Membrane switches are a popular solution for commonly used keyboard interfaces. Design engineers can simply order a stock solution, plug the unit into a driver or mother board, and assemble. But is this the most cost effective and reliable solution?
Flexible membrane switches can deteriorate due to mismatches in thermal expansion coefficients
For cost conscious designers (and who isn't?) membrane switches are low cost, widely available and are easy to design into a new product. Typically, they're made by laminating two polyester circuit layers that contain screen printed conductive inks, with a perforated insulator in between.
When the user presses the switch location, the front circuit layer flexes through the hole and completes the circuit. Where some confirmation of switch actuation is needed, designers can incorporate LED's or configure the display to react to user touch to provide feedback.
A flexible membrane type keypad. Note the flexible connector "tails"
Flexible polymer sheets are generally backed up with a metal plate, usually aluminum. The combination of flexible polymer circuitry and a metal backing plate would seem durable, but there are several failure modes that need to be addressed by the designer.
The flexible polymer is commonly polyester, (there are other options too) used because of its "memory" and resistance to permanent deformation. The grade of polyester in the acrylic adhesives used to assemble a possible membrane switch can vary widely in quality and performance, ranging from low-cost devices intended for limited use "throwaway" applications, to keypads designed for only hundreds or thousands of cycles. Designers can run into difficulty choosing an off-the-shelf keypad that will perform in service. And life cycle testing is expensive and time consuming.
Any engineering assembly that bonds dissimilar materials together experiences a mismatch in thermal expansion coefficients, and metal backed polyester membrane switches are no exception. Natural thermal cycling can cause delamination at the acrylic-metal interface resulting in a polyester circuit that peels away from the backing plate, eventually resulting in a failure.
Typical flexible and membrane switch construction.
The adhesive to backplate interface can be the site of
delamination and eventual failure
A similar problem occurs when LEDs are embedded in the flexible switch. Naturally, a polymer membrane switch cannot be soldered, so the LEDs are mounted with a small amount of conductive epoxy. That epoxy is subject to the same stresses and eventual cracking as the acrylic interlayer. For applications such as portable electronics that are subject to large numbers of cycles, bending or impact stress, and temperature variations, the problem can be unacceptable.
An important potential failure point is at the electrical connection between keypad and mother PCB. Low-cost flexible membrane pads use a connector that is an extension of the base layer polyester sheet in the laminated sandwich. It is more fragile than a rigid card edge connection or ribbon cable, and is easily kinked, creased or torn during assembly or repair operations.
Rigid PCB-mounted keypads are more durable
A durable, cost-effective solution to the thermal expansion problem is to design the switch circuitry onto a printed circuit board. This circuit is then mounted to the metal backing plate with high-performance adhesives in a manner similar to the flexible pad. This rigid laminate is much more resistant to cracking related to temperature swings or shock and vibration.
Typical rigid PCB type keypad. Fewer layers
and more connection options offer durability
without a cost penalty.
The printed circuit type design offers many advantages. LEDs can be soldered directly to the board for greater durability, and like all PCB designs, a wide variety of connector options can be utilized. The rear of the PCB is fully available for additional circuitry and can accommodate standard electronic components like resistors, capacitors, diodes and IC's.
One of the principal advantages of membrane switches is the wide variety of graphics that can be presented on the top overlay. That advantage is still available with a rigid PCB keypad. Custom elastomer overlays can be screen-printed with a wide variety of options to suit any application.
What type of membrane switch is lower cost?
For most applications, a more durable custom designed and rigid PCB option is no more expensive than the flexible membrane switch. A major reason is the overall cost savings that can be realized in assembly and integration of the keypad into the finished device.
A flexible membrane type keypad. Note the flexible connector "tails"
With multiple, durable connection options, it is possible to tailor the keypad to suit existing assembly operations, or change assembly techniques to "mistake proof" the installation with keyed connections. Without the flexible keypads' fragile "tail", packing, shipping, unpacking and internal handling of the parts is much safer, resulting in better production with fewer defects. In addition, the ability to utilize the back of the keypad's PCB allows the designer to reduce overall part count and possibly eliminate some supporting circuitry, resulting in a lighter, less costly and more reliable product.
In applications where high performance, durability and low overall system cost are important, the rigid option has significant advantages over conventional flexible membrane switches. Both offer highly customizable solutions that can create great looking products. Which is right for you? For any but the shortest production runs in the most benign user environments, rigid keypads hold the edge by combining higher durability without a significant cost penalty.
Epec Engineered Technologies has a very deep technology base and a long history in membrane switch design. Epec has paid a fee to ENGINEERING.com for promotion of their technology. They have had no editorial input to this post. All opinions are mine – Jim Anderton