Thick film resistor failure is rarely caused by a failure of the resistive element but is generally due to external environmental factors such as mechanical and electrical stresses and handling issues. Failures can be either classed as a degradation of performance or complete failure (usually as an open rather than a short circuit).
Six common causes of thick film resistor failure are:
Environmental – metal migration
Apart from handling damage leading to substrate cracks and chips most mechanical damage is caused by either vibration or inappropriate mounting of the device. Micro cracking of the resistor material caused by vibration or compression / extension of the resistor due to inappropriate mounting can lead to change in the resistance value, damage to the resistive element or component failure. In all cases the risk of failure is increased by presence of one or more of the stresses listed below.
Although a thick film resistor is often coated to protect it from moisture and aggressive chemical elements 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 a change in resistance value.
Most mechanical failure modes of thick film resistors are propagated by heat. It is therefore important to understand the heat dissipation properties of the resistor and substrate material. A low power resistor dissipates heat via conduction through its component leads or connections, while a high power resistor dissipates heat through radiation.
When current passes through a resistor it generates heat and the differential thermal expansions of the different material used in the resistor manufacturing process induces stresses in the resistor. Temperature Coefficient of Resistance (TCR) is the best known parameter used to specify a thick film resistor 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.
A continuous over-load of a resistor device degrades the insulation resistance and changes 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. It is therefore important to observe the resistor maximum specified voltage.
The key element in determining the surge survivability of a thick film resistor is the mass of a resistor element, which is directly proportional to its thickness multiplied by its surface area. 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. The increased surface area allows more heat dissipation which is important in power resistor applications.
The final factor contributing factor to a resistor surge capability is how the component is resistor trimmed to establish the final resistance value. The method used for trimming can create weak spots that cause failure under surge conditions.
Damage via ESD is a latent defect that can be difficult to detect. The resistor may be partially degraded by ESD but continue to perform its intended function. However, the chances of premature or catastrophic failure of the resistor device are increased, particularly if the device is exposed to one or more of the stresses listed above.
A resistor may be the lowest cost element in a system but failure can be just as catastrophic as 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 resistor technology and its manufacture can minimise the risks of failure.
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