# For a thick film resistive element does the surface area of electrical contacts affect resistive value?

I am learning about film-based heating elements and they are challenging my understanding of electronics a bit. I am used to thinking about resistors and wires on a schematic and I am not used to thinking about the contact area at the interface between a resistor and a wire.

Generally these heating elements consist of a carbon-based resistive ink that is first printed on a substrate, and after this a silver based higher conductance "wire" is printed over this, forming a circuit. You can see an example of such a circuit in the image below:

What I am wondering is for a given film resistor element, does the amount of contact area of the "electrode elements"/wires affect the resistance value of the element?

If you want some numbers to think about this, the resistance value of the carbon resistor ink is 1700ohm/sq (at 25um) and the rectangular element I'm thinking of might be something like 0.5mm width and 3mm length. If the wire/electrode overlap is 0.1mm, 0.5mm, 1mm, 1.49mm, is the resistance value affected?

• more contact = less resistance May 3, 2019 at 21:16

All that matters is the resulting gap length between conductors in your case since the electrode conductance is orders of magnitude greater.

Your % tolerance of this gap length times % tolerance of carbon conductivity results in your net R tolerance.

However, the width and thickness of the terminals does not affect as long as it is equal to or greater than carbon width.

This illustration of how width does not matter, but rather gap length.

For highly-conductive heater-resistors laser-trimming is used like this.

Yes, the resistance value will be affected. How could it not?

You need to characterize the interface between the carbon-based ink and the electrodes. Current flows vertically through this interface, not horizontally through the sheet resistance of the materials.

• Of course, with a highly-conductive layer over a lower-conductive layer, most of the current through that interface will be concentrated near the edge where they meet. Beyond a certain distance from that edge, additional overlap will have negligible effect. May 3, 2019 at 18:40
• @DaveTweed Sounds reasonable to me. So the resistance would be proportional to the perimeter length, I suppose, to first order. May 3, 2019 at 19:03