The bulk metal foil resistor was first developed by Dr Felix Zandman (who went on to found Vishay) in the early 1960’s and is now a mainstream resistor technology. In this post, we compare the performance of thick film resistors with bulk metal foil.
Construction Comparison
The bulk metal foil resistor manufacturing process involves attaching a metal foil, with a thickness of several micrometres, to a ceramic substrate using an adhesive. A photoetching process is then used to form the resistor pattern. The final step is to adjust the resistor value and tolerance using a laser.
Thick film resistors are manufactured by printing a paste containing a glassy frit carrier, metal oxides and a binder onto a substrate (usually Alumina). The paste is then fired at high temperature. 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.
Performance Considerations
The key parameters to consider when choosing a resistor device are:
- The resistor value.
- Tolerance – both initial and end of life.
- Stability over time.
- Voltage co-efficient of resistance (VCR).
- Temperature coefficient of resistance (TCR).
- Power handling capability.
- Pulse handling and ESD immunity.
- Noise.
- Size, weight and footprint.
- Cost.
Resistor Value: A resistor’s materials and construction directly impact its resistance value. Thick film resistors are available in the milliOhm to megaOhm range, with gigaOhm variants available from specialist manufacturers . In contrast, Bulk Metal Foil resistor values are in the milliOhm to the low hundreds of kiloOhm range
Tolerance: Thick film resistor tolerance tends to be in the ±1% to ±5% range; tighter tolerance values are available, but there are technical trade-offs to consider. Bulk metal foil resistors remain stable with temperature changes and ageing and offer tolerances in the 0.1% range
Resistor stability over time: Thick film resistors deliver adequate long-term stability for general applications, particularly where periodic calibration or trimming is possible. If resistance drift over time (especially load-life drift under applied power and elevated temperature) is a key consideration, bulk metal foil resistors tend to be the better choice.
TCR and VCR: Thick film resistors commonly have TCR in the hundreds of ppm/°C. Bulk metal foil parts have an order of magnitude lower TCR in the ±2 to ±25 ppm/°C range, making them preferred where temperature-driven gain/offset error must be minimised.
Metal foil resistors generally exhibit negligible VCR and therefore have better linearity in precision and voltage divider applications. Thick film resistor VCR depends on the resistor construction and its resistance value, but is generally an order of magnitude higher.
Power handling capability: Here, thick film resistors are the clear winner. Bulk metal foil resistors are more often optimised and selected for precision, so when power handling is the primary issue, thick film resistors families offer a wider choice and better cost-per-watt.
Pulse handling and ESD immunity: In environments where pulse and surge events are likely, thick film (particularly surge-rated series) is the best choice. Bulk metal foil is primarily selected for precision and may fail or drift permanently under pulses that a surge-rated thick film part would tolerate.
Noise: Bulk metal foil devices are low-noise devices, and in precision applications where noise matters, they are the clear winner.
Component size: For a given power rating, physical size is driven by thermal dissipation. Thick film tends to achieve very high resistance values in smaller footprints, while bulk metal foil is more commonly found in larger packages when precision/stability requirements dominate.
Cost: Here, thick film resistors are the clear winner. Bulk metal foil resistors are only economical when precision performance eliminates larger system costs such as calibration, drift or measurement uncertainty.
Conclusion
Bulk metal foil resistors excel in applications where precision and long-term stability are critical. Their main competition in such applications is thin-film resistor devices because the target requirements and the price vs performance tradeoffs are closer than thick-film.
In contrast, the low cost and high resistance density of thick film make them suitable for a wide range of general applications, particularly higher power applications. Thick film resistors are often the default choice for general-purpose and many higher power applications where absolute stability and noise are not as critical.