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Develop touch electrodes for non-uniform surfaces

Posted: 10 Oct 2013 ?? ?Print Version ?Bookmark and Share

Keywords:touch detection? non-uniform surfaces? discrete keys? 2D trackpads? PCB?

In these types of applications, the designer can remove the air gap by matching the curved surface. This is accomplished by fixing a flexible PCB (FPC) to the curved surface with double-sided adhesive tape. The designer may choose to create a complete FPC module, which includes the touch controller as well as the sense electrodes. Alternatively, only the sense electrodes could be placed on the FPC, which can connect to the controller through a cable connector or FPC tail.

Care should be taken when designing for 3D surfaces (curves in two or more directions C figure 5). Fitting FPC to 3D curved surfaces may require cutouts to fit smoothly without creating air gaps (more details to follow in the second article). In cases where there are large curves and a risk of air gaps appearing, the simplest option would be to use a fixed PCB with spring contacts.

Figure 4: FPC with self-capacitive touch electrodes and FPC tail.

Figure 5: Illustration of a 3D curved surface.

An alternative is to keep the PCB area flat (to enable the use of normal flat PCBs with no air gap to the overlay), and only have the curve on the other side, where the user will interact. This could also ease the manufacturing process, especially for more difficult materials such as wood overlays. This does create a non-uniform overlay thickness, and can be compensated for in electrode design. Different electrode designs are discussed later in this article.

The designer would do well to remember that thin FR4 PCBs (0.1 mm to 0.2 mm) provide a more cost-effective solution to FPC in applications where the required curve in the PCB is not too large.

When placing components on flexible PCBs, Care must be taken to place them in areas that will not be bent. This will reduce the risk of component failure such as capacitor cracking.

A variation of standard FPC is to use ITO or PEDOT films for transparent overlays requiring transparent electrodes for backlighting purposes.

How to compensate for curved surfaces
In some applications, it is not always possible to keep a constant thickness in the overlay material. This may be due to injection moulding of plastic parts to conform to a designed shape and stiffness, or the requirement for uniform backlighting to accommodate light diffusion layers.

Figure 6: Fitting a flat PCB to an overlay with a curved exterior creates a non-uniform thickness. Electrode layout can compensate for this.

Electrode shapes for thicker overlays
Although capacitive touch sensors from Azoteq are fully customisable for sensitivity settings on each channel, as well as having adjustable detection thresholds for set sensitivity selections, the designer could save time in fine tuning the sensitivity of each electrode by compensating for variations in the thickness of the overlay material in the electrode design. For projected capacitance, bigger gaps between the Tx and Rx electrodes will give more sensitivity (but reduce stability), or using thicker receivers in the electrode will allow for thicker overlays (but reduce conductive noise immunity). Figure 7 shows different electrode variations of the same design (for projected capacitive sensing) for different overlay thicknesses (in order of increasing sensitivity).

Figure 7: Projected button layouts in order of increasing sensitivity. Red represents the Tx electrodes, yellow represents the Rx electrodes and white represents the PCB cut-outs. The dimensions of all three buttons illustrated are the same.

For self-capacitive sensing, bigger electrodes will give more sensitivity, and avoiding sharp corners that form concentrations in field lines will allow for thicker overlays.

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