Silver migration is the ionic movement of silver between two adjacent traces. Silver migration inevitably results in a temporary electrical short. Silver migration is a phenomenon seen in many types of products and industries. It occurs in microelectronics, components, PCB assemblies and membrane switches.
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 and1980'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 usually required to create silver migration in a circuit using silver as the conductor. 1) A voltage potential between two traces. 2) The presence of an electrolyte such as moisture. The degree of contamination of lack of purity in the water can contribute to the rate and size of the migration pattern.
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.
- Modifying the silver composition with palladium or copper to reduce the tendency for silver migration, Cost and conductivity is the tradeoff. Other material suppliers have claimed to have developed a silver paste system that is not susceptible to silver migration. Such materials often have other issues that would pose a problem to a membrane switch designer and manufacturer, such as curing temperatures and mechanical properties.
- Covering the silver traces with an inert coating such as a protective carbon layer and/or an overcoat dielectric will prevent silver migration if effectively applied. Unfortunately the nature of the process and materials can allow pinholes, which provide avenues for silver migration to propagate. Proper material selection and process control can greatly reduce the chance of pinholes.
- Increasing the conductor spacing between traces that have a voltage potential will decrease the likelihood of silver migration. Reducing the voltage also reduces the risk.
- Preventing moisture penetration greatly reduces the risk of dendritic growth. Gasketing and sealing technology can stop the ambient penetration of moisture; elevated temperatures will make it necessary to utilize other methods to reduce migration.
- There are several areas of a membrane switch that are more susceptible to water damage because they are collection points for moisture or allow moisture to easily penetrate. Terminations, fold areas and reduced adhesion present higher potential for failure.
Silver Migration Tests
There are a number of tests that are recommended for silver migration..
- UL 796 specifies test conditions and methods. You can go to http://www.ul.com/pwb/silver.html for more detail on discussion of the UL task force to modify that document.
- Another industry standard available for Silver Migration is IPC F1996-01. This document applies to a Standard Test Method for Silver Migration.
- ASTM F1996-01 is also a test specific to testing for silver migration in membrane switches