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MPXM2102AS 数据表(PDF) 30 Page - Motorola, Inc |
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MPXM2102AS 数据表(HTML) 30 Page - Motorola, Inc |
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30 / 670 page  1–24 Motorola Sensor Device Data www.motorola.com/semiconductors Mechanical Failures The occurrence of mechanical failures include components of fatigue, environment assisted cracking, and creep. Packaging materials, process, and residual stresses are all contributors to mechanical failure. A summary of acceleration stresses is shown in Table 3. Contact with harsh media is an accelerating stress for all of the mechanical failure mechanisms. Table 3. Mechanical Failure Mechanisms Failure Mechanism Acceleration Stresses Fatigue crack initiation Mechanical stress/strain range Cyclic temperature range Frequency Media Fatigue crack propagation Mechanical stress range Cyclic temperature range Frequency Media Environment assisted cracking Mechanical stress Temperature Media Creep Mechanical stress Temperature Media PRESSURE SENSOR SOLUTIONS The range of solutions for pressure sensors to media compatibility is very diverse. Mechanical pressure sensors still occupy a number of applications due to this media compatibility concern. These devices typically operate on a variable inductance method and are typically not as linear as a piezoresistive element. Figure 10 shows a comparison between a mechanical pressure sensor and a piezoresistive element for a washing machine level sensing application. The graph shows a nonlinear response for the mechanical sensor and a corresponding straight line for the piezoresistive element. A common method of obtaining media compatibility is to place a barrier coating over the die and wire interconnection. This organic encapsulant provides a physical barrier between the harsh environment and the circuitry. The barrier coating can range from silicone to parylene or other dense films that are typically applied as a very thin layer. This technique offers limited protection to some environments due to swelling and/or dissolution of the encapsulant material when in contact with media with a similar solubility. When a polymeric material has a solubility parameter of the same value as the corresponding media, swelling or dissolution will occur. Stainless steel diaphragms backfilled with silicone oil provide a rugged barrier to most media environments, but generally are very costly and limit the sensitivity of the device. The silicone oil is used to transmit the stress from the diaphragm to the piezoresistive element. If a polymeric material is used as the die attach, the silicone oil will permeate out of the package. This concern requires a die attach that is typically of higher modulus than a silicone and may not adequately isolate the package stress from the die. Figure 10. Graphical comparison of the output from a mechanical pressure sensor compared to a piezoresistive sensor during a washing machine fill cycle. 0 1 23 45 6 140 145 150 155 160 165 170 175 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 TIME (MINUTES) WASHING MACHINE SENSOR PIEZORESISTIVE PRESSURE SENSOR Freescale Semiconductor, Inc. For More Information On This Product, Go to: www.freescale.com |
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