AN2869 datasheet(9/43 Pages) STMICROELECTRONICS

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AN2869

Main capacitive sensing guidelines

Doc ID 15298 Rev 6

9/43

It is not recommended to directly set the substrate against the front panel without gluing it by

pressure or by bonding. Some moisture or air bubbles can appear between them and cause

a change on the sensitivity. Indeed, if the substrate and the panel are closely linked together

this will avoid a varying sensitivity loss which is hard to predict (when the air bubbles are

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 Chapter 4.4.1).

2.2.2

Electrode and interconnection materials

Generally, an electrode is made with the following materials: copper, carbon, silver ink,

OrgaconTM 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 µ

Ω.

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

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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