Quantifying Microstrip Phase Delay Variance Driven by Prepreg Glass Yarn Geometry
Prepreg glass bundle spacing causes local permittivity shifts that introduce picosecond phase delay variance in high-speed microstrip traces.
Relative Permittivity Variation
Permittivity Fluctuation
Differences in the ability of a material to store electrical energy across a circuit board degrade transmission uniformity. The relative permittivity variation describes changes in the dielectric constant due to laminate non-uniformity or environmental conditions. This behavior affects trace impedance.
Physical Cause
Multiple factors contribute to these fluctuations in the dielectric constant within a single substrate sheet. The primary cause of relative permittivity variation is the heterogeneous nature of the substrate, which combines glass fiber reinforced fabrics and polymer resins with differing dielectric properties. Environmental factors like moisture absorption and temperature changes also shift the relative permittivity of the resin matrix during operation.
Additionally, variations in laminate thickness during the manufacturing press cycle can cause the local glass-to-resin ratio to fluctuate across the board area.
Electrical Consequence
Signal skew and impedance mismatches represent the primary electrical issues resulting from these dielectric changes. When a high-frequency differential signal propagates along a transmission line, any local relative permittivity variation alters the propagation delay and leads to phase distortion. This distortion degrades the digital eye pattern, causing receiver errors and reduced signal margin.
System designs for high-speed digital and RF boards use materials with very low variation to maintain signal integrity over long transmission paths. Choosing stable laminate materials is essential for high-frequency circuit performance.