Bonding Sensors in the Car
Sensors can generally be found anywhere in the car where information can be measured and then relayed. They often have the task of capturing physical variables such as temperature, pressure, position, or speed. Because these signals are then passed on to control units and evaluation electronics, the sensors must be reliably sealed against any aggressive media. Sensors that are used in harsh environmental conditions, e.g. for monitoring the oil level or pressure, must be composed of dependable materials. Encapsulants and adhesives that are particularly tolerant of high temperatures and have good chemical characteristics are especially suitable for these types of applications.
This article focuses on parking distance, tire pressure, speed, position, mass air-flow, and TMAP sensors.
Safe Parking Made Easy
When it comes to parking distance sensors, it is essential that the driver be able to depend on the accuracy of the data. This is because sensors help the driver safely maneuver their car into small parking spaces. If the sensor fails, it can be very expensive for the driver. So it is particularly important that the adhesive used has good acoustic characteristics such as the right oscillation behavior in addition to mechanical functionality, so that the ultrasonic signal is transmitted unadulterated and in high-quality. Due to their position in the bumper, the temperature requirements are moderate. The adhesives should be usable in a temperature range from -40 to +85 °C. In addition, they have to pass special automotive tests, such as temperature and salt spray tests.
Light-curing epoxy resins meet exactly these requirements and allow for in-line processes and short cycle times in series production, while enabling reliable curing of shadowed areas by preactivation. In view of the continuously increasing demand, ultrasonic sensor manufacturers can efficiently produce high volumes.
Under Pressure
Sensors are also used as alarm triggers as soon as the pressure drops. They are therefore very important with respect to safety, fuel efficiency, and wear. The incorrect tire pressure can create a huge risk of having an accident by significantly extending the braking distance or negatively impacting stability when cornering.
For die attach, sealing and fixing tasks, heat-curing epoxy resins are preferably used, while 2C materials are used for large-volume PCB casting. They all have in common that they are especially media-resistant and repellant against typical automotive fluids. In particular, they can withstand salt spray and cleaners. They also provide impressive results in thermal shock, humidity storage, vibration and drop tests. In order to ensure that pressure measurement always works correctly, the tightness of the sensor fixation to the housing is checked with a typical pressure of 7 bar for the long term and 9.5 bar for the short term.
Finding the Right Balance
Hall sensors are always applicable when position or speed need to be captured. These sensors are sometimes located directly on the wheel rim, or even on the wheel hub. From there, they provide accurate information on how frequently the wheel is rotating. For their assembly, dual-curing (light-/anaerobic-curing) adhesives are especially appropriate because they assure a rapid pre-fixation of the magnet, the magnet retainer and the Hall sensor. In addition, they secure final curing, even in shadowed areas. The adhesives are also highly resistant to media impacts like oil, gas, or even braking fluid, and also pass salt spray tests, reflow tests (JEDEC), vibration tests, and drop tests with flying colors.
Just the Right Mix
Mass air-flow and TMAP sensors help reduce emissions and are located in the engine compartment, or more precisely in the air intake system. The mass air-flow sensor, MAF for short, has the primary task of communicating the incoming air mass to the engine control unit. This quantity is needed in both gas and diesel engines to calculate various parameters. Mass air-flow sensors are somewhat more precise than TMAP sensors. A TMAP sensor measures the pressure and the intake temperature and can use this data to determine air mass.
In order to protect sensors, extremely reliable epoxy resins are used that pass all the typical automotive tests. They are resistant to oil, gas, and braking fluid and can survive in a temperature range of -40 °C to +155 °C. Even when stored at these temperatures, the encapsulant on the sensor does not display any stress cracks. There is also no interaction between the transmission oil and encapsulant.
Conclusion
As shown by the examples, the optimum adhesive depends very heavily on the thermal, mechanical, and chemical requirements of the sensor. There are even appropriate adhesives and encapsulants for the high-reliability sector that enable durable functionality under the most severe environmental conditions.
About the author:
Dr. Daniel Lenssen is Product Manager at DELO Industrial Adhesives.
Dr. Daniel Lenssen studied physics at the RWTH Aachen and achieved his doctorate degree in the field of semiconductor physics at the Research Centre Jülich. After a few years as development engineer for EPCOS, he joined DELO in 2003 as a Regional Sales Manager. In 2006, he became Business Development Manager. A few years ago, he moved into his current position in product management. Besides his special focus on light-curing products, he is responsible for adhesives and casting compounds for automotive sensors.
Images: DELO
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