In this post, we consider the impact of stressors and the passing of time on resistor performance. Thick film resistor ageing is not a concern where resistors operate in relatively benign environments. Long term data from many existing applications shows that resistor ageing is generally not a concern.
However, in more extreme environments such as down-hole, military, space and high specification industrial applications it can be a serious issue. Particularly if the resistor is expected to operate within specified limits over the long term and/or is difficult to access or replace.
One Time Events
One time events such as ESD, surge or pulse or one-time mechanical stressors can all degrade thick film resistor performance. In some cases, they can lead to resistor failure. The impact of these events is excluded from the discussion that follows.
The Impact Of Resistor Ageing
Resistor ageing causes a steady increase in resistance value over time. It impacts the tolerance of the resistor.
With a quantified change in resistance, the system designer may be able to make appropriate allowances. However, as discussed below, the information required to make calculations is either not available or uncertain.
One solution is to build enough safety margin built into the product that stress situations are never a problem.
Thick Film Resistor Ageing – Key Factors
Ageing is influenced by various factors. These include the materials used to manufacture the thick film resistor, the manufacturing process, the resistance value, the applied voltage and the encapsulation or coating.
The key factors that impact thick film resistor ageing are the temperature, the level of humidity and, of course, the passing of time. At ambient temperature, the impact of resistor ageing tends to be negligible.
Calculating the Impact Of Resistor Ageing
In principle, the impact of thick film resistor ageing can be calculated using a version of the Arrhenius equation. However, this assumes a known activation energy EA. The EA value varies depending on the materials used to manufacture the thick film resistor.
Sometimes the EA value is available from the resistor manufacturer. However, it can vary depending on the manufacturing lot and is often an estimate.
It is sometimes (wrongly) assumed resistors do not age after their initial infant period and resistors printed at the same time have the same ageing characteristics.
There are various methods of accelerating the ageing of thick-film resistors. These generally include some combination of heat and humidity. The problem is with a selection of the stress conditions and evaluating their impact.
Without a definitive method to calculate the impact of ageing the system designer can only make the best estimate. The engineering team of specialist thick film resistor manufacturers can often offer some help and guidance.