Failure Mechanisms in a Thick Film Power Resistor
Thick film power resistor product may be used in a wide range of applications from power supplies to braking systems in trains and mass transit. In general thick film resistor product is very reliable with low levels of failure. However it is important to be aware of potential failure modes so that environmental factors affecting reliability can be managed and resistor design modified to cope with any application specific issues.
There are three main ways a thick film power resistor can fail as follows:
€¬ A permanent change in resistance.
€¬ Disconnection / open circuit
€¬ short-circuit.
A description of the causes and effects of each failure mechanism follows.
Change in Resistance
Some changes in resistance are predictable and specified in the device datasheet. The designer must take account of electrical conditions (example - pulse load) and the environment (example - ambient temperature) in which the resistor is used. However, some environmental issues are less predictable and can have a major impact on the resistance value. Environmental issues include PCB contamination causing electrolytic migration and a permanent change in resistance value and ESD damage to the resistor.
Open Circuit
Mechanical or electrical overstress of a power resistor component can cause catastrophic failure and an open circuit. Constant vibration or mechanical shock may fracture ceramic substrates and ultimately the resistor material whereas continues surge conditions or over current may burn out conductor tracks. A resistor must be able to dissipate the heat it creates or ultimately burn out will occur. Also, the resistor device must be as closely CTE matched as possible to the substrate or this can lead to stress and mechanical failure. More subtle failure modes may be caused by chemical interactions that degrade silver in resistor terminals ultimately leading to open circuit conditions.
Short Circuit
Short circuit conditions in a power resistor are less common than open circuit or parameter change issues. The main cause is silver migration where silver is deposited ultimately causing a short circuit. Using appropriate metallurgy the impact of silver migration may be reduced or eliminated.
It is important to select the right thick film power resistor for the application and take appropriate advice on the resistor design if the threat of failure is to be reduced. Environmental, electrical and mechanical issues must be considered as all can have an impact. Power resistor devices may be comparatively cheap when compared with other devices on a PCB but their failure can cause damage to other, much more expensive, components.
There are three main ways a thick film power resistor can fail as follows:
€¬ A permanent change in resistance.
€¬ Disconnection / open circuit
€¬ short-circuit.
A description of the causes and effects of each failure mechanism follows.
Change in Resistance
Some changes in resistance are predictable and specified in the device datasheet. The designer must take account of electrical conditions (example - pulse load) and the environment (example - ambient temperature) in which the resistor is used. However, some environmental issues are less predictable and can have a major impact on the resistance value. Environmental issues include PCB contamination causing electrolytic migration and a permanent change in resistance value and ESD damage to the resistor.
Open Circuit
Mechanical or electrical overstress of a power resistor component can cause catastrophic failure and an open circuit. Constant vibration or mechanical shock may fracture ceramic substrates and ultimately the resistor material whereas continues surge conditions or over current may burn out conductor tracks. A resistor must be able to dissipate the heat it creates or ultimately burn out will occur. Also, the resistor device must be as closely CTE matched as possible to the substrate or this can lead to stress and mechanical failure. More subtle failure modes may be caused by chemical interactions that degrade silver in resistor terminals ultimately leading to open circuit conditions.
Short Circuit
Short circuit conditions in a power resistor are less common than open circuit or parameter change issues. The main cause is silver migration where silver is deposited ultimately causing a short circuit. Using appropriate metallurgy the impact of silver migration may be reduced or eliminated.
It is important to select the right thick film power resistor for the application and take appropriate advice on the resistor design if the threat of failure is to be reduced. Environmental, electrical and mechanical issues must be considered as all can have an impact. Power resistor devices may be comparatively cheap when compared with other devices on a PCB but their failure can cause damage to other, much more expensive, components.
