Ceramic thick film resistors have long been a mainstay in electronic applications, but they rely on brittle substrates that are prone to cracking or delamination. In view of this, Bourns, Inc. provides a steel based alternative for applications that require high power, thermal efficiency, and mechanical robustness.
Ceramic thick film resistors are reliable before cracking or delamination occurs, but the risk of cracking or delamination increases significantly as the equipment shrinks and power density increases. Bending, vibration, or thermal cycling of circuit boards can damage their performance and reliability, leading to potential on-site failures.
Traditional ceramic thick film resistors are low-cost and widely available, but their substrates are brittle and have low reliability in harsh environments. Stainless steel provides a hard but slightly compliant substrate that can absorb mechanical stresses generated by bending, vibration, and handling of circuit boards during assembly, reducing the risk of cracking or delamination.
Steel based thick film (TFOS) resistors provide a mechanically robust and thermally efficient alternative to demanding high stress designs, where even small amounts of circuit board bending, vibration, or thermal cycling can cause a decrease in ceramic resistor performance.
Bourns launched its first TFOS resistor TFOS30-150T in mid-2025 (Figure 1). Components produced with TFOS have excellent thermal conductivity, high power density, and strong mechanical durability, making them suitable for demanding applications. Many power or high-energy circuits have limitations on the ability of components to absorb, dissipate, and withstand energy pulses, in order to prevent cracking, drift, or premature failure.
Figure 1: Bourns' TFOS30-150T uses a stainless steel substrate, which is more reliable than thick film ceramic resistors. (Image source: Bourns Corporation)
Steel substrates have excellent heat dissipation performance, which can improve power dissipation and achieve higher power density in smaller packages. Apply a high integrity dielectric layer on the cleaned stainless steel substrate to prevent electrical energy from passing through the steel.
By transferring power processing and robustness to resistors, designers can reduce the use of heat sinks, decrease the number of parts, and improve on-site reliability. In short, according to Bourns, designers can achieve higher performance in a smaller space without the need for additional cooling hardware.
In the manufacturing process of TFOS components, thick film conductors and resistance patterns are drawn on the dielectric layer using screen printing technology. After each pass, the material needs to be fired and solidified in a high-temperature furnace to ensure adhesion and a strong conductive and resistive path. Finally, cover the conductor and resistor with a protective glaze layer to provide mechanical protection, environmental resistance, and electrical insulation from the underlying layer.
Advanced design considerations
TFOS resistors have high power and pulse processing capabilities, compact and small in size, and can maintain performance advantages under harsh conditions. This enables engineers to meet strict reliability and thermal management requirements without affecting the external dimensions.
TFOS30-1-150T complies with AEC-Q200 standards and is suitable for automotive grade applications such as battery energy storage systems, motor drives, inverters, fuel cell vehicle sensor boards, and other applications that require high power, thermal management, and mechanical robustness.
Bourns emphasized in an application note [1] on the use of this component in fuel cell stack sensor boards that TFOS is highly suitable for such applications due to its ability to handle high power densities. It can adapt to the pre charging and discharging circuits of fuel cell vehicles, ensuring efficient energy management even under variable frequency operation. Its low inductance and strict tolerance ensure accurate measurement of voltage, current, and temperature inside the fuel cell stack.
The TFOS30-1-150T measures 4.000 inches long x 2.756 inches wide (101.60 mm x 70.00 mm) and offers customizable termination options including solder pads, push in connectors, suspension leads, and termination cables. Bourns claims that this flat and sturdy steel substrate can be manufactured in various shapes and sizes, with a maximum size of 406 mm x 406 mm, and can be adapted to various customized layouts or directly installed on heat dissipation surfaces. Designers can also specify other ohm values, resistance tolerances, and integrate multiple resistors.
Its resistance is 150 ohms, with a tolerance of ± 10%, and precision optimization has been carried out. When installed on the radiator, its rated power is 260 W, while when using a fan to cool the radiator, the rated power can reach 900 W, making it suitable for applications that require a large amount of heat dissipation. TFOS30-1-150T has an extended operating temperature range of -55 ° C to+125 ° C. According to Bourns, TFOS can withstand extremely high component temperatures up to 350 ° C.