Silver is a metal commonly used in many electronics and microelectronic applications including hybrid electronics, microelectronic components and membrane switches. Silver is a very attractive metal to use because it has good conductivity, durability and relatively low costs. It is much less susceptible to oxidation than bare copper and in fact silver oxide is actually more conductive than the silver itself. It has been used in polymer thick film membrane switches for over 30 years. Silver based conductive compositions are usually screen printed to form the conductive traces in membrane switches.
One of the problems associated with silver is a phenomenon known as dendrite growth or silver migration. Silver migration is the ionic movement of metal, usually between two conductive traces where an electric potential exists. Given enough movement, the migration will cause a short between two traces and render the electronic device useless. Because silver has a free ion it tends to be more active than other metals, thus more susceptible to the dendrite growth.
Silver migration will occur if there is moisture present between two traces with a potential. The rate of migration depends on the amount of moisture, the temperature and the voltage. Silver migration can be created in a test lab by applying a droplet of water between traces with a voltage potential. The migration will happen within seconds.
Membrane switches that are exposed to high humidity or moisture conditions are more likely to experience silver migration. I am personally aware of a major silver migration field failure issue that occurred in a microwave oven. This was back in the early 1980’s and the application was stove top microwave oven. Boiling water on the stove created the moisture conditions. Additionally the configuration of the assembly made this particularly susceptible to moisture collection. The “pig tail” was crimped to the main body of the membrane switch, which actually formed natural collection points for moisture. A simple design change actually eliminated the crimps and the field failure dropped to near zero.
The following are some common techniques used to reduce or eliminate silver migration.
- There are several new polymer thick film pastes available that contain palladium, which has been demonstrated to significantly reduce (but not eliminate) the tendency of silver to migrate. Cost and performance may be a trade off with these pastes.
- Protecting the traces with a carbon coating or dielectric can be effective if the proper material is adequately applied. Some materials tend to be porous and are susceptible to small “air gaps” that allow moisture to penetrate.
- Increasing the conductor spacing between traces that have a voltage potential will decrease the likelihood of silver migration.
- Keeping moisture from penetrating the membrane switch can be effective in preventing migration. There are several methods and materials available that can effectively seal a membrane switch from its environment.
Silver migration is a potential problem that needs to be addressed in the design and manufacturing process.