What causes electrochemical migration (ECM)?

Electrochemical migration (ECM) is primarily caused by the movement of ions in the presence of an electric field. ECM occurs in electronic circuits, particularly on printed circuit boards (PCBs), and can lead to the formation of unintended electrical paths and electrical shorts. Several factors contribute to the occurrence of ECM:

1. Voltage Bias: ECM typically occurs when a voltage bias is applied to a PCB or electronic circuit. This bias can be the result of power supply voltages, signals, or other electrical potentials within the circuit.
2. Ionic Contaminants: The presence of ionic contaminants on the PCB or within its environment is a key factor in ECM. These contaminants consist of ions such as sodium (Na+), chloride (Cl-), and others. Contaminants can be introduced during the manufacturing process or as a result of environmental exposure.
3. Electric Field: The application of a voltage bias creates an electric field within the electronic circuit. This electric field exerts a force on the ionic contaminants, causing them to move.
4. Ion Migration: Under the influence of the electric field, ionic contaminants can migrate, moving along the surface or within the PCB substrate. The movement of ions is driven by the attraction of ions of opposite charges towards the electrodes of the bias.
5. Path Formation: As the ions move, they can create unintended electrical paths or bridges (dendrite or dendritic structure) between conductive elements on the PCB, such as between adjacent traces, solder joints, or vias.
6. Electrochemical Reactions: When migrating ions come into contact with metal surfaces or conductive elements, they can undergo electrochemical reactions. These reactions may lead to metal dissolution (metal ions entering solution) or metal deposition (metal ions forming solid deposits). This can further exacerbate the formation of conductive paths and short circuits.
7. Humidity and Moisture: The presence of humidity or moisture can accelerate ECM by providing a medium for ion mobility and facilitating ion movement. Water or moisture can serve as a conductor for ions, increasing their mobility.
8. Temperature and Environmental Conditions: Environmental factors, such as temperature and atmospheric conditions, can influence the rate and severity of ECM. Higher temperatures and more humid environments can promote ion migration.

To mitigate ECM, it's crucial to implement good manufacturing practices, including proper cleaning of PCBs to remove contaminants, the use of conformal coatings to protect against environmental exposure, careful design to minimize the risk of unintended electrical paths, and voltage control to prevent excessive bias that could accelerate ECM. Understanding the factors that contribute to ECM and taking appropriate preventive measures are essential for ensuring the reliability and longevity of electronic components and assemblies.