Physical Mechanism
Trace pathways of copper on a printed circuit board can slowly reshape when exposed to electrical currents and moisture. The electrochemical phenomenon of line migration occurs when metal atoms ionize and travel from an anode to a cathode, forming conductive filaments. These dendritic structures grow across the dielectric substrate, reducing insulation resistance and causing sudden short circuits.
This failure mode is a major reliability concern in high-humidity environments where boards are under continuous electrical bias. The spacing between adjacent tracks and the clean state of the board surface determine the voltage threshold at which this movement begins, making wash cycles after soldering necessary.
Testing Protocol
Reliability labs evaluate the risk of dendritic growth by applying high voltage under controlled thermal conditions. Testing for line migration requires placing the board in a temperature-humidity-bias chamber for hundreds of hours while monitoring current leakage. Sudden spikes in current indicate that a filament has bridged the gap between traces.
These studies allow manufacturers to certify their boards for demanding industrial or automotive applications.
Prevention Strategy
Protective measures stop the growth of metallic filaments by blocking exposure to water and contaminants. Designing assemblies with wider trace spacing and applying protective conformal coatings prevent line migration in harsh environments. Conformal coatings seal the copper traces against moisture, halting the electrochemical reactions.
Using solder masks and high-quality laminate resins also hinders the migration path of the metal ions.
