Quality Gets a Life Sentence
Sensors often play a vital role in automotive safety. That’s why their reliability must be assured throughout the service life of the vehicle. Bosch researchers are using experimental methods and computer simulations to make sure that micromechanical components will serve out their full “life term”.
This is a tremendous challenge, because these researchers have to predict the future behavior of sensors, and rule out possible sources of errors and deficiencies that might affect device functions. The sensor must, after all, work just as well in arctic winters as in subtropical temperatures. And it has to do this from day one throughout its product life of about 15 years.
That’s why Bosch thoroughly checks all sensors based on microsystems technology (MST) for reliability during every phase of product development – from the early design phase through structural and packaging technology to aspects of high-volume production.
This holistic, systematic approach has two advantages: Potential defects and costly reworking are ruled out early in the process. And that also holds true after the product is shipped: There is a very low risk of failure in vehicles during their lifetime.
Production experts discuss the frequency of component failures in terms of “ppm” (parts per million). Two ppm, for instance, means that two sensors out of a million would fail before reaching the intended product life of 15 years. That seems almost negligible. But in view of the large volume of MST sensors that are shipped, Bosch is committed to further reducing the failure rate. The main obstacle: There are nowhere near enough defective sensors available for a statistically relevant examination. Bosch therefore uses experiments and computer simulations to boost the reliability of sensors. To get an estimate of the resulting increase in product life, sensors are subjected to especially demanding tests and accelerated aging.
Take an angular rate sensor: This key component of the ESP (Electronic Stability Program) system consists of many narrow beams trenched out by an etching process. These delicate elements have to withstand even unforeseen, extreme stresses without breaking. The standard test procedure therefore includes a drop test of the sensor from a height of 1.20 meters. During the impact the sensor is subjected to a negative acceleration 30,000 times greater than the acceleration due to gravity. This test raises difficult questions for reliability experts: What stresses are generated? Where do these have critical effects? Researchers use test objects to investigate how materials behave under these extreme conditions. These micromechanical test structures are run through the same production process as an ESP sensor element.
First, polycrystalline silicon is deposited on a substrate. Then the structural element is trenched out of this layer by an etching process. Particular attention is paid to critical points – narrow sections, bends and angles, transitions from broader to particularly thin material regions. During the experiment these test structures are subjected to increasing mechanical stress until they break. This test provides researchers with valuable materials data they can apply in designing the sensor. They also use computer simulations. Simulations provide useful suggestions to help increase the fracture strength even further. A systematic combination of experimental and simulation methods provides a wealth of data that researchers use in the continuing improvement of sensors.
To optimize this sensitive component it’s equally important to study the system as a whole – for instance, packaged in its housing on a printed circuit board (PCB). That’s the way the sensor is usually installed in a car or in an automotive component, and the way it has to withstand the harsh environmental conditions: From -40 to +85 degrees Celsius in the car’s interior, as high as +140 degrees Celsius on the engine block. The sensor is subjected to all sorts of vibrations and shock. High humidity can also contribute to the aging process. Tests in a climate testing chamber are therefore part of the standard procedure: 1,000 cycles of a thermal shock exceeding 180 degrees Celsius can be equivalent to several years of aging. In addition, the sensors are tortured on a vibrator table. The entire frequency spectrum relevant to an automobile can be simulated here – up to several dozen kilohertz. A laser vibrometer can measure the way a PCB and an attached sensor housing resonate. If these vibrations occur in a vehicle and coincide with the natural frequency of an angular rate sensor, the sensor’s signal will be distorted.
Researchers can also use this laser method to ensure that no housing resonances are shifted toward the natural frequency of a sensor as a result of aging. Research has shown that if the housing frequencies are just slightly higher than the sensor frequency, aging-related softening of the solder can cause the housing frequencies to gradually converge toward the sensor frequency. This phenomenon is experimentally investigated by artificially manipulating the solder. The resonant system – PCB, solder connection, housing pins and sensor housing – is modeled in the computer and compared with the experimental data by using finite element simulation. Such “verified” models enable researchers to build reliability into the sensor elements – right up front in the design phase.
Bosch regards microsystems technology and the sensors based on it as a key technology, and the ultimate payoff is high reliability of sensor components. The immediate benefits are a detailed knowledge of materials parameters and extensive experience of production volumes in the millions of components. Expertise in design tools is complemented by an abundance of MST manufacturing know-how. Since Bosch controls the entire process chain, its professionals have easy access to all the right levers in every phase of product development to push reliability to ever higher levels.
That’s why Bosch thoroughly checks all sensors based on microsystems technology (MST) for reliability during every phase of product development – from the early design phase through structural and packaging technology to aspects of high-volume production.
This holistic, systematic approach has two advantages: Potential defects and costly reworking are ruled out early in the process. And that also holds true after the product is shipped: There is a very low risk of failure in vehicles during their lifetime.
Production experts discuss the frequency of component failures in terms of “ppm” (parts per million). Two ppm, for instance, means that two sensors out of a million would fail before reaching the intended product life of 15 years. That seems almost negligible. But in view of the large volume of MST sensors that are shipped, Bosch is committed to further reducing the failure rate. The main obstacle: There are nowhere near enough defective sensors available for a statistically relevant examination. Bosch therefore uses experiments and computer simulations to boost the reliability of sensors. To get an estimate of the resulting increase in product life, sensors are subjected to especially demanding tests and accelerated aging.
Take an angular rate sensor: This key component of the ESP (Electronic Stability Program) system consists of many narrow beams trenched out by an etching process. These delicate elements have to withstand even unforeseen, extreme stresses without breaking. The standard test procedure therefore includes a drop test of the sensor from a height of 1.20 meters. During the impact the sensor is subjected to a negative acceleration 30,000 times greater than the acceleration due to gravity. This test raises difficult questions for reliability experts: What stresses are generated? Where do these have critical effects? Researchers use test objects to investigate how materials behave under these extreme conditions. These micromechanical test structures are run through the same production process as an ESP sensor element.
First, polycrystalline silicon is deposited on a substrate. Then the structural element is trenched out of this layer by an etching process. Particular attention is paid to critical points – narrow sections, bends and angles, transitions from broader to particularly thin material regions. During the experiment these test structures are subjected to increasing mechanical stress until they break. This test provides researchers with valuable materials data they can apply in designing the sensor. They also use computer simulations. Simulations provide useful suggestions to help increase the fracture strength even further. A systematic combination of experimental and simulation methods provides a wealth of data that researchers use in the continuing improvement of sensors.
To optimize this sensitive component it’s equally important to study the system as a whole – for instance, packaged in its housing on a printed circuit board (PCB). That’s the way the sensor is usually installed in a car or in an automotive component, and the way it has to withstand the harsh environmental conditions: From -40 to +85 degrees Celsius in the car’s interior, as high as +140 degrees Celsius on the engine block. The sensor is subjected to all sorts of vibrations and shock. High humidity can also contribute to the aging process. Tests in a climate testing chamber are therefore part of the standard procedure: 1,000 cycles of a thermal shock exceeding 180 degrees Celsius can be equivalent to several years of aging. In addition, the sensors are tortured on a vibrator table. The entire frequency spectrum relevant to an automobile can be simulated here – up to several dozen kilohertz. A laser vibrometer can measure the way a PCB and an attached sensor housing resonate. If these vibrations occur in a vehicle and coincide with the natural frequency of an angular rate sensor, the sensor’s signal will be distorted.
Researchers can also use this laser method to ensure that no housing resonances are shifted toward the natural frequency of a sensor as a result of aging. Research has shown that if the housing frequencies are just slightly higher than the sensor frequency, aging-related softening of the solder can cause the housing frequencies to gradually converge toward the sensor frequency. This phenomenon is experimentally investigated by artificially manipulating the solder. The resonant system – PCB, solder connection, housing pins and sensor housing – is modeled in the computer and compared with the experimental data by using finite element simulation. Such “verified” models enable researchers to build reliability into the sensor elements – right up front in the design phase.
Bosch regards microsystems technology and the sensors based on it as a key technology, and the ultimate payoff is high reliability of sensor components. The immediate benefits are a detailed knowledge of materials parameters and extensive experience of production volumes in the millions of components. Expertise in design tools is complemented by an abundance of MST manufacturing know-how. Since Bosch controls the entire process chain, its professionals have easy access to all the right levers in every phase of product development to push reliability to ever higher levels.
A laser beam scans the housing vibrations of an angular rate sensor. The sensor is mounted on a vibrator table which simulates vibrations that are typical for an automobile.
