Which Membrane Switch Structure Is More Reliable?
From a structural perspective, membrane switches can generally be divided into two main types: tactile membrane switches and non-tactile membrane switches.
Non-tactile membrane switches usually include flat non-tactile structures and raised key-frame structures. In some designs, raised blind dots may also be used.
Non-Tactile Membrane Switches
Non-tactile membrane switches do not require a strong key feel, but they require high sensitivity. The switch should be activated with a light touch, while also avoiding accidental short circuits when the key is not pressed.
Therefore, proper spacer dots should be added to the lower circuit layer. In earlier non-tactile membrane switches, the key position, shape, and size were usually indicated by printed colors. Operators identified the keys visually.
To improve operation convenience and accuracy, raised key frames or raised blind dots were later added to the graphic overlay. In recent years, many flat non-tactile membrane switches have been replaced by capacitive membrane switches.
Tactile Membrane Switches
Tactile membrane switches have many structural designs. The most common and classic types are:
- Embossed tactile membrane switches
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Metal dome tactile membrane switches
Embossed Tactile Membrane Switches
Common embossed tactile designs include structures where both the upper circuit and overlay are embossed, or where only the overlay is embossed.
In some ultra-thin designs, the upper circuit layer may be removed, and the upper contact points are printed directly on the overlay. The embossed keys are formed by heating and hydraulic pressing through molds, allowing the key area to rise slightly above the panel surface.
This creates a three-dimensional key shape. PET material with excellent toughness and heat-shrink performance is usually selected. The rebound force is formed through hot pressing, so embossed tactile membrane switches usually offer a very good key feel and sensitive electrical performance.
However, they should not be used for long periods in high-temperature environments, because high temperatures may weaken the tactile response.
For embossed membrane switches, it is generally better to emboss both the upper circuit and the overlay. If the upper circuit is not embossed, air vents should be considered. Small holes may be added to the upper circuit as ventilation holes.
When necessary, proper spacer dots should also be added to the lower circuit contacts to prevent short circuits or unintended conduction.
If the contacts are printed directly on the overlay, the deformation of the overlay after high-temperature aging must be considered. If the baking time is too long, the overlay may deform significantly, making assembly and bonding difficult. If the baking time is too short, the contacts may not be aged enough and may wear out easily, shortening the product’s service life.
Metal Dome Tactile Membrane Switches
A metal dome tactile membrane switch uses a metal dome fixed on the lower circuit layer. The dome acts as one pole of the circuit. When pressed, it contacts another pole on the lower circuit layer and completes the circuit.
The key design of metal dome membrane switches can be highly flexible. This helps make the product more stable and reliable while also improving the appearance and decorative effect.
The overlay can be designed as:
- Flat surface
- Convex spherical surface
- Convex flat surface
- Round, square, triangular, or irregular key shapes
When the key is designed as a flat key, raised key frame, or blind dot, the adhesive spacer layer should be thickened to provide enough space for the metal dome to move freely.
If the tactile feel is not good enough, a small pad can be added to the back of the overlay key area.
The metal dome fixing layer can be designed in two ways:
- Hole-punched structure at the key position
- Embossed structure
For the hole-punched design, the hole diameter should be smaller than the metal dome. This structure usually provides a crisper tactile feel and more uniform actuation force. The embossed structure provides better overall sealing performance.
Another metal dome structure places the dome directly on the key position of the upper circuit to enhance tactile feedback. This design is usually used when there are many keys and the overall product size is small, leaving limited space for circuit routing.
LED-Integrated Membrane Switches
LED-integrated membrane switches can be designed with the LED circuit and lower circuit on the same layer or on different layers.
When the LED circuit and lower circuit are on the same layer, the structure is relatively simple. However, the LED window should be embossed. Otherwise, the LED may push up the overlay, and the LED itself may be damaged by heavy pressure.
A 0603 LED has a lower height, but its solder pads are smaller, making soldering more difficult. An 0806 LED is commonly about 0.8 mm high, with larger solder pads and fewer cold-solder issues.
When the LED circuit and lower circuit are on different layers, the structure is more complex, and the overall membrane switch becomes thicker. In this case, the LED window does not necessarily need to be embossed.
For LED membrane switches, an anti-static shielding layer is often needed on the overlay. Green and blue LEDs are especially vulnerable to electrostatic damage, particularly in winter.
Waterproof Membrane Switch Structures
Waterproof membrane switches are usually designed with an embedded frame structure.
One common structure is made from the same type of material in two separate parts. The circuit part is embedded into the panel part to form one complete product. This structure is widely used, and each adhesive layer should use waterproof adhesive.
Another structure uses the upper circuit, lower circuit, spacer layer, and other layers with the same thickness to form an outer frame together with the overlay. The circuit part is embedded inside to form a complete integrated structure.
Common waterproof adhesives include 3M 9495LE and 3M GTM920.
Light Guide Membrane Switches
A light guide structure means embedding a light guide film into a standard membrane switch. Side-emitting LEDs are used to illuminate the keys. EL cold light sheets can also be used.
This allows the membrane switch to be used at night or in low-light environments, while also improving its visual appearance.
When manufacturing light guide membrane switches, light leakage and light interference from the overlay and side edges must be avoided. All double-sided adhesives should use black adhesive.
After adding a light guide film, the tactile feel of the keys may become weaker. To improve the key feel, pads can be added to the back of the key area on the overlay.
Folded Circuit Structure
A folded circuit structure is usually considered when there are many keys, the overall size is small, and there is not enough space for circuit routing.
This structure is designed with upper and lower circuit layers. Special attention should be paid to the folding process, which should be performed under heat. The long-term effect of bending on service life must also be considered.
Conclusion
Each membrane switch structure has its own advantages and limitations. The most reliable design depends on the specific application, operating environment, tactile requirements, waterproof requirements, backlighting needs, and product size.
At the beginning of the design process, the most suitable structure should be selected from membrane switch manufacturer to ensure high product quality and stable long-term performance.
