Freescale Semiconductor, Inc.
“C” is intended to swell polymeric materials. Contaminants in
substances that can dissolve polymers or corrode metal. Copper
is added as a trace metal to accelerate the formation of free
radicals from the hydroperoxides. Table 2 details the various
gasoline/methanol mixtures with additives recommended by
the task force from Chrysler, Ford, and General Motors.
actualgasolinecanresultincorrosionormaterialdegradation,
so chloride ions or formic acid with distilled water are added
to create an aggressive fuel media. Gasoline can decompose
by a process called auto–oxidation that will form aggressive
Table 2. Fuel Testing Methods
Elastomer
Polymer
Metal
Alcohol/Fuel Blends
CMO
CMO
CM15
CM30
CM50
CM85
CM15
CM30
CM50
CM85
CM15
CM85
Aggressive Fuel, Add
Chloride ion
Distilled water
Chloride ion
Formic Acid
Sodium Chloride
t–Butyl Hydroperoxide
Formic Acid
Auto Oxidized Fuels, Add
t–Butyl Hydroperoxide
+
Cu
Recommended gasoline/methanol mixtures for material testing. The recommended testing for metals should include immersion in the liquid as
well as exposure to the vapor. The coding for the alcohol/fuel blends, CMxx is: C for Fuel C; M for methanol; and xx indicating the percentage of
methanol in the mixture.
The general question for the appliance industry
compatibility issues is not whether the media will contain
ions (as it most assuredly will) but at what concentration.
Tap water with no alkali additives contains ions capable of
contributing to a corrosive reaction [14]. A typical
application of a pressure sensor in the appliance industry is
sensing the water level in a washing machine. The primary
ingredients of detergent used in a washing machine are:
surfactants, builders, whitening agents and enzymes [15].
The surfactants dissolve dirt and emulsify oil, grease and
dirt. They can be anionic or cationic. Cationic surfactants
are present in detergent–softener combinations. Builders
or alkaline water conditioning agents are added to the
detergent to soften the water, thus increasing the efficiency
of the surfactant. These builders maintain alkalinity that
results in improved cleaning. Alkaline solutions at
temperatures indicated by the appliance industry range can
etch bare silicon similar to the bulk micromachining
process. Thus bare silicon could be adversely affected by
exposure to these liquids [16].
material properties results in a change of the sensor
performance. Failure mechanisms for pressure sensors in
harsh media application are listed below. The pressure
sensor allows a format for discussion, though the
mechanisms discussed are applicable in some degree to
all sensor and actuator devices.
Corrosion
Corrosion has been defined as any destructive result of
a chemical reaction between a metal or metal alloy and its
environment [17]. Several metal surfaces exist within a
pressure sensor package: metallic lines on the die,
trimmableresistors, bonding pads, wires, leadframes, etc.
Much of the die–level metal is protected by an overlying
inorganic passivation material (e.g., PECVD silicon
nitride);
however,
unless
some
package–level
encapsulantisused, bondpads, wires, and leadframesare
exposed to the harsh media and are potential corrosion
sites. Furthermore, an energized pressure sensor has a
voltage difference between these exposed metallic
surfaces, which compounds the corrosion problem.
Generally, corrosion problems are organized into the
following categories: uniform corrosion; galvanic
corrosion, and localized corrosion (including, crevice
corrosion, pitting corrosion, etc.) [17]. The factors that
contribute to corrosion are: the substrate (metallic)
material and its surface structure and composition; the
influence of a barrier coating, its processing conditions
and/or adhesion promotion; the cleanliness of the surface,
adhesion between a coating and the surface, solution
FAILURE MECHANISMS
The failure mechanisms that can affect sensors and
actuators are similar to that for electronic devices. These
failure mechanisms provide a means of categorizing the
varIous effects caused by chemical, mechanical,
electrical, and thermal environments encountered. An
understanding of the potential failure mechanisms should
be determined before media testing begins. The typical
industry scenario has been to follow a set boiler plate of
tests and then determine reliability. This may have been
acceptable for typical electronic devices, but the
applications for sensors are more demanding of a
thorough understanding before testing begins. The
sensitivity of the device to its physical environment is
heightened for a pressure sensor. Any change in the
concentration,
solution
components
(especially
impurities and/or oxidizers); localized geometry and
applied potential. In addition, galvanic corrosion is
influenced by specific metal–to–metal connections.
1–20
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