Revolutionizing Touch Sensing: Harnessing the Power of Custom PCB Springs

Building capacitive sensors using touch springs is a robust alternative to traditional mechanical switches. However, certain applications cannot directly place printed circuit boards (PCBs) under the overlay layer or connect them to the device’s housing. These applications include household appliances like stoves, washing machines, refrigerators, and microwaves, as well as various vehicle electronic devices such as radios, TV tuners, control panels, and passenger detection systems on seats.


By utilizing touch springs for touch sensing, the PCB can be positioned away from the housing, making backlighting easier to implement. The principle is similar to directly touching the PCB, where the touch spring forms a capacitive sensor. By measuring changes in capacitance, the touch button controls the on/off signal switch.


Theoretically, the shorter the distance between the touch springs on the PCB and the user’s hand, the less intermediate medium is involved, resulting in better tactile feedback.


Advantages of touch springs compared to PCB touch:


Provides a gap between the PCB and the housing.

Maintains good functionality even under vibration conditions due to the spring’s elasticity (e.g., in cars and washing machines).

Particularly suitable for thicker housings.

Offers a more linear sliding touch experience.

Facilitates backlight design.



The following design considerations should be taken into account:


The assembled spring needs to be preloaded between the PCB and the panel, typically around 3-7mm, depending on the size of the product. This minimizes the impact of vibration and temperature variations on its reliability.

Following ergonomic principles, the key shape should be circular with a diameter of 10-15mm, matching the spacing between human fingers. Going beyond the touch-sensitive area of the spring renders it ineffective. The key spacing should be around 18-22mm, as smaller spacing can lead to easily mistaken inputs.

The working height of the spring should be greater than 5mm, as lower heights would result in suboptimal performance.

Thinner panel and front shell thicknesses are advantageous for key functionality while ensuring flatness and preventing deformation.

Transparent/translucent effects are highlights of touch buttons. If an illuminated effect is desired upon power-on, a backlight source should be added to the backside. Surface-mount LEDs can be used to side-illuminate a 2-3mm thick PMMA substrate, which is generally displayed as a whole. The only drawback is that the sensitivity of touch may be slightly reduced, as the user’s hand needs to transmit through the panel, front shell, and backlight board to reach the touch spring. (If the housing is thicker, the spring needs to have a larger outer diameter, for example, if the housing is 4mm thick, the spring would need an outer diameter of approximately 10mm.)

The wire diameter of the spring ranges from 0.3mm to 1.5mm, and it does not have a significant impact on touch sensitivity.

If individual keys require individual backlighting, it is necessary to provide light shielding for each illuminated key to prevent light interference with other keys.

touch spring

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