Silver Migration in Membrane Switches
Silver Migration Explained:
Printed silver is a metal that has been used in the flexible circuit aspect of membrane switches for over 35 years. The conductive silver is screen printed on flexible substrates (typically polyester or polycarbonate) to form the conductive traces that are the electrical backbone of a membrane switch. Silver is still widely prevalent in membrane switch designs to this day due to its conductivity, usability and cost-effectiveness.
There are some disadvantages to using printed silver, however. Silver is a very active metal and is thus highly susceptible to silver migration or dendritic growth. Silver migration is the ionic movement of silver between two adjacent traces on the circuit. Silver migration will occur if there is moisture present between the two traces which results in a temporary electrical short. The rate of migration depends on the amount of moisture, the temperature and the voltage.
Preventing Silver Migration:
While silver migration can be a major headache for membrane switch users around the world, conductive silver certainly would not have been used in keypads for over three decades if there wasn’t a solution:
- Prevention is the ultimately the best solution. Preventing moisture ingress into the circuit will prevent silver migration from ever occurring. Properly designing the membrane switch using a frame seal is one of many methods CSI utilizes to prevent your keypad from silver migration.
- Utilizing copper flex circuitry instead of printed silver circuity.
- Covering the silver traces with a protective carbon layer and/or an overcoat dielectric.
- Increasing the conductor spacing between traces on the circuit.
- Reducing the voltage.