Membrane switches have become a staple in modern electronic interfaces, delivering reliable user input across industries ranging from consumer electronics to industrial equipment and medical devices. They are frequently chosen because they offer a combination of low cost, ultra-high reliability, and are completely customizable.
For many applications, their slim construction and low profile are a key differentiator, making them ideal when space is limited or when a sleek final product design is essential.
Still, as end products continue to shrink and aesthetic expectations rise, engineers often face the challenge of reducing the overall thickness of their membrane switches without compromising performance. Achieving this requires careful consideration of all available circuit materials, design priorities, and the options available in construction.
Circuit Materials in Membrane Switches
A membrane switch is a low-profile circuit that has domes laminated within the inner layers of the keypad. The foundation of a membrane switch lies in its base circuit material. Each circuit option has unique strengths and limitations that influence not only cost and durability but also the overall thickness of the finished membrane switch.
PET: Polyester Circuit with Screened Ink
Polyester circuits use screened conductive ink to form traces. This is an economical choice, especially for high-volume or cost-sensitive applications. While PET provides thin and flexible profiles, it has limitations when it comes to fine trace widths or integrating complex surface-mount components.
Example of a PET membrane switch.
FPC: Flexible Polyimide Circuit with Etched Copper
For applications demanding higher reliability and robustness, polyimide-based flexible circuits are an excellent option. They can support finer circuit traces and more complex designs. However, this material tends to be more expensive, particularly for larger switch designs. This can significantly impact pricing for cost-sensitive projects where the surface area is large.
Example of an FPC membrane switch.
Rigid PCB: FR4 Printed Circuit Board with Etched Copper
FR4 is widely used in electronics for its economical balance of cost and reliability. While rigid PCB circuits in membrane switches are thicker than PET or FPC, they offer excellent durability while still being a low-cost offering. The tradeoff is that rigid PCB circuits require traditional headers or wire harnesses rather than ZIF (Zero Insertion Force) terminations, which adds bulk and complexity.
Example of a Rigid PCB membrane switch.
Why Ultra-Thin Membrane Switches Are Popular
Engineers often require thinner membrane switches for very practical reasons:
- Integration with existing designs: Many times, the panel or enclosure that a membrane switch must mount to is already tooled. Re-designing or retooling these components is expensive and time-consuming, especially with injection-molded housings. The design is limited to only solutions that can be of a certain thickness.
- Ultra-thin systems: Consumer electronics such as tablets, smartphones, or flat-screen TVs demand nearly invisible input devices. A thick switch would protrude, disrupt the aesthetics, and potentially interfere with performance.
- System constraints: In some aerospace, automotive, and medical devices, internal spacing is limited. Every millimeter counts, so membrane switches must conform to stringent thickness requirements.
Choosing the Best Circuit Material for Low-Profile Designs
When reducing thickness is the priority, PET and FPC circuits are generally the best options. Both provide thin profiles, with PET being the more cost-effective solution and FPC offering greater robustness and design options.
Rigid FR4 PCB options can also be made in thinner constructions, sometimes as slim as 0.032 inches. However, these ultra-thin FR4 boards tend to be more expensive and come with longer lead times, so they are typically only used when necessary.
Design Options to Reduce Membrane Switch Thickness
Reducing thickness isn’t just about the circuit material. It requires a holistic design approach that considers every layer and feature in the assembly.
1. Keep It Simple
If your primary requirement is an ultra-thin membrane switch, simplicity is key. Avoid adding unnecessary functionality or bulky components like ICs, tall connectors, or shielding unless they are absolutely required. A streamlined design maximizes thinness and minimizes the risk of challenges due to added complexity.
2. Move LEDs to Another Circuit Board
LEDs can add thickness when mounted directly into a membrane switch. While most LEDs are not terribly tall, many still employ an embossed window or other raised feature that adds thickness. Relocating them to a separate PCB behind the switch is often a practical solution if eliminating them is not an option. By incorporating a transparent window or cutout in the switch, the lighting requirements can still be met while reducing the overall profile.
3. Select the Thinnest Available Materials
Nearly every layer in a membrane switch (pressure-sensitive adhesives, spacer layers, and graphic overlays) is available in different thicknesses. While the total thickness savings is not significant for each layer, by saving 0.003” on each, the overall thickness can be reduced when all these layers are stacked. By specifying thinner materials early in the design phase, engineers can make meaningful reductions in the overall profile without sacrificing functionality.
4. Communicate Early in the Design Process
One of the most overlooked but critical steps is early communication with your manufacturer. If ultra-thin requirements are known from the outset, suppliers can recommend material options, construction techniques, or alternate approaches to achieve the goals. Delaying this discussion until after tooling or the mating panel design is finalized often leads to expensive redesigns or compromises in performance.
Summary
Reducing the thickness of a membrane switch requires careful planning and prioritization. Engineers must weigh cost, reliability, and manufacturability against the need for a slim profile. Circuit material selection plays a central role, with PET and FPC leading the way for thin designs, while ultra-thin FR4 is an option for specialized cases. Beyond materials, strategies like simplifying the design, relocating LEDs, and choosing thinner adhesives and overlays can all help shave valuable millimeters off the assembly.
By clearly defining thickness requirements early and collaborating with experienced manufacturers, engineers can successfully integrate ultra-thin membrane switches into today’s compact, high-performance products.
Key Takeaways
- Membrane switches are chosen for their low cost, reliability, customization, and naturally thin profiles.
- PET and FPC circuits are typically best for low-profile applications, though thin FR4 can be used when required.
- Reducing overall thickness often means simplifying the design and relocating bulky components like LEDs.
- Material selection for adhesives, overlays, and spacers significantly impacts total thickness.
- Communicating thickness requirements early ensures manufacturers can propose the most effective solutions.
