There are many environmental and electrical factors that can cause the performance of a thick film resistor to degrade over time. To avoid compromising system performance it is important to be aware of the issues and to manage them appropriately.
Resistors are used in a wide range of applications where their stability over time is important. These include voltage divider circuits and resistors used as feedback or biassing elements where a change in resistance directly impacts on the performance of a circuit.
Factors that may degrade a resistor (change its resistance) may be environmental or electrical. They include:
- Constant overload.
- Surge and ESD.
- Mechanical damage.
- Application issues.
Resistors are specified to operate at a particular temperature. If that temperature is exceeded the resistance value will change.
Increasing temperature beyond specified limits will stress the thick film resistor film and substrate. Current flow through the resistor will generate heat which must be dissipated. There is also the temperature of the system environment to consider.
In the short term, if the temperature is brought back within specified limits the resistance value should return to its expected value but repeated temperature excursions beyond specified limits will eventually degrade the resistor performance.
A high moisture content in the environment can degrade the resistor performance. At the component level, the potential for corrosion, tracking and short circuit issues can be minimised by applying a suitable protective coating during resistor manufacturing.
Potential problems at the system board level should also be considered. Moisture and/or chemical elements in the environment can cause corrosion at the resistor/system board interface or growths that may eventually cause a short circuit.
A constant overload condition will generate excessive heat. This will stress the thick film resistor film and substrate beyond specified limits and will degrade the resistor performance over time. Or, in extreme cases, cause resistor failure.
Both surge and ESD effects disrupt the point to point contact of conductive elements in the resistor film. Depending on the number of occurrences of surge events and the severity of each surge the resistance of the resistor film will change over time.
Surge conditions and their potential impact on resistor performance have been discussed elsewhere on this blog. They are one of the most common causes of changes in the performance of a resistor over time. ESD events are extreme surge conditions and must be prevented wherever possible.
Mechanical damage and/or shock can happen during shipping, in manufacture, or in service. Inappropriate device mounting to the system board can mechanically stress the resistor device.
The damage is often immediately not obvious but it can compromise the correct operation of the resistor device over time. Temperature and/or electrical issues (mentioned above) can compound an issue that is initially minor and impact on the performance of the resistor device.
Many of the effects described above are transient conditions and they can be difficult for the system designer to identify and quantify. If a thick film resistor is impacted by one or more of these conditions during its in service life its performance will be degraded which can in turn impact overall system performance.
To minimise the impact it is important to choose a robust resistor that has been designed and manufactured with some capability to withstand the effects described above. Every effort should be made to identify potential environmental and electrical events that could degrade resistor performance and ensure they are addressed.