Steel-Based Resistors Out-Power Ceramic

Ceramic thick-film resistors, long a staple in electronic applications, rely on a brittle substrate that is vulnerable to cracking or delamination. Bourns, Inc. offers a steel-based alternative for applications that require high power, thermal efficiency, and mechanical robustness.

Ceramic thick-film resistors are reliable until they crack or delaminate, especially as devices shrink and power densities rise. Board flex, vibration, or thermal cycling can undermine their performance and erode reliability, which can lead to latent failures in the field.

Traditional ceramic thick-film resistors are cost-effective and widely available, but their brittle substrates make them less reliable in demanding environments. Stainless steel provides a rigid yet slightly compliant substrate that can absorb mechanical stress from board flex, vibration, and handling during assembly, reducing the risk of cracks or delamination.

Thick film on steel (TFOS) resistors offer a mechanically robust, thermally efficient alternative for demanding high-stress designs where even small amounts of board flex, vibration, or thermal cycling can degrade ceramic-based resistors.

Bourns introduced the first TFOS resistor, the TFOS30-1-150T (Figure 1), in mid-2025. TFOS yields components with exceptional thermal transfer, high power densities, and strong mechanical durability, making them suitable for demanding applications. Many power or high-energy circuits are constrained by how well a component can absorb, dissipate, and survive energy pulses without cracking, drifting, or failing prematurely.

Figure 1: Featuring a stainless-steel substrate, Bourns' TFOS30-1-150T provides more reliability than thick-film ceramic resistors. (Image source: Bourns, Inc.)

Steel substrates provide superior heat spreading, which improves power dissipation and enables higher power density in smaller footprints. A high-integrity dielectric layer is applied to the cleaned stainless-steel substrate to prevent electrical conduction through the steel.

By shifting power handling and ruggedness into the resistor, designers can reduce heatsinking, cut down on part count, and increase field reliability. Simply put, designers can pack more performance into less space without extra thermal hardware, according to Bourns.

During TFOS component manufacture, thick film conductor and resistor patterns are applied to the dielectric layer using a screen-printing process. After each pass, the materials are solidified by firing in a high-temperature furnace to ensure adhesion and robust conductive and resistive paths. Finally, a protective overglaze layer is applied over the conductor and resistor for mechanical protection, environmental resistance, and electrical insulation to the underlying layers.

Premium design considerations

TFOS resistors offer high power and pulse-handling capabilities in a compact, low-profile form factor to maintain performance margins under challenging conditions. This enables engineers to meet stringent reliability and thermal management requirements without compromising form factor.

The TFOS30-1-150T is AEC-Q200 compliant for automotive-grade applications such as battery energy storage systems, motor drives, inverters, fuel cell vehicle sensor boards, and other applications where high power, thermal management, and mechanical robustness are essential.

In an application note on utilizing the component in a fuel cell stack sensor board,^[1] Bourns highlights the TFOS's suitability for such applications due to its ability to handle high power densities. It can accommodate pre-charge and discharge circuits in fuel-cell vehicles and ensure efficient energy management even under variable frequency operations. Its low inductance and tight tolerance ensure accurate voltage, current, and temperature measurement within the fuel cell stack.

Available in a 4.000 in. L x 2.756 in. W (101.60 mm x 70.00 mm) form factor, the TFOS30-1-150T offers customizable termination options including solder pads, push-on connectors, flying leads, and termination cables. Bourns says the flat, sturdy steel substrate can be manufactured in various shapes and sizes up to 406 mm x 406 mm to accommodate custom layouts, or be directly mounted to heat-spreading surfaces. Designers can also specify alternative ohmic values, resistance tolerances, and integration of multiple resistors.

With a resistance value of 150 ohms and a tolerance of ±10%, it is optimized for precision. It boasts power ratings of 260 W when mounted on a heat sink, and up to 900 W with a fan-cooled heat sink, making it suitable for applications requiring substantial energy dissipation. The TFOS30-1-150T operates over an extended temperature range of -55°C to +125°C, and according to Bourns, the TFOS can withstand extremely high element temperatures up to 350°C.

Conclusion

TFOS isn’t a universal replacement for ceramic resistors, but it provides a strategic upgrade path when thermal margins are tight, reliability is paramount, or substrate fragility poses a risk. By rethinking the substrate, Bourns has turned a fundamental passive component into a more durable, thermally capable, and adaptable building block for modern electronics.

Resources

1. Application Note: Effective Monitoring, Protection and Energy Transfer Solution for Fuel Cell Vehicles