Overcoming 5 Design Challenges for Capacitive Touch Backlighting

As automotive designers continue to make the in-vehicle experience more closely match that of our mobile device usage, backlighting of capacitive switches is becoming more ubiquitous. Automotive OEMs find that adding backlighting to their touch switches, dials and buttons is a way to differentiate themselves from their competition and add premium aesthetics to their cars’ interiors.

There are, however, challenges associated with integrating backlighting into capacitive interfaces, and navigating them can increase design time and costs. What follows are a few of those challenges and their potential solutions.

Electronic functions that are uniformly illuminated give a control panel a high-quality look. Additionally, capacitive functions that are dimly lit due to poorly placed LEDs or have hotspots that obscure indicators hinder user comprehension of how to use in-vehicle electronics. Therefore, good-quality, uniform lighting also improves dashboard navigation, helping guide users through operations.  

One method to produce uniform lighting is to back up the LED from the capacitive surface to provide enough room for the light to fan out over the designated area. However, this might be difficult if not impossible to do in space-constrained designs requiring a thin capacitive switch.

Side-fire LEDs are an effective method of delivering uniform lighting without having to create a thick capacitive switch. Because they illuminate from the side, rather than vertically, and use light guides to redirect the light upward, side-fire LEDs don’t need to be distanced from the designated area. As a result, light uniformity can be achieved while still restricting the thickness of the capacitive switch.

Because light guides diminish brightness a bit, it’s important to use high-quality, high-efficiency LEDs. And the design itself affects the quality of light emitted from side-fire LEDs, as well. The choice of light guides along with their placement in relation to the LEDs can impact brightness.

Light bleed occurs when the light from a capacitive button or switch leaks into the one next to it. This diminishes the aesthetics of a luxury car’s dashboard and hinders switch navigation. In contrast, there should be a space between the illuminated areas that serves as a transition area between capacitive functions. Making sure lighting stays within designated graphics can be especially challenging when buttons and other illuminated areas are close together in space-constrained applications.

In automotive applications where thinness of the capacitive switch is essential, multiple side-fire LEDs can enable precise lighting within designated graphics. This solution may be too costly for some design budgets, however, in which case automotive interior designers can work with switch engineers to modify graphics to back up a lower number of LEDs to precisely illuminate the capacitive function.

Everyone has experienced difficulty reading their smartphone screens on a sunny day. Because backlighting often helps drivers/passengers navigate applications and alerts them when they are activated, it needs to be seen at all times of the day, not just in the dark of night. Making automotive capacitive backlighting sunlight readable, however, is a formidable challenge – it must be brighter than daylight. Then, when the sun has gone down, backlighting must adjust, becoming dim enough to avoid blinding the car’s occupants.

Incorporating LEDs with more lumens makes backlighting sunlight readable. Stronger LEDs, however, are more costly. Additionally, sensors will need to be integrated into the circuit to adjust the LEDs’ brightness in response to ambient light. In order to stay within budgetary restraints, other design adjustments must be made to offset the cost of high-lumen LEDs and sensors.

A cutting-edge trend in car interiors is ambient LED lighting, such as illuminated strips bordering the dashboard and center console. For aesthetic purposes, the backlighting of buttons and switches should match the ambient lighting. However, capacitive circuitry can make such color matching tricky.

PEDOT is translucent, making it an effective material for backlit capacitive interfaces. Capacitive switch designers working with PEDOT should assess the exact tint of the vehicle’s ambient lighting and work with suppliers to procure LEDs with the precise wavelength to compensate for the slight tint of the PEDOT and color match the ambient lighting. The result will be homogenous lighting throughout the car’s interior for a finished, aesthetically pleasing look.  

Due to safety concerns, haptic feedback is desirable for in-vehicle electronics. While visual feedback alone is acceptable in smartphones and tablets, it can present a danger to drivers, who should keep their eyes on the road rather than glancing at dashboards. According to studies cited by a Frontiers in Psychology white paper, haptics have been shown to reduce reaction time when using in-vehicle electronics and to ease the cognitive load of driving, ultimately increasing driver safety.

However, the number of electronic components needed for both haptic response and backlighting creates space concerns for designers. Also, haptic components might not be translucent, so designers would need to offset LEDs or use side-fire LEDs with light guides, both of which would only add to space constraints.

Flexible printed circuits (FPCs) enable engineers to find ways to integrate both backlighting and haptic feedback into a capacitive switch. FPCs are thinner and lighter than other circuits such as PCBs, which can compensate for the space and weight of haptics and LEDs. Additionally, the thin, bendable circuit can then be formed into a 3D shape, giving engineers more options to fit the circuit within dashboards, center consoles or armrests.  

• Black overlays to avoid light-bleeding effects
• Backlighting performance improvement by integration of light diffuser film or ink
• Design-to-PET film production process optimization (selection of materials, stack-up, testing)
• Selection of suitable ink materials (Silver, PEDOT, carbon, dielectric)
• Circuit routing and design (1- or 2-sided with vias, feature sizes, circuit layout)

Molex’s core competency lies with its expertise in printed circuitry and the understanding of the electronics behind capacitive interfaces, which enables our engineers to provide design-to-cost and design-to-quality printed electronics solutions. As a result, Molex delivers today’s industry-leading backlighting displays, printed electric circuits and PEDOT conductive sensors on a global scale and offers a roadmap that anticipates and overcomes tomorrow’s challenges. Contact us for more information on how Molex can make your in-vehicle backlighting ideas a reality.