Freescale Semiconductor, Inc.
Mechanical Failures
PRESSURE SENSOR SOLUTIONS
The range of solutions for pressure sensors to media
compatibility is very diverse. Mechanical pressure sensors still
occupy a number of applications due to this media compatibility
concern. These devices typically operate on a variable
inductance method and are typically not as linear as a
piezoresistive element. Figure 10 shows a comparison
between a mechanical pressure sensor and a piezoresistive
element for a washing machine level sensing application. The
graph shows a nonlinear response for the mechanical sensor
and a corresponding straight line for the piezoresistive element.
A common method of obtaining media compatibility is to
place a barrier coating over the die and wire interconnection.
This organic encapsulant provides a physical barrier between
the harsh environment and the circuitry. The barrier coating
can range from silicone to parylene or other dense films that
are typically applied as a very thin layer. This technique offers
limited protection to some environments due to swelling
and/or dissolution of the encapsulant material when in contact
with media with a similar solubility. When a polymeric material
has a solubility parameter of the same value as the
corresponding media, swelling or dissolution will occur.
Stainless steel diaphragms backfilled with silicone oil
provide a rugged barrier to most media environments, but
generally are very costly and limit the sensitivity of the device.
The silicone oil is used to transmit the stress from the
diaphragm to the piezoresistive element. If a polymeric
material is used as the die attach, the silicone oil will permeate
out of the package. This concern requires a die attach that is
typically of higher modulus than a silicone and may not
adequately isolate the package stress from the die.
The occurrence of mechanical failures include components
of fatigue, environment assisted cracking, and creep.
Packaging materials, process, and residual stresses are all
contributors to mechanical failure. A summary of acceleration
stresses is shown in Table 3. Contact with harsh media is an
accelerating stress for all of the mechanical failure mechanisms.
Table 3. Mechanical Failure Mechanisms
Failure Mechanism
Acceleration Stresses
Fatigue crack initiation
Mechanical stress/strain range
Cyclic temperature range
Frequency
Media
Fatigue crack propagation
Mechanical stress range
Cyclic temperature range
Frequency
Media
Environment assisted cracking Mechanical stress
Temperature
Media
Creep
Mechanical stress
Temperature
Media
175
170
165
160
2
1.8
1.6
1.4
1.2
1
155
150
0.8
0.6
0.4
0.2
0
145
140
0
1
2
3
4
5
6
TIME (MINUTES)
WASHING
MACHINE
SENSOR
PIEZORESISTIVE
PRESSURE
SENSOR
Figure 10. Graphical comparison of the output from a mechanical pressure sensor compared to a piezoresistive
sensor during a washing machine fill cycle.
1–24
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