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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 wirebonds 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
(c) 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 wirebonds. 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 125C for 1000 hrs at full biasing voltage. The THB is also another 60C 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
How to Reach Us:
Home Page: www.freescale.com Web Support: http://www.freescale.com/support USA/Europe or Locations Not Listed: Freescale Semiconductor, Inc. Technical Information Center, EL516 2100 East Elliot Road Tempe, Arizona 85284 1-800-521-6274 or +1-480-768-2130 www.freescale.com/support Europe, Middle East, and Africa: Freescale Halbleiter Deutschland GmbH Technical Information Center Schatzbogen 7 81829 Muenchen, Germany +44 1296 380 456 (English) +46 8 52200080 (English) +49 89 92103 559 (German) +33 1 69 35 48 48 (French) www.freescale.com/support Japan: Freescale Semiconductor Japan Ltd. Headquarters ARCO Tower 15F 1-8-1, Shimo-Meguro, Meguro-ku, Tokyo 153-0064 Japan 0120 191014 or +81 3 5437 9125 support.japan@freescale.com Asia/Pacific: Freescale Semiconductor China Ltd. Exchange Building 23F No. 118 Jianguo Road Chaoyang District Beijing 100022 China +86 10 5879 8000 support.asia@freescale.com For Literature Requests Only: Freescale Semiconductor Literature Distribution Center P.O. Box 5405 Denver, Colorado 80217 1-800-441-2447 or +1-303-675-2140 Fax: +1-303-675-2150 LDCForFreescaleSemiconductor@hibbertgroup.com
Information in this document is provided solely to enable system and software implementers to use Freescale Semiconductor products. There are no express or implied copyright licenses granted hereunder to design or fabricate any integrated circuits or integrated circuits based on the information in this document. Freescale Semiconductor reserves the right to make changes without further notice to any products herein. Freescale Semiconductor makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does Freescale Semiconductor assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation consequential or incidental damages. "Typical" parameters that may be provided in Freescale Semiconductor data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including "Typicals", must be validated for each customer application by customer's technical experts. Freescale Semiconductor does not convey any license under its patent rights nor the rights of others. Freescale Semiconductor products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the Freescale Semiconductor product could create a situation where personal injury or death may occur. Should Buyer purchase or use Freescale Semiconductor products for any such unintended or unauthorized application, Buyer shall indemnify and hold Freescale Semiconductor and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that Freescale Semiconductor was negligent regarding the design or manufacture of the part. FreescaleTM and the Freescale logo are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners. (c) Freescale Semiconductor, Inc. 2008. All rights reserved.
AN3728 Rev. 1 07/2008

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