Chapter 4:
Stackups and layouts of capacitive touch sensors
4.2 Basics of touch sensors layout
Touch Sensor Layout
The Layout (shape and dimensions) of a Capacitive Touch Sensor is determined by a number of different factors that are taken into consideration during the design phase. These factors include, but are not limited to:
- Type of application
- Cost of materials
- Manufacturers’ specs and availability
Common 0D and 1D Touch Sensor Layouts
The Layouts of these applications are usually directed by the guidelines provided by the IC vendors. A typical guideline referring to the Layout of a simple button is shown below:
Figure 1. Guidelines example of a Simple Capacitive Touch Button. (Source)
Each IC vendor has different guidelines and also different ways that these guidelines are expressed. A typical collection of the diverse 0D and 1D applications layouts follows below:
Button (0-Dimension)
This is the layout of a typical circular Capacitive Touch Button (light blue color). Buttons can have hundreds of different geometry shapes and dimensions. The guidelines usually recommend that a shielding entity (dark blue) is added either in the same plane as the buttons or in a different plane close to them, in order to avoid false touch detection by the influence of the trace routing. The small circular entities (red and purple) are called “Vias”.
Vias are usually copper-plated holes in a printed circuit board (PCB) that allow the conductive layers to connect with each other. In capacitive touch sensor electronics, their usual role is to help the trace-electrode interconnection.
Vias require a minimum amount of copper on a layer for a proper connection so, in most instances, a via pad (circle of copper called an annular ring) is attached to the end of narrow traces to increase the material available for a connection.
Figure 2. Detailed look of the Electrode-Via-Trace connection
Utilizing vias, the traces are routed in a separate layer than the electrodes. The intermediate dielectric layer (usually FR-4) ensures that the influence of the conductive material of the traces is not affecting the correct touch detection of the electrodes.
Capacitive Touch Buttons can be found in a number of different applications such as home appliances (microwave ovens, refrigerators, remote controls), automotive interiors, consumer electronics (laptops, cameras) etc. and are generally considered to be the next generation of mechanical buttons, replacing them day by day.
Figure 3. Detailed look of the Electrode-Via-Trace connection
Slider (1-Dimension)
This is the layout of a typical Capacitive Touch Slider with chevron segments (dark purple color). The slider has a lot in common with the Button (described in 5.2.1.1), as it also frequently utilizes Vias for the electrode-trace connection. Sliders can also appear in a lot of different variations (size, shape, number of segments) according to the diversity of the application.
Capacitive Touch Sliders are mostly used in the same variety of applications as the previously mentioned Buttons (e.g. home appliances, automotive interiors, consumer electronics). Their main function is adjusting the levels of a quantity gradually (for example increase/decrease the volume in a music device).
Figure 4. Simple Capacitive Touch Slider Layout (chevron segments)
Wheel (1-Dimension)
This is the layout of a typical Capacitive Touch Wheel. Once more, the wheel application shares most of its properties with the button and slider (previously described).
Capacitive Touch Wheel applications are very similar to the previously mentioned Slider ones (adjusting the levels of a specific entity).
Figure 5. Simple Capacitive Touch Wheel Layout
Common 2D Touch Sensor Layouts (Screens)
Going through the Touch Sensor Designs of the Capacitive Touch Screens industry, we realize the vast number of different approaches. What this basically means is that there are thousands of different designs out there, each one of them being Intellectual Property of the company that owns it. However, there is also a collection of some very typical designs that dominate a great part of the industry.
In order to make things more clear, we would separate these typical designs based on the number of layers the X and Y electrodes are located.
Double-Layer approach
Figure 6. Double Layer Bars and Stripes (“Manhattan”) Layout
This is a typical, basic layout of a double-layer approach, where both the X (purple) and Y (pink) electrodes are shaped like bars. The electrodes are located in different Z-layers and are usually separated by one dielectric layer (or even more).
Figure 7. Double Layer Diamond Layout
This is another double layer approach, where the electrodes are diamond-shaped. The electrodes are overlapping with each other at a smaller part compared to the previous case, which should make it more sensitive than the Manhattan one, in theory. However, the complexity of the physics involved in the capacitive touch technology means that there is also a number of other factors that affect the performance of the design (cell size, bridge gap, vertical distance of the electrodes etc).
Coplanar approach
Figure 8. Coplanar Diamond Layout
In this case both the X and Y electrodes are located in the same layer and are shaped like diamonds. This means that one group of electrodes should be connected through conductive bridges in order to ensure the continuity of the sensor. As we can see in this case, bridges are used to connect the X electrodes (purple) with each other.
Figure 9. Detailed look of the X Electrodes bridge
As shown above, the bridge is crossing over the Y electrodes (pink) and ensures the continuity of the X electrodes in the co-planar approach. Its width, length and height may vary, according to the specifications of the project.
True single layer approach
Figure 10. True Single Layer Layout
This case uses only one group of electrodes which are located in the same layer. The challenge here is that this approach usually requires more traces than the previous ones (which utilize the rows/columns idea), as each electrode needs to be connected separately with the controller. However there are a lot of custom-made solutions that overcome this challenge (e.g. by grouping some electrodes together and connecting them with the same trace).
Key takeaways from this section:
Double Layer Capacitive Touch Screen layouts are the most common designs in the industry for years, but they lack in compactness. Coplanar and True Single Layer approaches improve this property, but they increase the complexity at the same time, making the decision of the layout an overall tradeoff.
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