Capacitive Touch Sensors

The sensor itself is an electrical capacitor, and the controller measures the capacitance of the sensor.  When your finger (or anything that holds an electrical charge) approaches and touches the sensor, the capacitance of the sensor changes.  The “partnership” between the sensor and the controller is critical, so the controller knows how to interpret a change in capacitance.  This requires a match between the electrical conductivity of the sensor material, the efficiency of the electrical traces that connect the sensor to the controller, and the controller itself.

At the heart of a touch sensor is a material that is clear and at the same time electrically conductive. At Xymox, we use a material called PEDOT – a very flexible polymer that adheres very well to polyester base films. Diving deeper, we offer two customizable types of sensors for consideration, each with their own characteristics:

Translucent Sensors

• Used mainly in backlighting applications
• Discrete button patterns for either self-capacitive or mutual-capacitive designs

Transparent Sensors

• Used over displays
• Full matrix multi-touch sensors and discrete button sensors
• Kodak HCF – Kodak’s expertise in film technology helps them coat PEDOT perfectly onto super-clear polyester. Xymox then processes that HCF (Highly Conductive Film) into a custom touch sensor.

Sometimes customizations can get complicated. You know what you want in the end, but you aren’t exactly sure how to get there. That’s why our engineers and design experts are perfect for guiding you through the full array of choices and customization options, including:

Sensor Pattern

• Matrix – a double diamond pattern that works great for gestures
• Discrete – a cost-effective option requiring less production time, they also offer specific touch points that help prevent false touches and are often favored in harsh environments
• Slider – maximizes versatility by allowing the entire X axis to serve as a touch point, while still maintaining a cost-effective approach

Screen Size

• Standard – off-the-shelf sensors come in standard sizes (4.3″, 5.0″, 7.0″, 10.4″, etc.)
• Custom – allows for any shape and size required

Lens Choice

• Acrylic – available in standard thicknesses, it’s a durable polymer material with a scratch-resistant coating
• Chemically Strengthened Glass – provides an excellent touch surface and looks great
• Polycarbonate – provides greater impact resistance at the expense of a softer surface that can be scratched more easily

Tail Design & Location

The tail connects the electrodes of the sensor to the inputs of the controller. Off-the-shelf sensors use a bonded tail and the length and location are fixed, often requiring the electronics to be reconfigured. A custom sensor can place the tail in most any location on the sensor and can be the exact length that best meets the requirements. Moving the tail on the sensor is often much easier than moving electrically noisy components or adding specialized shielding to those components.

The length and location of the tail can make a significant difference in the cost of the overall part, both through material costs and physical dimensions pertaining to the manufacturing configuration. By strategically positioning the tail in the optimal location, more parts can be printed per sheet, thus optimizing the cost, and integration with the circuitry can happen in the best manner.

There are two main ways to create the tail:

• Integrated Tail – printing conductive traces on the same polyester sheet that the sensor is made from. Think of it like an extension of the sensor itself. Integrated tails provide the most reliable connection to the sensor. Kodak HCF film with PEDOT allows for an extremely tight bend radius for final assembly – meaning the finished assembly is very durable.
• Bonded Tail – made from a different flex circuit and bonded to the sensor. This is the standard configuration for ITO sensors. You typically see the orange colored polyimide tail connected to the transparent sensor using conductive bonding materials to make the electrical connections. Generally, more expensive to produce, bonded tails can also include the sensor controller directly on the tail itself (called chipon-flex or COF).

Controller Choices

The controller is the brains of the operation. The sensor detects changes in capacitance, and the controller chip is required to interpret those changes. Most standard sensors feature a controller on a flex circuit bonded to the sensor, however that may not always be the best configuration.

When choosing a controller, be sure to ask:

• Is this the best controller for the application?
• Are the settings in the controller optimal for the application?

In most instances, it is most economical to have the controller be part of the main control board. Placing the controller directly on the tail is only economical when space on the main board is limited.

Touch sensors are quickly becoming the preferred technology interface for many modern applications. However, when choosing between a membrane switch and touch sensor for your device, the answer ultimately depends on the specifics of the situation and the needs of your application. Before moving forward with your interface of choice, consider:

• How will it be switch/sensor be used in relation to the host product?
• What’s the expected operating environment?
• How long is the product’s life expectancy?
• What is the desired user experience?
• What is the production budget and schedule?

A successful capacitive touch interface requires many separate components working in unison. When these components are designed together and work as one, the result is a hassle-free user experience – one that can sometimes prove problematic for off-the-self sensors to achieve with a high level of consistency.

There are four key factors to consider when deciding whether to use an off-the-shelf solution or an application-specific approach:

• Customer User Experience
• Design Compromises
• Supply Chain Realities
• Longevity of Supply

We do! Using state of the art technology and conductive silver ink made in house, Xymox can produce parts with 200 micron line weights. And the biggest advantage of partnering with Xymox is our ability to tailor the exact sensor pattern to meet your needs. From discrete touch point designs to functional slider systems to full-touch, high-definition sensors and everything in between, Xymox is the best choice for delivering a cap touch interface that leaves a lasting impression on your customers.