Freescale Semiconductor
Application Note
AN3728
Rev 1, 07/2008
Media Compatibility for
IPS PRT Pressure Sensors
by: Ryan Hooper
John Young
Freescale Semiconductor, Inc., Tempe, AZ
INTRODUCTION
What is the term Media Compatibility, and how does it affect
Pressure Sensors? Media is any chemical compound, in
liquid, solid or gas which interacts with the sensor where the
sensing of pressure is done either directly through this
chemical, or by measuring the pressure of a gas containing a
certain amount of this chemical. Media Compatibility is
presented here as the various media that our sensor can
sense pressure with, either directly or through an air column.
While this document will lay groundwork for compatible
chemical media, it will not serve as a qualification guarantee.
The only method we endorse fully is the utilization of a
pressure sensor with clean, dry air.
This document will cover the Media Compatibility of these
Integrated Pressure Sensors (IPS) Piezo-Resistive
Transducer (PRT) technology Pressure Sensors. Looking at
the cross section of a Unibody pressure sensor in
Figure 1,
the gel layer is evident over the die. Force or pressure is
transferred through the gel to the sensing element on the die.
The gel coat protects the die and wire bonds that are
encapsulated within and is available as a standard
Fluorosilicone or Media Resistant gel. The Media Resistant
option is the default choice in sensor selection for an
application that may call for added protection over the
Fluorosilicone gel. When a corrosive is introduced to the
sensor that is not compatible, it has the potential to dissolve,
or be absorbed by the gel and corrupt the die surface or lead
to wire-bond breakage. Some types of media will take longer
to corrupt the die than others, so a disposable application is
possible. Knowing how to make the sensor last for longetivity
with/without media can accelerate or slow down the lifetime of
the sensor.
Figure 1. Cross Section of Unibody Pressure Sensor
RELIABILITY
Reliability of the sensor can be improved by Pulse biasing
the pressure sensor. This turns the sensor on and off to
improve the lifetime of the sensor and is a typical standard
practice for PRT sensors. With corrosive media, this will not
improve sensor lifetime if the media is expected to eventually
corrupt the sensor. The sensor will also not survive in a
corrosive or non-corrosive environment with direct contact
with liquid media. Eventually the die or wire-bonds will be
compromised by the chemical interaction in the long term.
This is not to say that Pulse biasing should be avoided
completely. This is standard practice to improve typical sensor
lifetimes in typical applications.
Another method to prolong lifetime is to un-bias the sensor
during sudden cold to hot cycling. The gel layer is a good
protection layer against liquid moisture from contacting wire-
bonds or die level. However if this is saturated in a harsh
environment during cold-hot cycling, condensation can build
up. During the hot cycle this can lead to the possibility of
© Freescale Semiconductor, Inc., 2008. All rights reserved.
shorting wire-bonds. Waiting to take a reading at the warmer,
temperature after the moisture may have evaporated is ideal.
The gel is designed to protect against this instance, but while
we are focusing on media and harsh environments, this is an
issue to consider. A lower bias voltage also tends to prevent
the faster decay of wire-bonds that have been compromised.
Think of the sensor with compromised saturated gel as a
simple electolytic corrosion cell. Wire-bonds that are biased
and grounded act as anodes and cathodes respectively. The
gel that has absorbed a chemical can act as an electrolyte,
and corrode wire-bonds. In the lower voltage bias, the lower
current will slow the electrolysis, prolonging broken wire-
bonds.
How is the ideal application of a PRT IPS pressure sensor
to interact with media of questionable nature? The ideal
method is the media separated from the sensor by a column
of clean, dry air as seen in
Figure 2.
Some vaporization of the
media will occur in the air column, but for most applications
Media Resistant gel will provide adequate protection here.
Corrosive media that would normally corrupt the sensor, when
implemented with an air column, has been proven by
customers to survive product life cycles in their application.
RELIABILITY LIFETIME TESTING
Our sensors are qualified and undergo an Accelerated
Lifetime testing. This incorporates the following conditions:
High Temperature Output Lifetime (HTOL) and Temperature
Humidity Biasing (THB) testing. The HTOL test is
representative of a 10 year accelerated lifetime test. It
requires the part to be tested at 125°C for 1000 hrs at full
biasing voltage. The THB is also another 60°C at 90% relative
humidity for 1000 hrs. These tests prove reliability of
Freescale sensors for high humidity and for high temperature,
and give an indication of how long the sensor can last in such
an environment. They are tested at 100% duty cycle of biasing
the sensor. If the parts are tested at a lesser duty cycle, it will
prolong the life cycle and last much longer.
Pressure Sensor
Sealed tube of air/liquid
column
Liquid being measured
Figure 2. Media Separated by a Column of Clean Dry Air
AN3728
2
Sensors
Freescale Semiconductor
Media Evaluations and Risks
Media
Water
Ethanol (Ethyl
Alcohol)
Diesel
Brake Fluid
Windshield Washer
Fluid
Transmission Fluid
Power Steering Fluid
Engine Coolant
Differential Fluid
Fuel C
Florosilicone
Gel
Not Tested
Not Tested
Tested-Worse
Result
Not Tested
Not Tested
Not Tested
Not Tested
Not Tested
Not Tested
Tested--Poor
Result
Tested--Poor
Result
Tested
Tested
Okay
Okay
Need More Data
Media Resistant
Gel
Tested
Tested
Tested
Tested
Tested
Tested
Tested
Tested
Tested
Tested
Tested
Tested
Tested
Okay
Okay
Need More Data
Long-Term
Direct Exposure
Likely to have
issues
Likely to have
issues
Likely to have
issues
Likely to have
issues
Likely to have
issues
Likely to have
issues
Likely to have
issues
Likely to have
issues
Likely to have
issues
Likely to have
issues
Likely to have
issues
Likely to have
issues
Need More Data
Okay
Need More Data
Likely to have
issues
Air
Column
Unlikely to have
issues
Unlikely to have
issues
Unlikely to have
issues
Unlikely to have
issues
Unlikely to have
issues
Okay
Unlikely to have
issues
Unlikely to have
issues
Unlikely to have
issues
Likely to have
issues
Likely to have
issues
Unlikely to have
issues
Unlikely to have
issues
—
Okay
Need More Data
Disposable - 24hr Application
(SSOP ONLY)
Probably Okay
(Customer to Validate)
Probably Okay
(Customer to Validate)
Probably Okay
(Customer to Validate)
Probably Okay
(Customer to Validate)
Probably Okay
(Customer to Validate)
Probably Okay
(Customer to Validate)
Probably Okay
(Customer to Validate)
Probably Okay
(Customer to Validate)
Probably Okay
(Customer to Validate)
Likely to have issues
Likely to have issues
Probably Okay
(Customer to Validate)
Probably Okay
(Customer to Validate)
Probably Okay
(Customer to Validate)
Okay
Need More Data
Fuel C + M
Engine Oil
Detergent and Bleach
Human Breath
Gases- Nitrogen
Gases-High Oxygen
AN3728
Sensors
Freescale Semiconductor
3
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© Freescale Semiconductor, Inc. 2008. All rights reserved.
AN3728
Rev. 1
07/2008
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