There are many critical aspects to consider when designing a membrane switch. The electrical portion of the keypad is one of these critical aspects. There are few considerations to consider when laying out the circuit including: the pin out sequence; tail length & position; multilayer circuitry; and use of LEDs.
Connector Pin Out Sequence:
CSI produces membrane switches with both row & column matrix, or common ground circuit layouts. Customers should consider allowing CSI to determine the pin out sequence if the sequence is not critical to the customer’s design. Allowing CSI to produce the pin out will result in a superior design including potentially lower cost, higher reliability, shorter lead time.
A matrix layout is when the circuit is designed in a grid type format using and “X” (rows) and “Y”(columns) axis. The benefit of this layout is there are less tracks allowing for a larger number of connections which then reduces the number of pinouts and connector size. The drawback however is that there are multiple print layers required in order to produce the full circuit.
– Common Bus:
For a common bus layout on a membrane switch one common lead is used for all switch locations. A lead is used to “ground” the circuit. Each switch will have its own individual trace. The benefits of this layout is that only one layer of print is required to complete the circuit. The drawback to this design is it means there is an increase in the number of tracks, pinouts and connector size.
Tail Length & Exit Positioning:
Membrane switches are typically connected to the customer’s printed circuit board by a flat cable or flex tail. The flexible tail is cut from the circuit layer(s) of the switch. The tail exit position and tail length must be specified before CSI can begin the design of the membrane switch. In volume production tail length, and exit position can substantially affect productions costs. CSI can suggest the best tail exit locations to ensure the most manufacturability is as economical possible.
Multilayer membrane switches circuits can be designed as screen printed conductive inks on heat stabilized polyester (typically .005”), or with circuit boards. With screen printing technology, multiple circuit layers are created (if necessary to achieve needed trace routing) with dielectric insulating ink printed in areas where circuits cross over one another. With flexible or rigid printed circuit boards feed throughs are used to: create multiple circuit layers, as well as to produce two sided circuits.
Light emitting diodes or LED’s are typically assembled to a circuit layer that lies below the static layer that carries the key functionality due to vertical height issues and space constraints. The tail from the upper layer provides the key interface, while a second tail from the lower layer supplies power for the LED’s. In some cases CSI can include light emitting diodes and key functionality on the same layer, and through the same connector, thereby eliminating the cost of the second circuit layer, and connector.