Freescale Semiconductor, Inc.
The designer could match an application’s requirements, for
INDUSTRY STANDARDIZATION
media compatibility, with the available sensor products thus
takingpriceandperformanceintoaccount. Thiswillenablethe
designer to minimize the total cost of an application. A
standard will also enable suppliers to provide products
warranted to defined criteria. Once a standard is adopted, the
suppliers may rationalize their test efforts and pass the
savings on to their customers.
A standard should provide a designer with a simple,
coherent, complete definition of a media’s effects on a sensor.
The standard should included an accepted test methodology,
test equipment guidelines, life time model, acceleration
factors model, and a definition of failures. A proposed list of
criteria to include in a model are shown in table 4.
Why an industry standard? The increasing use of electronic
sensors in everyday life has designers wrestling with the
complexity of defining the compatibility of a sensor with the
mediathey are measuring. A designer may decide to solve the
question of media compatibility by choosing to isolate the
sensor from the media via a stainless steel diaphragm. While
this solution provides very good media isolation, it is not
without some drawbacks such as cost, size of packaging,
decreasedsensitivityandlongtermdrift. Withoutarecognized
standard for defining media compatibility, the designer is left
to a series of ad hoc test methods and conflicting
specifications.
An industry media compatibility standard will provide the
designer with a method of evaluating sensor performance.
Table 4. Suggested Criteria for Media Compatibility
Media Contact — Front or Back
Pressure Range
Supply Voltage
Solubility Parameter
Conductivity of Media
pH
Supply Voltage Duty Cycle
Voltage Potential within Media
Frequency Output is Measured
Relative Motion of Media (e.g., Flow)
Temperature Range
Recipe of Media and Contaminants
Sensor to Media Interconnection
Lifetime Expectancy
These criteria must be included not only for the media, but
also for the contaminants in the media. An example is a
washing machine level sensor which must be compatible with
water vapor (the media) and detergent and chlorine (the
contaminant). To create a standard, a series of tests which
benchmark the criteria must be designed and performed. The
results would form the basis of the life time and acceleration
factor models.
There are several ways to create a standard, each of which
have their own associated pros and cons. Three possible
ways to create a standard are: an industry association
committee, a panel of industry representatives, or a de facto
standard set by one or more industry suppliers. To define a
standard for media compatibility may require more than one
of these methods. An industry leader may define a standard
form to which they deliver product. This may stimulate the
formationof a committee which defines a broader standard for
the industry. As this standard becomes more accepted by the
industry, the committee may work with an industry association
to “legitimize” the de facto standard. No matter how the
standard is formulated, receiving broad industry acceptance
will require meeting the customers’ needs.
CONCLUSION
Investigation of media compatibility for pressure sensors
has been presented from a physics–of–failure approach. We
have developed a set of internal standard test and reliability
lifetime analysis procedures to simulate our customers’
requirements. These activities have incorporated information
from several fields beyond sensors and/or electronics,
including: electrochemistry and corrosion, polymers, safety
and environmental, automotive and appliance industry
standards, and reliability. The next critical step to elevating the
awareness of this problem, in our opinion, is to develop an
industry–wide set of standards, driven by customer
applications, that include media testing experimental
procedures, reliability lifetime analysis, and media
compatibility reporting to allow easier customer interpretation
of results.
ACKNOWLEDGMENTS
Many individuals have contributed to the media
compatibility initiative and are deserving of an
acknowledgment. The individuals include Debi Beall, Gordon
Bitko, Jerry Cripe, Bob Gailey, Jim Kasarskis, John Keller,
Betty Leung, Jeanene Matkin, Mike Menchio, Adan Ramirez,
Chuck Reed, Laura Rivers, Scott Savage, Mahesh Shah,
Mario Velez, John Wertz, MEMS1, MKL, Reliability Lab,
Characterization Lab, and the Prototype Lab.
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