
800
1000
TEMPERATURE CYCLES
Figure 8. Pressure leakage measurements through the metallic leadframe/polymeric housing material interface on a
pressure sensor as a function of temperature cycles between –40 and 125°C.
Adhesive Strength
Strength Components
Packaging of the sensor relies on adhesive material to
maintain a seal but not impart stress on the piezoresistive
element. Polymeric materials are the primary adhesive
materials which can range from low modulus material such as
silicone to epoxy with a high modulus. An example of a typical
joint is shown in Figure 9. The joint has three possible failure
locations with the preferred break being cohesive.
Contributors to a break include whether the joint is in tension
or compression, residual stresses, the adhesive material,
surface preparation, and contamination. An adhesive failure
is accelerated by media contact, cyclic or static temperature,
and cyclic or static stress (e.g. pressure).
DIE TO MAT’L
ADHESIVE
STRENGTH
COHESIVE
STRENGTH
DIE TO EPOXY
ADHESIVE
STRENGTH
Figure 9. Failure locations for an adhesive bond of
dissimilar materials.
Motorola Sensor Device Data
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1–23
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