Dielectric Degradation
Ion migration across the surface or through the bulk of a printed circuit board forms a metallic pathway between biased conductors. This phenomenon, known as conductive anodic filament growth, occurs when copper ions dissolve from an anode and redeposit at a cathode under the influence of an electric field. The pathway forms along glass fibre bundles or at the interface between the resin and reinforcement within the epoxy matrix.
Humidity and elevated temperature accelerate the dissolution of metal, while the presence of halides or other ionic contaminants provides the electrolyte needed for ion transport.
Mechanism Detail
Moisture absorption by the substrate resin lowers the glass transition temperature and creates a path for ionic species to migrate. Acidic flux residues trapped during assembly often react with the epoxy matrix to weaken the bond between the glass fibers and the resin. Once the bond degrades, these gaps act as capillary channels for moisture to penetrate deep into the board internal structure.
Electrical potential drives copper ions through these microscopic channels until a solid metallic bridge connects two isolated circuits. Short circuits happen suddenly when the filament bridges the gap, which leads to catastrophic failure in high density interconnect boards. The growth rate remains proportional to the applied voltage gradient and the level of ionic contamination present at the interface.
Reliability Impact
Dense circuit patterns increase the probability of connection failures because the spacing between features shrinks below the threshold required to withstand typical operating voltages. Qualification testing involves exposing boards to high temperature and high humidity while applying a continuous bias voltage for long durations to measure insulation resistance drops. Boards pass these tests only if the leakage current stays below specified limits throughout the duration of the test.
Design engineers mitigate risk by increasing the physical distance between conductors and selecting substrate materials with superior resistance to hydrolysis. Selection of laminate materials with low moisture absorption properties prevents the formation of conductive anodic filament growth.
