Backlighting capacitive touch circuits

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By Steve Baker | Jun 02, 2021
A backlit capacitive switch

Projected capacitive (PCAP) touch technology has become a popular user interface option for many industries in recent years. Not only do they offer a sleek, intuitive user experience, but the possibilities for backlighting a capacitive touch circuit are nearly endless.

While capacitive touch technology incorporates well with a variety of backlighting options, the design of the circuit is an important consideration. If designed improperly, the switches can potentially impede parts of the lighting, resulting in an uneven or inconsistent look.

How can capacitive touch circuits affect backlighting?

Capacitive circuits work by projecting a capacitive field and measuring any changes to the capacitance. This capacitive field is most commonly generated using circuits printed with conductive ink. Standard conductive inks, such as silver, carbon, or dielectric ink, can pose challenges when printing backlit PCAP circuits. Due to the opacity of the inks, they can block backlighting and result in uneven lighting or shadowing on the switch.

Fortunately, there are several methods to ensure that backlit capacitive touch circuits illuminate uniformly every time. Below, we’ll be going over the three most common techniques we use at GMN to ensure consistent backlighting.

Methods for backlighting capacitive switches:

  • Selective printing

One of the simplest ways to backlight a PCAP switch is by selectively printing around any backlit areas or iconography. When using an inexpensive carbon or other opaque ink, icons or symbols can be left unprinted within the design. As the backlighting rises through the switch, the light only comes through the unprinted area, resulting in a user-intuitive illuminated icon.

However, there are a few requirements to employ this backlighting technique. First, the switch area needs to be large enough for iconography to be left unprinted. In addition, there needs to be enough conductive ink surrounding the unprinted area to complete the circuit and result in an effective switch. This method is ideal for large or geometrically simple switch designs.

  • Backlighting through clear conductive ink

For smaller or more complex switch designs, a solution that has been recently gaining popularity is using clear, polymer-based conductive inks (such as PEDOT ink). These inks run from translucent to nearly transparent and allow the circuit to be lit from directly underneath. Unlike a switch that uses opaque ink that potentially blocks lighting, clear ink conducts electricity the same way but allows light to pass through the circuit unobstructed. While transparent inks are more expensive than opaque alternatives, they can be applied the same way through screen printing.

Another advantage of using these inks is that the translucency can be altered based on the type of ink and thickness of the deposition. Less transparent inks also act as a lighting diffuser, thereby eliminating hotspots.

  • Altering the capacitive touch stack-up

Another solution is to engineer the stack-up so that the backlighting source sits above the capacitive circuit. While most capacitive touch circuits are backlit from underneath, rearranging the backlighting source (typically light-guide film or fiber optic bundles) to sit above the PCAP switches can ensure that the circuits do not impede any lighting.

While this is an effective method, the sensitivity of the PCAP switch needs to be tuned to accurately register inputs through the backlighting layer. Lighting hotspots are also a potential concern as the backlighting sits directly underneath the overlay, but this can be easily solved by adding a diffuser.

The above solutions are often mixed and matched depending on the design to ensure that each part of the interface is consistently lit. GMN offers a host of different backlighting solutions for nearly any project. To discuss your specific backlighting needs, schedule a consultation with our experts.