CSI Keyboards uses copper flex circuitry in the majority of our custom keypad designs due to its excellent dielectric strength, thermal stability, chemical resistance and flexibility. Copper flex, also known as Kapton circuits, have become the superior choice over printed silver especially for outdoor applications.
A printed silver circuit can be replaced with a copper and polyimide construction with minimal additional cost. Copper flex circuitry construction designs offer a significant advantage over printed silver.
Copper Flex membrane switch panels are produced using polyimide (Kapton) as the base material. Copper flex keypad switches are manufactured by laminating a thin sheet of copper to a flexible film substrate. The copper is then chemically etched away, leaving the copper traces. An additional layer of polyimide is laminated to the circuit leaving the gold contacts exposed.
The biggest case against using printed silver circuitry and thus using copper flex circuity is a phenomenon known as silver migration which occurs in microelectronics, components, PCB assemblies and membrane switches. Silver migration is the ionic movement of silver between two adjacent traces that inevitably results in a temporary electrical short.
Silver is a very active metal and is thus highly susceptible to silver migration or dendrite growth. Yet it is also a very cost effective metal for the electronic industry because of it’s conductively and usability. With the reduction or elimination of lead in electronics, silver is a very attractive choice because of its solderability and conductivity.
Silver Migration with Membrane Switches
Silver migration in membrane switches was a much bigger problem in the 1970’s and 1980’s; mainly because of the technical inability of the membrane switch manufacturers. In some cases these manufacturers were graphic screen printers who could screen print silver paste, but had little understanding of electronics or reliability issues associated with the electronics industry.
Today, with competent membrane switch manufacturers, silver migration is less of a problem. However, there are situations such as severe environments or design constraint issues where silver migration is still a risk. As in all aspects of electronics, the industry drive to reduce space and reduce costs with increased functionality continually pushes the envelope for designers and manufacturers of membrane switches.
Causes of Silver Migration
Two factors are typically required to create silver migration in a circuit using silver as the conductor:
1) A voltage potential between two traces.
2) The presence of moisture.
Ways to Reduce or Prevent Silver Migration
Some or all of the following solutions can be used to reduce or prevent the occurrence of silver migration.