Although thick film resistors are often the cheapest components on a system board failure can be just as disastrous as the most expensive component. Specifying the device correctly is crucial to long term performance to specification and to minimize the chances of failure. However, correct specification can be a challenge and often requires measurement of actual circuit values and careful calculation using any known values.
To ensure the resistor performs as required under both steady state and transient conditions requires a partnership between the customer and the chosen supplier. The manufacturer delivers detailed knowledge of materials, design and manufacturing limitations while the customer provides the detailed knowledge of the application.
The customer should define the project requirements including delivery dates and schedules, specified component life, the end application, project quality requirements and prototype and production quantities.
Specific application information may then be delivered including maximum available component area, dimensional constraints, termination type, length of pins and finish and component marking requirements.
With the above in place the manufacturer may consider technical requirements including:
• Operating voltage.
• Peak voltage and transient conditions.
• Operating frequency.
• Instantaneous power or surge.
• Resistor tolerance.
• Nominal resistance
Parameters that are often overlooked but are key to long term performance include the peak voltage, instantaneous power or surge and transient conditions. In most applications the only way to determine these characteristics is via actual circuit measurements or calculation. Failure to specify these parameters correctly will impact on the long term reliable operation of the resistor device.
If the resistor is for a consumer application the long term impact on performance due to environmental factors is often irrelevant. However, in industrial and high reliability application they require consideration if the resistor is expected to continue to perform without performance degradation over the long term. Factors to consider include:
• The environmental temperature.
• The presence of any aggressive elements or gases.
• Temperature co-efficient of expansion.
• Maximum operating temperature.
• Levels of moisture.
Again, many of these parameters can only be obtained via measurement and observation.
To define a resistor to operate in a specialist application can therefore take considerable time and effort. There are many factors to consider that may not be immediately obvious but if overlooked will either result in a change in resistance or component failure. Experience shows that most application specific resistor failures are due to transient conditions or environmental factors that were not considered appropriately at the design stage.