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16
CapTIvate™ Touch Getting Started Manual
Copyright © 2020, Texas Instruments Incorporated
4.1.2.1
Copper-clad sensor (PCB)
This solution uses copper pads etched on the surface of the PCB as sensors, which is the most common
implementation method for capacitive touch. Rigid PCBs are the most widely used and are suitable for flat front
panels, while flexible PCBs are suitable for curved front panels. It should be noted that the flexible PCB is thinner
than rigid PCB, which means that the distance between the button and the ground plane is relatively short.
Usually, the density of the grid ground plane should be appropriately reduced.
4.1.2.2
Conductive washer/spring type sensor
Figure 4-1 Schematic diagram of spring structure
This solution realizes the expansion of the touch button space through conductive material washers/springs. It is
mainly suitable for situations where the touch front panel cannot be attached to the PCB, and it is also suitable for
curved, inclined or irregular front panel solutions.
4.1.2.3
Electronic ink type sensor
The electronic ink-type sensor uses conductive ink to form a capacitive sensor. Generally, close integration with
the front panel is achieved by spraying or printing. The main features are flexible patterns and shorter design
cycles, which are suitable for irregular front panels. Another application scenario is the ITO liquid crystal display,
which mainly utilizes the transparency characteristics of the ITO film. Since the film resistance of electronic printing
ink is larger than that of copper, the series resistance needs to be appropriately reduced.
4.1.3
Mechanical design checklist
There are three main aspects of the mechanical structure that will affect the touch sensitivity: coating, housing,
and surrounding devices. The unreasonable coating design mainly affects the capacitance change caused by finger
touch. For the self-inductive solution, grounding the shell will improve the noise immunity of the product, but it
will increase the capacitance to ground and reduce the sensitivity. For the mutual-capacitive solution, the
grounding of the surrounding environment has little effect on the sensitivity. As capacitive touch is a sensitive
device and also a source of interference, a grid ground plane is required for protection and isolation. Table 4-2 is a
checklist of mechanical structure design.
Table 4-2 Mechanical structure design checklist
Number
Component
Recommendation
1
Overlay
Material
Avoid the use of conductive materials and conductive paints.
2
Thickness
10mm or thinner, 2-3mm is recommended, depending on the
material and sensor size.
3
Cascade
Avoid gaps between the overlay and the buttons, use non-conductive
adhesive materials.
4
Metal shell
In the case of ensuring sensitivity, the shell should be grounded as
much as possible to improve noise immunity.
5
Surrounding devices
Keep the PCB with button function away from the internal radiation
noise source of the product as far as possible.
Keep the PCB with button function away from sensitive devices as
much as possible.
Keep the PCB with button function as far away as possible from the
PCB with a lot of ground.
