Transmission Stability
Electrical parameter variation measures the shift in the impedance of a transmission line from its designed value due to manufacturing or environmental factors. High-frequency signals are sensitive to characteristic impedance drift because any mismatch causes signal reflections and degrades the eye diagram. This drift often occurs due to variations in the dielectric constant of the substrate or changes in the width of the etched trace.
Consistency in the lamination process is required to keep these variations within the standard 10 percent tolerance.
Dielectric Variation
Material properties change when the circuit board is exposed to moisture or temperature fluctuations. Characteristic impedance drift is often driven by the absorption of water into the resin system, which increases the relative permittivity of the dielectric. High-humidity environments can cause the impedance to drop below the design target.
Selecting materials with low moisture absorption helps to mitigate these shifts in outdoor applications.
System Impact
Signal integrity depends on maintaining a constant impedance along the entire length of the signal path. When characteristic impedance drift occurs, it creates discontinuities that increase return loss and reduce the power delivered to the load. Testing with time domain reflectometry allows technicians to locate the specific areas where the impedance has shifted.
Proper stackup design minimizes the sensitivity of the circuit to these manufacturing variables.
