AN2869 datasheet(11/43 Pages) STMICROELECTRONICS

11 / 43 page

AN2869

Main capacitive sensing guidelines

Doc ID 15298 Rev 6

11/43

Figure 5.

Flexible PCB (FPC)

Figure 6.

FR4 (2-sided epoxy-fiberglass)

2.2.3

Panel materials

You can choose the panel material which best suits your application. This panel material

MUST NOT be conductive. The material characteristics impact the sensor performance,

particularly the sensitivity.

Dielectric constant

The panel is the main item of the capacitor dielectric between the finger and the electrode.

Its dielectric constant (

R) differentiates a material when it is placed in an electric field. The

propagation of the electric field inside the material is given by this parameter. The higher the

dielectric constant, the better the propagation.

Glass has a higher

R than most plastics (see Table 3: Dielectric constants of common

materials used in a panel construction). Higher numbers mean that the fields will propagate

through more effectively. Thus a 5 mm panel with an

R of 8 will perform similarly in

sensitivity to a 2.5 mm panel with a relative epsilon of 4, all other factors being equal.

A plastic panel up to 10 mm thick is quite usable, depending on key spacing and size. The

circuit sensitivity needs to be adjusted during development to compensate for panel

thickness, dielectric constant and electrode size.

The thicker a given material is, the worse the SNR. For this reason, it is always better to try

and reduce the thickness of the front panel material. Materials with high relative dielectric

constants are also preferable for front panels as they help to increase SNR.

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