Thick film resistors are used in a wide range of automotive applications. They are found in the car cabin, in audio and in-car navigation systems. More demanding applications include ‘under the hood’ engine control, oil and air temperature and fuel efficiency sensors.
With the transition to electric vehicles, battery management systems are now a common application for automotive resistors. As self-steering, self-parking and other driver assistance aids are introduced, the demand for automotive electronic components, including resistors, is expected to increase.
Thick film resistors in automotive applications are subjected to a variety of stresses. The cabin is a relatively benign environment. In other areas, resistors are exposed to high temperatures, severe temperature cycles and mechanical stresses, including vibration and shock. High moisture levels and corrosive elements including salt and sulphur are common problems to overcome.
The AEC-Q200 automotive electronic component standard has five quality ratings. These cover the range of potential environments found in a typical vehicle. A component found in the car cabin requires a lower level qualification than one used in an engine temperature sensor. Electronic components found in specialist automotive applications can have their own qualification standards.
Automotive Applications Impact On Resistor Performance.
Temperature extremes, mechanical stresses, high moisture levels and corrosive elements can all impact the performance of electronic components, including resistor devices.
Temperature
Resistor heat dissipation is a concern, particularly in high-power applications. Various methods, including heatsinks, are used to transfer heat from the resistor to the surrounding environment. In high-temperature environments, this is not possible and, unless steps are taken to derate the resistor, the resistor film can overheat and fail.
Mechanical Stresses
The most common mechanical stress in automotive applications is constant vibration, but mechanical shock can also be an issue.
In extreme cases, mechanical shock can cause the thick film resistor substrate to fracture and fail. A more common (and subtle) problem is microcracking of the substrate, disrupting the resistor film and causing changes in resistance value.
Constant vibration can also cause microcracking and/or disrupt the composition of the resistor film over time. The interface between the resistor and the system board (the solder joint) and the connection between the resistor lead and resistor substrate can be degraded by vibration over an extended period.
Moisture And Corrosive Elements
Water ingress can damage the resistor material. Chemical elements can react with elements in the resistor film and thereby cause changes in resistor value. The reaction of sulphur with silver is a common concern. The solution, in most cases, is to coat the resistor with a suitable barrier compound.
Increased temperature, coupled with moisture and/or chemical elements, can also affect the resistor to system board interface. This can cause cracking or growths and migrations resulting in resistance changes.
Resistor Technologies In Automotive Applications
The choice of resistors in automotive applications depends primarily on the vehicle application and cost. For example, audio systems are internal to the car. The primary concern is, therefore, not robustness but performance. Low noise metal foil resistors are common in car audio systems.
In contrast, in high-power applications in the car engine compartment Wirewound or thick film resistors will often be the first choice. Both technologies are robust and relatively low-cost.
In sensor applications, precision thin film devices could be the first choice. However, as they may not survive extreme environmental conditions, they are often replaced by thick film resistors.
Resistors For Specialised Automotive Applications
Racing cars and robot vehicles are examples of automotive applications where safety, reliability or performance cannot be compromised. Resistor components operate in extreme versions of the demanding environmental conditions discussed above. Where space to fit electronic systems is limited application-specific resistor components are often the only viable option.
In racing cars 100’s of sensors monitor systems and telemetry beams data back to the pit team. LED lighting is used as a driver and pit crew aid. Resistors are found in these systems and in the engine compartment. Robotic vehicles have similar performance demands as racing cars, with the added complexity of command and control circuitry (to replace a driver) and battery management.
Many advancements in specialist vehicles will eventually move to mainstream vehicles as car manufacturers incorporate the technology into their designs. This can only increase the demand for high-performance thick film resistor devices.