Dielectric Loss
Material property shifts in epoxy resins characterize the loss of insulating efficiency as a board ages under high thermal loads. Dielectric constant degradation describes the reduction in the ability of a laminate to store electrical energy (relative to a vacuum) due to chemical decomposition. This process occurs when the polymer chains in the resin break down and alter the capacitive behavior of the substrate.
Polymer Aging
Thermal stress causes the release of volatile compounds and the oxidation of the resin matrix which changes the polarity of the molecules. Dielectric constant degradation happens faster in environments where the board operates near its glass transition temperature for extended periods. The speed of the change depends on the specific chemistry of the resin and the presence of moisture within the board layers.
Impedance Shift
As the dielectric constant drops, the characteristic impedance of the traces increases and changes the signal timing of high-speed circuits. Engineers monitor dielectric constant degradation by testing sample coupons that have been subjected to accelerated life testing in environmental chambers. Because the change is permanent, the board may eventually fail to meet signal integrity requirements even if the temperature returns to normal.
Long-term reliability models use these measurements to predict the functional lifespan of the electronics in harsh industrial applications.
