Application Specific Power Resistor Manufacturing Limitations

When considering a custom resistor for any demanding application it is important to recognise resistor manufacturing limitations that may force design compromises. Unfortunately, no resistor is perfect, including those designed specifically to address the demands of a particular application.

The relatively low cost vs performance of Thick Film Resistor technology makes it an ideal choice for most application specific requirements. The technology offers high packaging density, a robust construction, excellent heat dissipation and low inductance.

Causes Of Power Resistor Failure
A number of stresses can contribute to power resistor failure, they include:
  • Thermal stresses.

  • Electrical Surge & ESD.

  • Mechanical stresses.

It is vital the power resistor manufacturing process does not leave the resistor device more prone to damage by one, or more, of these stresses. Minor mechanical defects can make the resistor less resistant to mechanical damage. Track thinning (hot spots) due to inappropriate track abrasion can contribute to failure under surge conditions and poor choice of materials and dimensions can result in thermal performance issues.

The Resistor Manufacturing Process

A resistive paste consisting of a mixture of metal oxides, a carrier and a binder is applied to a substrate (usually Alumina) and then fired at high temperature (typically 850°C). During firing the carrier material burns off, the metal oxides combine to form the resistor film and the glassy frit binder melts to hold the resistor material in place. Resistive layers may then be added sequentially to create the required resistance pattern and value.

The resistance value is determined by the point to point contact between the granular film of metal oxides. To determine the final resistance value (trim) the final resistor pattern is often abraded.

Resistor Manufacturing Compromises

It is important the resistor manufacturing process is designed to ensure the continual firing process at high temperature does not build in potential defects to the resistor device that will be exacerbated by high temperatures or temperature variations in the final application.

While there may often be a demand to minimise the size of power resistor device this must not compromise heat dissipation or the resistive element may be damaged. An appropriate choice of substrate material may maximise heat dissipation and minimise component size but this must be weighed against material cost and complications in the manufacturing process.

Designing 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 (both size and material) is important to ensure the pulse energy can be dissipated. Optimising the thick film firing process can also improve the surge and ESD survivability of the high power resistor component.

Susceptibility to mechanical stresses can be minimised by choosing appropriate materials and minimising the stresses involved in the manufacturing process. An experienced thick film power resistor designer will know how far it is possible to push materials and processes to meet demanding application specifications without compromising long term, reliable power resistor operation.