These days, up to 100 sensors may be used in a single vehicle. And while that may seem like a lot, this trend will continue to grow as automobiles become more and more automated. Sensors help to perform a variety of functions including safety checks and engine steering. Many of these sensors have components that are fixed with adhesives and are protected with encapsulants. Chemical, thermal, and mechanical requirements are extremely demanding in vehicles, so adhesives must work reliably and efficiently for extended periods of time in harsh environments.

Sensors are found anywhere in a car where information can be recorded and 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, sensors must be reliably sealed against aggressive media like gasoline or gear oil. Sensors that are used in harsh environmental conditions must be made from dependable materials. Encapsulants and adhesives that are particularly tolerant of high temperatures and have good mechanical characteristics are especially suitable for these types of applications.

What follows is a closer look at the role sensors play in measuring parking distance, tire pressure, speed, position, mass air-flow, and manifold absolute pressure.

Figure 1: Parking distance 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 they are seeing and hearing. This is because sensors help the driver safely maneuver the car into small parking spaces. If the sensor fails, so does the driver—leading to an expense he or she was not counting on. That being said, it is important that the adhesive has good mechanical functionality and good acoustic characteristics, such as the right oscillation behavior, so that a high-quality ultrasonic signal can be transmitted in an unadulterated way.

By putting adhesives through a salt spray test, engineers can determine which products are suitable and which are non-suitable for this specific application. It is also important to note that due to the sensor’s position in bumpers, the temperature requirements of the adhesive do not need to be high and can have an operating temperature range of -40 °C to +85 °C to function properly.

As customer demand increases for park distance control sensors in their vehicles, production processes will need to speed up. Light-curing epoxy resins fulfill these demands, allowing for short cycle times in series production. These resins also provide impressive results during standard automotive tests like temperature change and salt spray testing. Light-curing epoxy resins are cured by preactivation; they are first dispensed and then receive irradiation, which slowly starts the curing process. Once this begins, parts are joined, and the polymerization progresses further, resulting in a quick handling strength.

Under Pressure

Figure 2: tire pressure monitoring sensors have been legally required for some time now.

Sensors are also used as alarm triggers with regard to tire pressure. They relay information from the tire to send a signal both visually and acoustically as soon as the pressure drops and are therefore very important with respect to safety, fuel efficiency, and wear. The incorrect tire pressure can create the risk of having an accident by significantly extending the braking distance or negatively impacting stability when cornering.

The one-component, heat-curing epoxy resins used here are for die-attach, as well as sealing and fixing the two-component products for large-volume casting of the PCB. These resins are especially media-resistant, are repellant against typical automotive fluids, and can withstand salt spray and cleaners. They also provide impressive results in vibration and drop tests. The bonded sensors are put through everyday situations that a car is exposed to in extensive, simulated laboratory tests.  

Figure 3: a hall sensor measures rotation, among other things.

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 the wheel hub. From there, they provide accurate information on how frequently the wheel is rotating. In these cases, dual-curing (light/anaerobic-curing) adhesives are especially appropriate because they assure a rapid pre-fixation and secure final curing of the magnet, the magnet holder, and the hall die even in shadowed areas. The adhesives are highly resistant to media impacts like oil, gas or even braking fluid and also pass salt spray tests, reflow tests according to the semiconductor standardization body (JEDEC), vibration tests, and drop tests with flying colors.

Figure 4: important for exhaust control, MAF sensors have become a topic of interest only recently.

Just the Right Mix

Mass air-flow (MAF) and temperature and manifold absolute pressure (TMAP) sensors help reduce emissions and are located in the engine compartment or, more precisely, in the air intake system. The MAF sensor 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. MAF sensors are somewhat more precise than TMAP sensors, which measure 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 the encapsulant.

As evident by the above mentioned examples, the optimum adhesive depends very heavily on the thermal, mechanical, and chemical requirements of the sensor. There are also appropriate adhesives and encapsulants for the high-reliability sector that enable durable functionality under the most severe environmental conditions.

Figure 5: TMAP sensors are exposed to the most severe environmental conditions.