Thick Film Power Resistor Failure Modes
Thick film power resistor failure is often related to environmental or transient conditions that were not considered during the component design phase. Mechanical stresses, thermal issues, constant overload or surge conditions and mounting issues can all degrade the power resistor device or, in some cases, cause catastrophic failure. Handling issues, including ESD, can also have a major impact on the reliable operation of the resistor device.
RESISTOR MECHANICAL FAILURES
One of the most common reasons for thick film power resistor damage is the combination of the component mounting method and mechanical stress. Vibration and/or shock can cause micro cracking of the resistor material or, in severe cases, substrate cracking. Any stress induced by the component mounting method will increase the probability of damage.
Substrate cracking can cause complete device failure while micro cracking can lead to an irreversible change in resistance value. This resistance change can cause errors in system performance, the cause of which can be difficult to identify at a system level.
POWER RESISTOR THERMAL ISSUES
When current passes through a resistor it generates heat. This, in turn, causes a differential thermal expansions of the different material used to manufacture the resistor component and induces relative mechanical changes (stresses) in the resistor.
The primary heat dissipation mechanism of a high power resistor is via radiation. It is important to cool the resistor component appropriately by using either forced air cooling and/or an appropriate heat sink to prevent damage. The thermal operating environment and heat management must be considered carefully at the component design phase to ensure the device is fit for purpose.
Power Coefficient of Resistance (PCR) quantifies the resistance change due to self-heating when power is applied and is particularly important for resistors used in power applications.
The Temperature Coefficient of Resistance (TCR) changes the resistance value with temperature. TCR is a known factor and, depending upon materials used, is typically ± 100 ppm / °C or better for Thick Film Resistors. TCR is usually specified as hot or cold TCR by the material manufacturer. Normally only hot TCR applies, which causes an increase, in resistance. Cold TCR is a decrease, in resistance and only comes into play either in extremely cold operating environments or in ‘cold start’ situations.
If considered during the high power resistor design phase many environmental factors such as moisture and chemical elements can be addressed by applying a suitable coating to the resistor device. However, it is important to consider the surface treatment of the system board and the resistor mounting method as the temperature can cause metal migration between the terminals of the resistor leading to potential short circuit or change in resistance value.
RESISTOR OVERLOAD AND SURGE CONDITIONS
A continuous over-load of a resistor device tends to degrade the insulation resistance and changes the resistor parameters over time. It is therefore important to observe the resistor maximum specified voltage.
All possible surge events must be considered at the power resistor component design stage. The mass of the resistor element and its geometry both have a major impact on pulse event survivability. An increased surface area also allows more heat dissipation.
The final factor contributing factor to a thick film resistor surge capability is the component adjustment for final resistance value. Laser resistor trimming can create weak spots that can later cause failure under surge conditions.
Damage caused by ESD is a latent defect that can be difficult to identify. The resistor may be partially degraded, yet continue to perform its intended function. However, the chances of premature or catastrophic failure are increased particularly if the device is exposed to one or more of the stresses listed above.
A resistor can often be the lowest cost element in a system but failure can be just as catastrophic as a failure of any other system element. It is therefore important to understand potential failure modes and how they may be addressed. A partnership with a specialist manufacturer with long term experience of thick film power resistor technology and its manufacture can minimise the possibility of resistor failure.