AN3728 Datasheet, PDF (1/4 Pages) Freescale Semiconductor, Inc – Media Compatibility for IPS PRT Pressure Sensors

Freescale Semiconductor
Application Note
Media Compatibility for
IPS PRT Pressure Sensors
by: Ryan Hooper
John Young
Freescale Semiconductor, Inc., Tempe, AZ
AN3728
Rev 1, 07/2008
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.