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When designing systems for high-reliability applications, resistor selection is critical. The two main manufacturing approaches are high volume and custom thick film resistor production.

High-volume manufacturing excels at delivering consistent quality at low costs. This is achieved via standardised, highly automated processes.

In contrast, custom thick film resistor manufacturers tailor materials and processes to meet stringent performance specifications. They tend to manufacture only in low to medium volume.

This article contrasts custom versus high-volume thick film resistor manufacturing.

Introduction to Thick Film Resistors

There are many types of resistors. Some are general purpose, while others have power, voltage and environmental limitations.

Thick film resistor technology applications include power, instrumentation and medical electronics. High-reliability applications include military, oil and gas, automotive electronics and sensors.

Thick film resistor technology covers a wide range of resistance values and has a high current-carrying capacity. The technology is stable under a variety of environmental conditions. Thick Film is a robust and relatively low-cost technology.

Comparison of Manufacturing Processes

The first step in the thick film resistor manufacturing process is substrate and resistor material selection.

Substrate Selection

The substrate selection can have a direct impact on the performance and reliability of the final product. The key issues to consider are thermal conductivity, surface smoothness, mechanical strength and electrical insulation. The thermal expansion coefficient of the substrate and the resistor paste should be a close match.

Substrate material selection involves balancing these factors to fit with the application. Cost and availability are always important considerations

Resistor Material Selection

The resistor paste is formulated by mixing conductive materials (e.g., ruthenium oxide, bismuth oxide) with insulating materials (e.g. glasses, ceramics) and organic binders. The paste composition and ratio of conductive to insulating materials determine the desired resistance value.

The choice of conductive and insulating materials determines the electrical and thermal characteristics of the thick film resistor. Ruthenium oxide has high resistivity, excellent TCR and good electrical stability. Bismuth oxide is a lower-cost alternative.

Glass frits and ceramic fillers provide structural integrity and influence the resistor’s power dissipation capability. They also impact resistance drift with temperature. Organic binder selection affects the paste’s printability and substrate attach.

Manufacturing Process

The resistor paste is screen-printed or onto the prepared substrate. Multiple layers can be printed to achieve the desired resistor thickness.

The printed substrates are dried to remove the organic solvents and binders from the paste. The final step is to fire the dried substrates in a controlled atmosphere furnace at high temperatures (>800°C). Firing fuses the conductive and insulating materials and forms a solid resistive film. The firing process also burns off any remaining organic materials

The resistance value can be finalised by selectively removing portions of the resistive film using lasers or abrasion. Terminals or contacts are then added depending on the application. The final stage is to test, package and ship.

High Volume Manufacture

In high-volume manufacturing, the focus is on standardisation. The aim is maximum throughput in the shortest possible time without compromising quality. Once a proven design is available, any changes to materials or equipment setup are avoided wherever possible.

Economies of scale are all important in high-volume manufacturing. The aim is to make the end product suitable for as wide a range of applications as possible to keep the volume high.

Stringent statistical process control and automated testing processes deliver consistent quality across large production volumes.

The prime consideration when selecting materials is cost. The high volumes give the manufacturer maximum bargaining power with material suppliers.

Custom Thick Film Resistor Manufacturing

With custom thick-film resistors, the issues are often the opposite of those in high-volume production. The focus shifts towards matching unique design specifications and performance requirements.

Devices for non-standard applications tend to be used in lower volume. Achieving the specification often requires specialist materials and processes. Combined, these factors make many processes used in high-volume manufacturing impractical.

Automation set-up costs are hard to justify for medium-quantity production runs. Specialist processing, materials and in-process selections are impractical on high-volume lines.

Material cost is often an overriding factor in high-volume manufacturing. Whereas in specialist applications, performance can be more important than cost.

Where process standardisation is paramount in high-volume manufacture, the ability to make fine adjustments to the process is more important for customised resistors.

Cost Implications

In high-volume applications, the costs of establishing and maintaining process controls and automated testing are high. The only way to justify these costs is to push a high volume (hence high value) of resistors through the process in the shortest possible time. Changeover between products, processes and production runs are time-consuming and are avoided.

In custom resistor manufacturing, the focus is on delivering a product to match a specific specification. Hence, this is the opposite of the high-volume approach. The aim is to optimise the process and materials, this can involve trial runs of prototype batches.

High volume throughput can force down material and processing costs. Hence, in general, a product delivered by a high-volume manufacturer will be cheaper than a custom thick film resistor product. However, product cost is not the only consideration.

It is important to consider the overall system cost. A custom thick film resistor could save on the system-level cost. Potential failure rates in service are another issue to review.

In conclusion, when selecting thick film resistors, engineers must consider factors such as lead times, minimum order quantities, and the total cost of ownership.

In some systems, component failure is not an option. The ability to optimise thick film resistor characteristics like high power handling, tight tolerances, stable temperature coefficients, and long-term drift can outweigh economic factors.

The high-volume approach lacks flexibility. The custom approach is often at the expense of higher costs and longer lead times. Lead times can vary significantly. Minimum order quantities also differ, with high-volume manufacturers often requiring larger orders. There is a lot to consider.