Parasitic Parameter
Transmission lines located on the outer layers of a printed circuit board experience a localized capacitive coupling to the reference plane. The microstrip surface capacitance represents this charge storage capability, which is influenced by the surrounding air and solder mask. It affects the characteristic impedance of the trace by lowering the impedance when the capacitance increases.
This value is critical for predicting the propagation velocity of high-frequency signals.
Solder Mask Effect
Applying a protective liquid photoimageable solder mask alters the dielectric constant of the medium surrounding the copper traces. Solder mask has a higher dielectric constant than air, which increases the surface capacitance of the microstrip. This additional capacitance lowers the characteristic impedance of the trace, which must be compensated for during design by narrowing the trace.
Computational models must include the solder mask thickness and dielectric properties to avoid impedance errors. This step is necessary to achieve high signal fidelity in production.
Circuit Loading
High surface capacitance can load the signal path and degrade rise and fall times in fast digital circuits. This capacitive loading can also cause impedance discontinuities at via transitions and connector launch points. Design verification must account for this parameter during pre-layout simulation.
