Numerous environmental and electrical factors can cause a thick film resistor to degrade over time. To safeguard system functionality, it is crucial to recognise these potential issues and mitigate their impact.
Resistance stability over time is important in many applications. These include voltage divider circuits and resistors used as feedback or biassing elements. It is evident here that a change in resistance will impact a circuit’s performance.
Various factors can degrade a resistor (change its resistance), including environmental, electrical, mechanical, and application issues. They include:
- Moisture.
- Heat.
- Consistent overload conditions.
- Surge and ESD.
- Mechanical Stresses.
- Handling Issues.
In general, the longer the exposure to one or more of these stresses, the higher the level of degradation.
Environmental Factors
Resistors are specified to operate at a particular temperature. Increasing temperature beyond those limits will cause a change in the resistance value.
As current passes through a power resistor, it generates heat. Differential thermal expansions of the resistor materials can cause mechanical stresses in the thick film resistor film material and substrate.
If heat is dissipated and the temperature falls within specified limits, the resistance value should return to its expected value. However, repeated temperature excursions will eventually degrade the resistor performance.
The primary heat dissipation mechanism of a thick film power resistor is through radiation. Therefore, air temperature, resistor location and airflow are all vital considerations. Components close to the resistor should be reviewed and their impact on the ambient air temperature considered.
Heatsinks are one potential thermal management solution, but a strong mechanical bond between the rear of the power resistor component and the heatsink is critical to thermal performance. Hence power resistors are often bolted to the heatsink, but mechanical stress that causes cracking (or microcracking) must be avoided.
Moisture or chemical elements in the environment can cause an increase in resistance at the resistor/system board interface. Over the long term, they can cause corrosion and change the resistance value. In extreme cases, they can cause organic growths that eventually cause a short circuit.
Coating the power resistor and/or system board can minimise the impact of environmental factors. As the entrapment of contaminants can make the situation worse, it is important to clean the components and system boards before coating.
Electrical Factors
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.
Surge conditions are one of the most common causes of changes in the performance of a resistor. ESD events are extreme surge conditions and must be prevented wherever possible.
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.
Mechanical Stresses
While choosing the right power resistor is important, it’s essential to consider mounting to the circuit board. Improper resistor mounting can lead to a range of issues. They include thermal failure, signal interference, or physical damage that can, over time, cause resistor failure.
Mechanical damage and/or shock can occur during shipping, or manufacture. Inappropriate device mounting to the system board can mechanically stress the resistor device.
A potential weak point is where the lead exits the resistor device. Using undue force on the leads when mounting to the system board can damage the power resistor track and/or the ceramic substrate. It is important to provide strain relief when lead forming. Avoid bending too close to the resistor device, twisting, or excessively splaying the resistor leads.
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 thick film resistor performance in a given application is vital. Without this information, it is difficult to choose the best resistor for the application. TSEC engineering team is always available to help.