When choosing a high power resistor for any demanding application Thick Film resistor technology offers high performance at a relatively low cost. The technology delivers high packaging density, excellent heat dissipation and low inductance.
However, no high power resistor is perfect. It is important to understand resistor manufacturing limitations and their impact on the design process. To ensure a high power resistor continues to operate in extreme environments with minimal performance degradation over it’s entire in service life requires a detailed knowledge of potential failure modes including:
Mechanical stresses and environmental factors.
It is vital the power resistor manufacturing process does not leave the resistor device more prone to damage by one, or more, external stresses. Minor mechanical defects can make the resistor less resistant to mechanical damage. Track abrasion can cause track thinning (hot spots) and lead to failure under surge conditions. A poor choice of materials can result in thermal performance issues.
There are two components of heat management the system designer must manage. Heat in the high power resistor must be dissipated without impacting on nearby temperature
sensitive electronic components. Thick Film Resistors can reach temperatures which exceed safe operation within seconds. It is therefore important to keep the operating temperature range within maximum specified limits – typically 50 – 60C.
Heat is generated in the high power resistor as current passes through the resistor element. This heat must be dissipated to prevent damage to the resistor materials. At the power resistor component level, the classic solution is to manufacture a resistor with sufficient mass to dissipate the heat. However, this solution can compromise component packaging density at the system level.
The solution is to use a appropriate combination of materials with superior thermal properties. But high performance materials all come at a cost. At the system level, cooling using heat sinks, forced air cooling, water cooling and heat pipes, either individually or in combination, to keep the operating temperature range within specified limits.
Heatsinks are sometimes used in isolation but require excellent thermal bonds between the resistor element and heatsink to avoid thermal fractures of the resistor. Choosing an appropriate resistor material may also improve thermal performance.
While small package size is desirable this must not compromise heat dissipation of the resistive element. Choosing a specialist substrate material may maximise heat dissipation and minimise component size but this must be weighed against material cost and potential complications in the manufacturing process
Surge And ESD Solutions
To survive a surge condition a high power resistor must be able to dissipate the surge energy. Design for surge conditions involves choosing appropriate dimensions for the resistive element and selection of the best (performance vs cost) resistive material.
Selection of the substrate (size and material) is important to ensure its mass can dissipate the pulse energy. Optimising the thick film firing process can also improve the surge and ESD survivability of the resistor component.
Mechanical And Environmental Issues
Susceptibility to mechanical damage can be minimised by appropriate material selection and minimising stresses in the manufacturing process. An experienced resistor
designer will know the limits of materials and processes and how to meet demanding application specifications without compromising long term, reliable resistor operation.
If coated with a suitable barrier material Thick Film resistor technology is resistant to moisture and most common chemical elements. However, inappropriate handling can damage the substrate material and/or the interface with the system board. Mechanical and environmental issues can be mitigated to a degree but not eliminated.
A controlled manufacturing process that minimises minor defects in the resistor layer, pinholes and resistor track thinning resulting from the final resistor trimming process can reduce the risk of failure. However, to ensure safe and reliable long term operation the system designer has a responsibility to keep operational power dissipation and temperature within specified limits.