AN4312 Datasheet, PDF (9/36 Pages) STMicroelectronics – Guidelines for designing touch sensing applications with surface sensors

AN4312
Main capacitive sensing guidelines
2.2.2
greater than 2 mm diameter). Hence, the way used is to strongly glue them all mechanically
or with a suitable bonding material.
It is possible to construct sensors that do not rely on a substrate. These are described in this
document under separate sections (Section 2.2.7, Section 3.5.3 and Section 3.5.4).
Electrode and interconnection materials
Generally, an electrode is made with the following materials: copper, carbon, silver ink,
Orgacon™ or Indium Tin Oxyde (ITO).
The resistance to electric current of a material is measured in ohm-meters (Ωm). The lower
this degree of resistivity the better, as well as a good RC time constant. That is why
interconnections will be made with low Ωm material. e.g. a printed silver track at 15.9 nΩm
that is 100 mm long, 0.5 mm wide and 0.1 mm thick (so the area is 0.05 mm²) will have a
resistance of 32 µΩ.
About metal deposition, another well-known approach is to consider the Ω/□ ^(a) of a
material. For instance, you can compare silver and ITO (which is about 10 times greater)
and deduce which material is well suited for the connections.
Figure 3. Clear ITO on PET with silver connections
a. Pronounced “Ohms per square” and also called sheet resistance; if you know this constant (given by the
manufacturer) and how many squares are put in series, you can deduce the overall resistance of the line.
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