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 bulk metal foil resistor technology with thick film.
The bulk metal foil manufacturing process involves attaching a bulk metal foil to a ceramic substrate using an adhesive. The foil generally has a thickness of several micrometers.
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.
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 co-efficient of resistance (TCR).
Power handling capability.
Size, weight and footprint.
Although custom thick film resistor devices produced by specialist manufacturers can deliver long term stability and tolerances comparable to metal foil devices the stability over time, tolerance, TCR, ESD survivability and noise performance of the foil resistor devices is generally significantly better than standard thick film resistor products.
However, thick film technology offers a greater variety of resistance values, significantly improved power handling capability, improved size per unit resistance (smaller size), improved high frequency performance and significantly lower cost.
The limitations of bulk foil resistor in power applications coupled with their excellent stability, TCR and noise performance means they are better suited to precision applications. In contrast, the low cost and high resistance density of thick film make them more suited to a wide range of general applications, particularly higher power applications. Bulk metal foil resistors often compete more with thin film resistor devices than thick film.