Develop sense electrodes for 3D touchpad surfaces

Over the last couple of years, touchpads have become a standard input method for next-generation user interfaces for everyday applications, from white goods to smart phones, computer peripherals, and remote controls. Touchpad design requires a thorough understanding of capacitive sensing, from electrical to mechanical stack, as the entire device and the user interaction with it need to be understood, especially for hand-held or battery-operated devices. It can be challenging to add touchpads to designs that are not flat or are made with three-dimensional shapes and multiple parts.

Even vs. uneven surfaces
A touchpad is a collection of individual touch buttons arranged in rows and columns, called channels. As the user moves his or her finger over these channels, a delta is sensed in the measured count values and can be used to determine the XY touch co-ordinate.

Traditionally, touchpads are flat with a uniform overlay thickness. One of the main reasons for this is that, with a uniform pattern and a uniform thickness, the touch strength when the user's finger touches the surface will be uniform over the entire touchpad. If, however, the overlay is not of uniform thickness, the mutual capacitance and therefore the touch deltas will no longer be uniform.

This article explains the most important design choices that need to be made when designing a touchpad for a 3D surface.

Selecting the substrate
The choice of substrate is no longer limited only to flexible circuit boards (FPCs). Depending on the overlay shape and the interconnects to the main board, the designer also has the option of an FR4 PCB. The high sensitivity of the Azoteq Trackpad ICs allows the designer to place the touchpad or slider on the main PCB. This would lower the cost of the solution significantly, as the additional FPC and connector would no longer be needed. However, there are a few factors that must be considered by the designer when selecting the substrate, and these are described in the following sections.

The table below is a list of dielectric constants/permittivity for materials that are commonly used for touchpads.

Equation 1 below shows the parallel plate capacitance equation, where A is the area of the pad, 蔚₀ is the permittivity of the air, 蔚_r is the relative permittivity of the overlay material, and d is the thickness of the overlay.