The most common cause of power resistor ESD damage is a direct transfer of an electric charge from either a human body or a charged material to the power resistor device. ESD damage may be divided into three main categories
1. Parametric failure
The ESD event alters the resistance of the power resistor device causing it to shift outside its specified tolerance
2. Catastrophic Damage
The ESD event causes the device to immediately stop functioning. In some cases, this failure may be the cumulative effect of several ESD events.
3. Latent Damage
The ESD event causes some damage to the power resistor but insufficient to cause complete failure. The device may continue to function as specified but the load life of the device may be significantly reduced with each ESD event.
Thick film resistors almost always experience negative resistance changes when subjected to ESD. The damage caused depends on the power resistors ability to dissipate energy. Power resistor ESD sensitivity is therefore directly related to the dimensions of the resistor as the smaller the size the less area is available to spread the energy delivered by the ESD event. Energy concentration in a small area of a resistors active element generates heat which can lead to irreversible damage.
The best method to protect a power resistor is to establish systems and processes to reduce the probability an ESD event can occur. However, if it is not possible to eliminate all ESD risks working with a specialist thick film resistor manufacturer can minimise the risks of ESD damage.
Design and manufacturing options include:
· Improvements to the thick film paste – The amount of ESD damage is directly related to the conductive mechanism of the resistive material, which is in turn related to the composition of that material. The choice of thick film paste can, therefore, influence the ESD performance of the resistor device.
· Changes to the track profile – Modifications (widening) to the track profile will improve the ESD performance of the device hence an element of ESD survivability can be designed in if the nature of the ESD threat is known.
· Firing temperatures and profile – Firing is one of the most critical elements of the thick film process. Optimising the process can have a direct effect on the power resistor ESD sensitivity.
Wherever possible action should be taken to reduce the possibility of an ESD event that may cause a thick film power resistor device to fail or (perhaps worse still) to continue to function but with a degraded performance. However, in some applications, the potential for an ESD event cannot be removed entirely. Where the level of threat is known thick film power resistor devices may be designed that will survive an ESD event by modifying the substrate and paste materials, the track profile and /or the firing temperature.