Many environmental and electrical factors 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.
In many applications resistance stability over time is important. These include voltage divider circuits and resistors used as feedback or biassing elements. Here, a change in resistance will directly impact the performance of a circuit.
Factors that may degrade a resistor (change its resistance) may be environmental, electrical or application related. They include:
- Consistent overload conditions.
- Surge and ESD.
- Mechanical damage.
- Application issues.
In general, the longer the exposure to one or more of these stresses the higher the level of degradation.
Current flow through the resistor will generate heat. There is also the temperature of the system environment to consider.
Resistors are specified to operate at a particular temperature. Increasing temperature beyond those limits will stress the thick film resistor film and substrate. This, in turn, will cause a change in the resistance value.
If heat is dissipated and the temperature falls back within specified limits the resistance value should return to its expected value. But repeated temperature excursions will eventually degrade the resistor performance.
High moisture content in the environment can degrade the resistor performance. At the component level, it can cause corrosion, tracking and short circuit issues.
Corrosion is also a potential issue at the system board level. Moisture or chemical elements in the environment can cause an increase in resistance at the resistor/system board interface. In extreme cases, they can cause or growths that eventually cause a short circuit.
Coating the resistor and/or system board can minimise the impact of environmental factors. But, it is important to clean the surface of the component thoroughly before coating. The entrapment of contaminants can make the situation worse.
A constant electrical overload condition will generate excessive heat. This will stress the thick film resistor film and substrate beyond specified limits and 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 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 occur during shipping, in manufacture, or in service. Inappropriate device mounting to the system board can mechanically stress the resistor device.
Often damage is not immediately obvious but it can compromise the correct operation of the resistor device over time. Temperature and/or electrical issues can compound a minor issue and degrade resistor performance.
Many of the effects described above are transient conditions. As a result, they can be difficult for the system designer to identify and quantify.
Identification of what could degrade resistor performance in a given application is vital. Without this information, it is difficult to choose the best resistor for the application.
Should you need any help deciding on the best resistor for your application TSEC engineering team is always available to help.