The Main Causes Of Thick Film Resistor Failure
Thick film resistor failures are often caused by external environmental factors such as electrical and mechanical stresses and handling issues but seldom occur due to a failure of the resistive element itself. Failures are either classed as a degradation of performance or complete failure (usually as an open rather than a short circuit).
We consider several common causes of thick film resistor failure, many of which may not be immediately obvious to the circuit designer. Failure modes include:
· Mechanical stress
· Environmental – metal migration
· Thermal issues
· Constant overload
Apart from the obvious precautions required to prevent handling damage leading to cracks and chips to the resistor material most mechanical damage is caused by stresses induced by vibration or inappropriate mounting of the resistor device.
Ongoing vibration can cause micro-cracking of the resistor material leading to change in the resistance value, damage to the resistive element or (often if combined with other stresses such as thermal) component failure. Inappropriate mounting of a device can cause ongoing compression or extension of the resistor increasing its susceptibility to the other stresses listed below.
Most thick film resistors are coated in some way to protect them from common environmental issues such as moisture and some chemical elements but environmental factors such as moisture and contamination still require careful consideration. Both can cause metal migration between the terminals of the resistor leading to potential short circuit or change in resistance value.
As noted above mechanical failure modes of thick film resistors are often propagated by heat. It is therefore important to properly understand resistor heat dissipation properties. A low power resistors primary heat dissipation mechanism is via conduction through its component leads or connections, while a high power resistor dissipates heat primarily through radiation.
When current passes through a resistor it generates heat and the differential thermal expansions of the different material comprising the resistor induces relative mechanical changes (stresses) in the resistor.
Temperature Coefficient of Resistance (TCR) is the best known parameter used to specify a resistor’s stability and defines the resistive element’s sensitivity to temperature change. 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.
It is important to note the thick film resistors maximum specified voltage as a continuous over-load condition will tend to degrade the insulation resistance and change the resistor parameters over time. Voltage stress can cause conduction from normally non-conductive materials in the resistor film leading to deterioration and occasionally failure due to hot spots.
The key element in determining a thick film resistor surge survivability is the mass of a resistor element, which is directly proportional to its thickness multiplied by its surface area. Hence, thick film technology has a significantly higher surge handling capability than thin film resistors but surge survivability still needs careful consideration.
The geometry of a resistor also affects its surge withstand capability. A larger surface area results in a higher film mass, and ultimately an improved surge performance. In addition, this increased surface area allows more heat dissipation which is important for power resistor applications.
The final factor contributing factor to a thick film resistor surge capability is how the component is adjusted for final resistance value. Resistor trimming and the method used for trimming can create weak spots that ultimately cause failure under surge conditions.
Damage via ESD can be difficult to identify. The resistor may be partially degraded, yet continue to perform as expected. 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 may be the lowest cost and most basic component in a system but failure can be just as catastrophic as a failure of any other component. It is therefore important to understand potential failure modes and how they may be addressed. A partnership with a specialist resistor manufacturer with long term experience of thick film resistor design and manufacture can minimise the potential for resistor failure.