Isotropic versus Anisotropic Dielectric Models in Packaging Substrates
Anisotropic dielectric tensor modeling eliminates up to 8 ohm trace impedance errors and timing skew in high speed package substrates compared to isotropic models.
Trace Edge Fringing Fields
Electromagnetic Coupling
Electric and magnetic field lines that extend beyond the physical boundaries of a conductor influence the electrical behaviour of nearby circuits. The presence of trace edge fringing fields affects both the characteristic impedance and the level of crosstalk in high density PCB designs. These fields represent the energy stored in the space surrounding the copper features.
Crosstalk Mechanism
Energy escapes the direct path between the trace and the reference plane and spreads into the surrounding dielectric material. The intensity of trace edge fringing fields depends on the thickness of the conductor and the distance to the nearest return path. Thicker traces or those placed far above a ground plane exhibit more extensive spreading of the field lines.
Lateral extension of the energy can couple into adjacent traces even if they are physically separated by a gap. Numerical solvers are used to calculate the capacitance and inductance associated with these complex field distributions.
Impedance Control
Maintaining signal integrity requires careful management of the spacing between high speed nets to prevent interference. Large trace edge fringing fields increase the effective width of the signal path and lower the impedance of the line. Using thinner dielectrics or lower profile copper can help constrain the fields closer to the conductor.
Correctly accounting for this effect ensures that the final manufactured board performs as expected at high frequencies.