Thick film resistor failure is rarely caused by resistive element failure, but is usually due to external environmental factors such as mechanical and electrical stresses and handling problems. Failures can be classified as either a performance degradation or a complete failure (usually as an open circuit rather than a short circuit).

Six common causes of thick film resistor failure are:

thermal problems

mechanical stress

constant overload

Increase

Environmental – metal migration

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MECHANICAL FAILURES

Aside from handling damage that results in cracks and chips in the substrate, most mechanical damage is caused by vibration or improper mounting of the device. Microcracks in the resistor material caused by vibration or compression/extension of the resistor due to improper mounting can lead to changes in the resistor value, damage to the resistor element, or component failure. In all cases, the risk of failure is increased by the presence of one or more of the stresses listed below.

ENVIRONMENTAL FACTORS

Although a thick film resistor is often coated to protect it from moisture and harsh chemicals, environmental factors such as humidity and contamination still require careful consideration. Both can cause metal migration between the resistor terminals, which can lead to a short circuit or a change in resistor value.

THERMAL PROBLEMS

Most of the mechanical failure modes of thick film resistors are propagated by heat. Therefore, it is important to understand the heat dissipation properties of the resistor and the substrate material. A low power resistor dissipates heat by conduction through its components or connections, while a high power resistor dissipates heat by radiation.

When current passes through a resistor, it generates heat, and the differential thermal expansions of the different materials used in the resistor manufacturing process induce stresses in the resistor. The temperature coefficient of resistance (TCR) is the best known parameter used to specify the stability of a thick film resistor and defines the sensitivity of the resistive element to temperature change. The Power Coefficient of Resistance (PCR) quantifies the change in resistance due to self-heating when power is applied and is particularly important for resistors used in power applications.

OVERLOAD CONDITIONS

A continuous overload of a resistance device degrades the insulation resistance and changes resistance parameters over time. Voltage stress can cause conduction of normally non-conductive materials in the resistor film, leading to deterioration and sometimes failure due to hot spots. Therefore, it is important to observe the maximum specified voltage of the resistor.

SURF CONDITIONS

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 times its surface area. The geometry of a resistor also affects its ability to withstand surges. A larger surface area results in a higher film mass and ultimately improved surge performance. The increased surface area allows for greater heat dissipation, which is important in power resistor applications.

The final factor that contributes to a resistor’s surge capability is how the component’s resistance is adjusted to establish the final resistance value. The method used to clip can create weak points that cause failure under overvoltage conditions.

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Damage via ESD is a latent defect that can be difficult to detect. The resistor can be partially degraded by ESD but continue to perform its intended function. However, the chances of premature or catastrophic failure of the resistive device are increased, particularly if the device is exposed to one or more of the stresses listed above.

CONCLUSION

A resistor may be the lowest cost element in a system, but its failure can be just as catastrophic as the failure of any other element in the system. Therefore, it is important to understand the possible failure modes and how they can be addressed. A partnership with a specialist manufacturer with long-term experience in thick film resistor technology and manufacturing can minimize the risks of failure.