Capacitive Touch

Mechanicals

The mechanicals are the mechanical characteristics of the design. The mechanicals include the overlay material, ink on top of the overlay, any adhesives used to bond the electrode to the overlay or enclosure, and any transition materials used to remove air gaps between the electrode and the overlay. Mechanicals also include the types of materials used for the electrodes. The mechanicals affect both the signal and the parasitic capacitance.

The goal of this section is threefold:

1. To understand the benefits in terms of both aesthetics and robustness
2. To understand how the materials on top of the electrode and the electrode material itself influence the layout of the electrodes
3. To avoid mistakes in the mechanicals that are detrimental to the electrical performance

Typical Stackup

The following figure shows a typical stackup for a capacitive touch solution. One of the main goals of this stackup is to reduce (or eliminate, if possible) any low-dielectric (air) gaps between the electrode and the area where the touch takes place. The capacitance associated with the stackup has a very strong effect on the signal (change in capacitance from a touch). The signal is directly proportional to the dielectric of the materials. If possible, high-dielectric materials should be used, but at a minimum the stackup should eliminate any air gaps. Air gaps can also contain moisture, which can influence performance or even damage the stackup as temperatures vary and the contents of the gap expand and contract.

Another critical attribute of the stackup is that it should be non-conductive. This is not usually a problem with the overlay material but can be overlooked when choosing adhesives, labels, or inks. Popular adhesives for capacitive touch solutions include 200MP products from 3M™ such as 467MP and 468MP.

Overlay

Why you want to have an Overlay?

Overlays are very important part of the capacitive touch system, it allows the designers to give their product a sleek industrial design and also provide the ESD and environment protections like the pictures showing below.

What is the trade-off?

It provides many benefits to product designers by adding an overlay on top of the capacitive sensor however the benefits come with a trade-off. When an overlay is placed between a sensor and the point of contact for a finger, a sensor’s sensitivity is greatly reduced because the overlay reduces the effective electric field going out from the sensor. The dielectric constants determines how efficiently electric Field passes through material and the distance between the overlay and the sensor determines how much electric field left in target point of contact area. As the diagrams show below, adding an overlay over a sensor reduce the electric field going out from the sensor which reduces the strength or range of a sensor to a touch condition.

The following figure shows the relationship between the thickness of the overlay and the sensitivity of the circuit. From the parallel plate capacitance equation, the capacitance in inversely proportional to the material thickness (C ~ 1/d).

The thickness and dielectric of the material influence the electrode design. The electrode area is a function of the area of interaction (a fingertip or the palm of the hand) while the spacing (to adjacent electrodes or ground fill) is related to the thickness of the overlay. For example, with a 2-mm overlay that has a dielectric of 3, the spacing should be approximately 1 mm (half of the thickness). Using a higher dielectric material (for example, Er = 6) the thickness could be doubled while maintaining the same level of performance. The following table shows dielectric values for various materials used as overlays.

Table: Material Dielectric and Breakdown Voltage

1. Relative permittivity
2. http://www.corninggorillaglass.com

The table above also includes the breakdown voltage for different overlay materials. This should be considered when designed for ESD protection. ESD solutions should be system solutions, and any additional components should complement the protection provided by the overlay.

Electrode and Trace Materials

The performance is affected by the conductive materials that are used for the electrode and for the trace between the electrode and the microcontroller. Most applications use copper on a PCB, and copper has a resistivity of 1.7×10-6 Ohm-cm (1). As the resistivity of the conductor increases, the ability to move charge to and from the electrode decreases. This has the same effect as an increase in parasitic capacitance. This increase in resistivity, like an increase in parasitic capacitance, reduces the system sensitivity. The table below shows the resistivity for materials that are commonly used in touch applications.

Table: Resistivity of Materials

1. Resistivity is given in Ohm-cm so that resistance is equal to the resistivity times the length divided by the cross-sectional area : R = p x L / A.
2. This resistivity is for a film thickness of 270 nm. Typically, vendors provide sheet resistance instead of resistivity for ITO, which is on the order of 10 to 100 Ohms per square.

When using high-resistivity materials, generally the recommendation is to increase the area of traces to reduce the resistance (at the cost of capacitance). ITO solutions provide lower sensitivity, which must be compensated for in the capacitance measurement algorithm by longer measurement times.

Other Situations

Not all applications fit into the typical category, and this section describes two special cases. The first is intentional air gaps that are greater than 2 mm between the electrode and the overlay material, and the second is the use of gloves.

Gaps

Why you want to have a gap?

In some applications, components are on the same layer as the electrode. This prevents the overlay from being directly applied to the electrode. A common example of this is when an LCD is mounted near the electrode (see diagram below). Another scenario is when the overlay material is not a uniform surface and, therefore, the electrode cannot make direct contact with the overlay.

What is the trade-off?

As we discussed in the Overlay section, air has really low dielectric constant and therefore does not pass the electric field very efficiently.

As the diagram shows, theoretically the sensitivity for 1mm air gap without any moisture will be similar to an 8mm thick glass. Typical dielectric values for Air is 1. (Without moisture) Typical dielectric values for Glass is 8.

To keep the air gap without affecting too much of the sensor performance, we must bridge the gap for the sensor area with a non-conductive filler (typically adhesive) or a conductive extension. When the gap is in excess of 2 mm, then a conductive extension, either foam or metal, should be used. The metal or foam must be malleable to conform to the shape of the surfaces and prevent the formation of gaps.

When customer choice their bridge materials please take in consideration of dielectric values, transparency, mechanical support, manufacture cost, noise tolerance. The diagram below shows several materials we experimented to help to bridge the air gap and the sensitivity increase goes to an order of: Metal Spring > Conductive Foam > Carbon Fiber > Polycarbonate > ABS > Silicone

In either case, the gap must be filled or bridged with a non-conductive filler (typically adhesive) or a conductive extension. When the gap is in excess of 2 mm, then a conductive extension, either foam or metal, should be used. The metal or foam must be malleable to conform to the shape of the surfaces and prevent the formation of gaps. As shown in the Figure above, the area created by the foam or metal in contact with the overlay is now the area that influences the capacitance.