www.ti.com
CapTIvate™ Touch Getting Started Manual
15
Copyright © 2021, Texas Instruments Incorporated
4
Mechanical structure and hardware design
As the first step of capacitive touch design, the mechanical structure and hardware design greatly affect the
sensitivity and noise immunity of the capacitive touch solution. Therefore, before reading this chapter, it is
recommended to read the first chapter to understand the basic principles of capacitive touch in order to better
understand this chapter. In general, the mechanical structure and hardware design are mainly trade-offs between
structural constraints, sensitivity, and noise immunity. This means that it is often necessary to repeatedly modify
and compromise based on test feedback.
4.1
Mechanical structure design
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 formulas 1-3 that the
dielectric constant and thickness of the covering material will affect the sensitivity of the capacitive touch. The
dielectric constants of different materials are shown in Table 4-1.
Table 4-1 Dielectric constant of covering material
Material
𝜀
𝑟
Air
1.00059
Glass
4 - 10
Sapphire glass
9 - 11
Mica
4 - 8
Nylon
3
Plexiglas
3.4
Polyethylene
2.2
Polystyrene
2.56
Polyethylene terephthalate (PET)
3.7
FR4 (Fibe Epoxy)
4.2
PMMA (polymethyl methacrylate)
2.6 - 4
For panels stacked from different materials, the touch capacitance can be calculated equivalently according to the
following formula:
𝐶 = 𝜀
𝑟
𝜀
0
𝐴
𝑑
= 𝐴𝜀
0
∑
𝜀
𝑟𝑖
𝑑
𝑖
(4-1)
A: The contact area between the finger and the sensor pad covering layer.
𝑑
𝑖
: The thickness of the different covering layers.
ε
0
: The dielectric constant of air.
𝜀
𝑟𝑖
: Dielectric constants of different covering layers.
Since the dielectric constant of air is only about 1, it can be seen from the formula 4-1 that the presence of the
variable phase thickens the panel, so the air between the sensor and the cover layer should be eliminated as much
as possible. In addition, in many cases, the sensitivity of the buttons is low, mainly because the panel is too thick.
Because the conductor will interfere 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.
4.1.2
Sensor structure selection
According to actual application requirements, various materials can be used to construct capacitive sensors. The
following will briefly introduce the common sensor structure.
