Signal Deviation
The gradual change in the characteristic impedance of a transmission line across its length or over its service life causes signal distortion in high-speed printed circuit boards. This deviation, known as trace impedance drift, occurs when physical properties like trace width, dielectric height, or resin moisture levels change. Small shifts in these parameters disrupt the uniform signal path, causing signal reflections that degrade high-frequency performance.
Environmental Driver
Moisture absorption by the substrate resin represents a primary driver of this impedance shift in finished boards. As the laminate absorbs humidity from the surrounding air, the dielectric constant of the resin increases, lowering the impedance of the trace over time. This effect is especially pronounced in environments with fluctuating temperature and humidity, which cycle the material through phases of expansion and contraction.
Even a small amount of absorbed moisture can shift the impedance of a fifty-ohm line outside its five-percent design tolerance, leading to transmission errors.
Fabrication Control
Fabricators minimize this drift by utilizing low-moisture resins and protecting boards with protective conformal coatings. They also utilize advanced impedance testing during quality control, using time-domain reflectometry to measure the uniformity of the trace along its entire path. This inspection helps identify out-of-tolerance boards before they are shipped to the customer.
