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Used when membrane switch circuit real estate is tight, active components are involved, and highest possible performance is required.
The copper used in FPCs has much higher conductivity than silver on PET. FPCs can have multilayer constructions and allow for the soldering of active and passive components. Flexible PCBs are more costly than either silver on PET and rigid PCBs.
Minimum trace width is .0028” and minimum pitch .0047”. Single layer circuit thickness is typically .001” and two sided .002”. Multilayer circuits are up to .007” thickness.
Melrose manufacturers membrane switches on three circuit types:
- Flexible silver on polyester
- Flexible printed circuit board FPC
- Rigid printed circuit boards PCB
Conductive Silver Ink on Polyester Circuits
Screen printed silver on polyester circuits in membrane switches are most common because of their low cost.
- Trace dimensions: Circuit traces must be >.010” and are typically between .0003 and .0005” in height.
- Surface Resistivity: Between 10 and 20 milliohms/sq/.001”; Traces are usually 0003 to .0005” in height
- PET used in membrane circuits is typically .005” and heat stabilized
- Two connector types: 1) Zero insertion force (zif) connectors; pitch > .020” 2) .100” pitch connectors and solder tabs
- Passive components; typically LEDs, and also resistors, and diodes are mounted with conductive adhesive paste. Active components are not available with silver on PET circuits.
- Circuit crossovers are created with screen printed dielectric.
- Through hole conductors create two sided circuits, and can be used to eliminate trace crossovers.
- Higher resistance carbon inks can be used reduce costs in high volumes (e.g., shielding), and to enhance mechanical performance (zif tail overprint)
Melrose manufactures membrane switches applied to Rigid PCBs. Copper has much higher conductivity than silver on PET, and allows for multilayer constructions and soldering of active and passive components. Rigid PCBs are more costly than silver on PET circuits, but less so than flexible PCBs.
Flexible Printed Circuit (FPC)
More on membrane switches
Membrane Switch Circuit Types