Capacitive Touch Switch Principles and Design Guidelines

Capacitive touch keys are widely used in household appliances and consumer electronics markets, offering several major advantages:

1. No mechanical components, resulting in a longer lifespan.
2. Non-contact sensing, eliminating the need for openings in the panel.
3. Enhanced aesthetic and simplicity in product design.
4. Excellent waterproof capabilities.

This article briefly introduces the principles of capacitive keys and considerations for PCB design.

Principle: Capacitive touch sensing works by measuring changes in capacitance between the panel and the surrounding environment to detect touch events. When a human finger touches, it creates a certain level of capacitance, forming one pole of the capacitor, while the other pole is the copper foil of the PCB. An insulating medium is added between these two layers, effectively creating a capacitor, as illustrated in the diagram below：

The touch sensor circuit can measure the total capacitance on the touch pin pathway. When the total capacitance on this pathway changes beyond a certain threshold, it is determined as a “finger touch”. The following diagram shows the distribution of capacitance along the circuit board pathway：

Capacitance Composition / Clarification

Cgroung  : The capacitance between the touch reference ground and the main ground.

Ccomponet ：Parasitic capacitance inside the chip.

Ctrace  ： The capacitance between the trace and the circuit reference ground.

Celectrode  ： The capacitance between the touch electrode and the circuit reference ground.

Ctouch  ： The capacitance formed between the finger and the touch electrode relative to the main ground.

The readings of a touch sensor result from the combined effect of all the capacitances listed above. Ccmponet, Ctrace, and Celecrode are often referred to as parasitic capacitance C (i.e., the capacitance when no touch action occurs). Ctouch is the capacitance that occurs during a touch action. When the parasitic capacitance Cp is smaller, and Ctouch is larger, the touch action is more easily detected by the system, meaning higher sensitivity.

Conclusion: The most important aspect of designing capacitive touch keys is to reduce parasitic capacitance Cp while increasing touch capacitance Ctouch. Methods to reduce parasitic capacitance Cp include:

1. Shortening the length of the traces.
2. Optimizing the PCB layout.

Methods to increase touch capacitance Ctouch include:

1. Reducing the thickness of the circuit board and the panel, or the cover layer.
2. Ensuring a close fit between the cover layer and the capacitors, and choosing a cover layer with a high dielectric constant.
3. Increasing the area of the touch electrodes.

Choice of Cover Layer: The formula for calculating the capacitance of the capacitor plate is:

C_touch = ε * S / (4 * π * k * d)

ε: Dielectric constant of the cover layer

S: The mapped area of contact between the finger and the touch electrode through the cover layer k: Electrostatic constant

d: Thickness of the cover layer

Dielectric constants of different materials: The dielectric constant of air is the lowest. To ensure high sensitivity, materials with a higher dielectric constant should be used.

Material	Dielectric Constant (ε)	Withstand Voltage (V/mm)
Air	1.0	1200-2800
Teflon®	4.6 – 4.9	18000
Silicon Rubber (High Temp)	7.6 – 8.0	7900
Silicon Rubber (Room Temp)	6.0	13000
PET	3.2	280000
Porcelain	2.9 – 3.0	16000
Polypropylene	2.8	13000
ABS	2.4 – 4.1	16000
Non-woven Fabric	1.2 – 2.5	3900
Paper (High Temp)	2.5 – 6.0	–

When installing the cover layer, it is important to note: When the cover layer is mounted closely, an insulating adhesive can be used for bonding to eliminate gaps and ensure stability. When there is a distance, metal springs can be used for connection, and the springs should maintain their deformation.

PCB Design Requirements

Factors influencing parasitic capacitance: trace length > ground plane around the touch electrode > spacing between the trace and ground > number of vias > trace width. It is recommended that the PCB thickness is between 0.5mm and 1.6mm. If the board thickness is less than 1mm, the ground plane on the reverse side should be reduced.

Trace requirements:

• Trace length should not exceed 300mm.
• Trace width (W) should not be greater than 0.18mm (7mil).
• Trace corner (R) should not be less than 90°.
• The spacing between the trace and ground (S) should be between 0.5mm and 1mm.
• The gap between the touch electrode and ground should be between 1mm and 2mm.
• The diameter of the touch electrode (D) should be between 8mm and 15mm.
• Traces should not be used in parallel with high-frequency interference lines.
• The touch electrode and traces should be surrounded by a grid ground.

Ground Plane Treatment

• Using a grid ground is a compromise to enhance anti-interference capability while ensuring high sensitivity.
• A grid ground should be used around the touch electrode; there should be no ground plane on the back side where there is no interference.
• The ground plane should be at least 1mm away from the electrode.
• The total area of the grid ground should not exceed 40% of the total area; a 5mil line width and 50mil spacing can be set.
• There should be no solid ground within a 10mm radius around the touch electrode and traces.

Key Pad Shape

The optimal shape is circular, but rounded rectangular pads can also be used. Sharp points, which concentrate the electromagnetic field, should be avoided. The key diameter should be between 8mm and 15mm, with the typical value being 12mm. If the cover layer is thick, a larger electrode diameter may be chosen. The spacing between two adjacent keys should be at least 5mm to avoid mutual interference.

Design for Sliders/Rollers

Combining multiple touch keys together forms a slider. Accompanied by software, it can detect the direction and position of the slide movement. The slider can be arranged in a straight line or a circular pattern.

The human finger has an approximate diameter of 9mm; therefore, it is recommended that the slider width (W) be 8mm, with a spacing of 0.5mm between sliders. As the finger slides along the slider, the signal change on two adjacent electrodes is opposite: the electrode where the signal strengthens indicates the position the finger is moving towards, and the electrode where the signal weakens indicates the position the finger is leaving from. Therefore, the distance between two adjacent touch electrodes should not be too large, otherwise, it would not be possible to accurately calculate the finger’s position.

Parameter	Minimum Value	Maximum Value	Typical Value
Width (W)	4	–	8
Height (H)	8	15	12
Spacing (A)	0.5	2	0.5
Spacing to Ground (S)	1	2	Recommended Value

Touch Spring Design

Many designs use springs to connect the electrode with the cover layer. When choosing a spring, the following should be noted:

• The spring installation height should be greater than 5mm.
• The spring diameter should not be less than 10mm.
• The order of spring sensitivity performance is metal strip-shaped > bell-shaped > straight barrel-shaped.
• The distance between adjacent spring keys should not be less than 10mm.

Contact WeGlow immediately for a variety of cost-effective capacitive touch switch solutions. You can also learn more about WeGlow’s capacitive touch switch products.