Membrane switches are everywhere, but how many people truly pay attention to the circuit design? Typically, HMI(Human Machine Interface) engineers and designers focus on the visible design elements first, such as overlays, pantone colors, and backlighting. Next, the focus is usually on the keypad design, such as the switching mechanism.
Flexible circuit options include polyester ( PET) that uses silver ink as the circuit element, and flexible PCB( FPC) that uses copper as the conductive element. Form, fit and function is very similar for both though their respective costs and reliability can drastically differ.
Layer1: Starting from the outer most layer, the “graphic overlay” is formed with specialized PET or polycarbonate films with printed artwork. This layer serves as the barrier to the environment while containing graphical text, images, and the colors scheme of the membrane switch.
Layer2: the overlay adhesive is a pressure sensitive adhesive layer that bonds the overlay to the spacer layers. The overlay adhesive layer will not be impacted whether a PET or FPC circuit is used.
Layer 3 and 4: the next two layers moving inward are the spacer layers. These are also made from PET films. The spacer layers typically serve as electrically insulating layers within the stack-up and help build the overall thickness.These layers can also help contain dome switches, sandwiching them to the circuit layer.
Layer5: The circuit layer is the next layer moving inward and resides between the spacer layers and the membrane switch rear adhesive. The circuit layers primary responsibility is to carry the switches electrical signals and provides the most critical functionality of the membrane switch.Discrete components such as LEDs, resistors, capacitators and some ICs can be affixed directly to the circuit layer, depending on the circuit design.
Layer6: The rear adhesive is a pressure sensitive adhesive layer that bonds the membrane switch to the installation mounting surface. This adhesive layer design is critical since it typically serves as the ingress protection barrier and helps ensure no moisture, dust, or other contaminants can enter the mounting surface or housing.