Using Polyester And Flexible Pcb In A Membrane Pcb Keypad Design

By Author: elecflex
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When a company begins the process of introducing a product to market that contains a Human Machine Interface, membrane switches are one of the common selected technologies because of their high reliability, level of customization, and relatively low cost.

Membrane PCB keypads are everywhere, look around, but how many truly pay attention to the circuit design? Typically, Human Machine Interface engineers and designers focus on the visible elements first, like overlays, backlighting, etc. Next, the focus is usually on the keypad design, including the keypad material, tactile response, and the switching mechanism. But they often fail to provide enough attention to one of the most critical design features of a membrane switch, the flexible circuit behind the overlay.

Flexible Circuit Options

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. To learn the impact of using the two circuit designs, it is important first to analyze the construction of a typical membrane switch.

Layer 1:

Beginning ...
... from the outermost layer, the graphic overlay is formed with specializing polyester or polycarbonate films with printed artwork. The layer acts as a shield to the environment while containing images, graphical text, and the color schemes of the membrane switch. The overlay layer will not be impacted whether polyester or flexible PCB circuit is used.

Layer 2:

The overlay adhesive is a pressure-sensitive adhesive layer that bonds the overlay to the space layers. The overlay adhesive layer will not have an impact on whether polyester or flexible PCB is used.

Layer 3 and 4:

The next two layers moving inward are the space layers that are made from polyester films. They typically serve as electrically insulating layers within the stack-up and help build the overall thickness.
The layers can help to contain dome switches, sandwiching them to the circuit layer. There is no impact to the space layers, whether a flexible PCB or polyester circuit is used.

Layer 5:

The circuit layer is the next layer moving inward and resides between the space layers and the membrane switch rear adhesive. The primary responsibility of circuit layers is to carry the switches electrical signals and provide the most crucial functionality for the membrane switch. Discrete components, like capacitors, resistors, LEDs, and some ICs can be affixed directly to the circuit layer, depending on the circuit design.

This layer is one of the most crucial layers within the stack-up in terms of reliability, manufacturability, and cost. Changing the circuit type within the membrane switch should not put any effect on any other layers.

Layer 6:

The rear adhesive is a pressure-sensitive adhesive layer that bonds the membrane switch to the installation mounting surface. The adhesive layer design is crucial as it typically serves as the ingress protection barrier and makes sure no dust, moisture, or other contaminants can break through the mounting surface. The rear adhesive layer will have no impact when a polyester or flexible PCB circuit is used.

At Elecflex, we recommend performing a careful assessment of the program objectives, installation environment, and membrane switch design details so that the most appropriate flexible circuit design is chosen.

We offer custom Membrane Switches, PCB Keypad, and Panels featuring robust construction and support a wide variety of designs and applications.