SLAU857A May   2021  – July 2021 MSP430FR2476 , MSP430FR2512 , MSP430FR2522 , MSP430FR2532 , MSP430FR2533 , MSP430FR2632 , MSP430FR2633 , MSP430FR2672 , MSP430FR2673 , MSP430FR2675 , MSP430FR2676

 

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4.1.1 Cover Layer Design

In the design of capacitive touch, capacitive sensors are placed under the cover layer to reduce environmental impact and prevent ESD problems caused by direct finger contact. It can be seen from Equation 6 that the dielectric constant and thickness of the covering material affect the sensitivity of the capacitive touch. Table 4-1 lists the dielectric constants of different materials.

Table 4-1 Dielectric Constant of Covering Material
Materialεr
Air1.00059
Glass4 - 10
Sapphire glass9 - 11
Mica4 - 8
Nylon3
Plexiglas3.4
Polyethylene2.2
Polystyrene2.56
Polyethylene terephthalate (PET)3.7
FR4 (fiberglass and epoxy)4.2
PMMA (polymethyl methacrylate)2.6 - 4

For panels made from different materials, the touch capacitance can be calculated according to the following equation:

Equation 6. C=εrε0Ad=Aε0∑εridi

Where:

  • A = The contact area between the finger and the sensor pad covering layer.
  • di = The thickness of the different covering layers.
  • ε0 = The dielectric constant of air.
  • εri = Dielectric constants of different covering layers.

Because the dielectric constant of air is approximately 1, it can be seen from Equation 6 that the presence of the variable phase thickens the panel, so air between the sensor and the cover layer should be eliminated as much as possible. In many cases, the sensitivity of the buttons is low because the panel is too thick. Because the conductor interferes with the electric field mode of capacitor charging and discharging, conductive materials and paint or adhesives containing metal particles cannot be used. Typical recommended mixtures are 3M™ 200MP, 467MP, and 468MP.