A Definition of Silver Flex Membrane Switches
Silver flex membrane switches are among the most popular electronic interface solutions due to their affordability. Designers and engineers choose silver flex membrane switches because their thin flexible layers preserve a low profile without compromising versatility in design options. Because the conductive silver inks are printed directly onto a flexible film substrate, silver flex membrane switches are more cost effective than those that contain chemically-etched copper.
Silver Flex Membrane Switch Construction
- Graphic overlay – Typically, polyester is the material used due to its superior chemical resistance and flex life compared to polycarbonate.
- Overlay adhesive – The adhesive layer, typically created with an acrylic adhesive, bonds the graphic overlay to the top circuit layer.
- Top circuit layer – This is a 0.005” – 0.007” heat stabilized polyester printed with silver conductive inks.
- Circuit spacer – This layer separates the top circuit from the bottom circuit so the switch remains open until the keypad is pressed. The circuit spacer is a polyester spacer with adhesive on both sides.
- Lower circuit layer – This is a 0.005” – 0.007” heat stabilized polyester printed with silver conductive inks. This layer terminates a flexible tail that acts as the interconnect to controller PCBs or other electronics.
- Rear adhesive layer – This adhesive layer bonds the entire membrane switch package to the product enclosure or housing, or to a rigid support panel.
- Rigid support layer – This optional layer adds structural integrity to the membrane switch assembly.
The Resistance of Printed Inks
The resistance of traces and printed patterns is determined by the ink used, trace length, width, and thickness of the printed layer. Typically, the resistance of conductive inks is specified in “ohms per square per mil,” where mils is the printed ink layer thickness. The resistance of a trace is represented by R = L/W x ohms/sq/mil, with L being the length of the trace, W being the width, and ohms/sq/mil being the resistance of a square element printed at a layer thickness of 1 mil of printed ink.
To protect against silver migration and to increase durability, carbon can be printed on silver. The silver controls the overall trace resistance because the silver and carbon are in parallel. Keep in mind that the contact resistance of an overprint carbon ink will be higher.
The Challenges Posed by Silver Migration
At times, silver migration may occur when a DC voltage is applied across nearby printed silver traces in the presence of an electrolyte such as water caused by moisture. In these instances, silver dendrites can grow between the traces, leading to a short. There are ways to mitigate silver migration, including…
- increasing the minimum spacing between traces or features with a DC voltage between them (> 30 mil min) and avoiding sharp points which concentrate the electric field and increase the likelihood of migration
- overprinting the silver traces where they are near each other with dielectric or carbon ink
- designing vents and openings so that moisture is away from silver traces; it is especially important to avoid moisture traps that allow water to accumulate between and over silver traces. Liquid water accelerates migration by a factor of about 10,000
- if the other mitigations cannot be used, breaks between traces, such as slits or barriers, can block the migration path
- use a lower DC voltage because higher voltage accelerates migration
Features & Benefits of Silver Flex Membrane Switches
The features and benefits of silver flex membrane switches make them the most common construction for custom flexible membrane keypads. When compared to costly electromechanical switch assemblies, silver flex membrane switches stand out for their affordability, plus a host of other features and benefits:
- Extremely thin construction saves the all-important design space
- Low cost, especially when compared to electromechanical switch assemblies
- Sealed construction is resistant to moisture and contaminants
- Low-profile surface is cleaned easily
- An easy, one-piece, one-operation assembly
- Offer a variety of design options
Design Options Offered by Silver Flex Membrane Switches
- Tactile and non-tactile with either metal or polyester tactile domes
- Pillow or rim-embossed graphic overlays
- Embedded LEDs
- Fiber optic backlighting
- EL (Electroluminescent) backlighting
- Rigid backers such as aluminum and FR4
- EMI/RFI shielding
- Graphic overlays that can be digitally printed, screen printed, or a combination of both
- The graphic overlays can incorporate transparent and tinted display windows, as well as selective textures and UV hardcoat surface finishes
- Standard connectors on .100” centers or prepared for zif connections