Silver is a metal commonly used in many electronic 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 more conductive than silver itself. It has been used in polymer thick film membrane switches for more than 30 years. Silver-based conductive compositions are generally screen printed to form the conductive traces on the 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 the metal, generally between two conductive traces where an electrical potential exists. With 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, making it more susceptible to dendrite growth.
Silver migration will occur if moisture is present between two potential traces. The migration rate depends on the amount of humidity, temperature, and voltage. Silver migration can be created in a test lab by applying a drop of water between traces with a voltage potential. The migration will happen in a few seconds.
Membrane switches that are exposed to high humidity or damp conditions are more likely to experience silver migration. I am personally aware of a major silver migration field failure problem that occurred in a microwave oven. This was in the early 1980s and the application was a stovetop microwave oven. The boiling water on the stove created the humid conditions. Furthermore, the configuration of the assembly made it particularly susceptible to moisture build-up. The “pigtail” was crimped to the main body of the membrane switch, which actually formed natural moisture collection points. A simple design change really did eliminate the kinks and the field fault was reduced to almost zero.
The following are some common techniques used to reduce or eliminate silver migration.
- There are several newer polymer thick film pastes available that contain palladium, which has been shown to significantly reduce (but not eliminate) the tendency of silver to migrate. Cost and performance can be a tradeoff with these pastes.
- Protecting traces with a carbon or dielectric coating can be effective if the proper material is properly applied. Some materials tend to be porous and are susceptible to small “air gaps” that allow moisture to penetrate.
- Increasing the separation of conductors between traces that have a voltage potential will decrease the probability of silver migration.
- Preventing 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 issue that must be addressed in the design and manufacturing process.