Via impedance plays a critical role in signal integrity. When the via impedance doesn’t match the transmission line impedance, reflections can occur, with their magnitude depending on the difference at the transition point. To minimize impedance discontinuities, consider implementing a coaxial via structure with a signal pad surrounded by multiple ground vias. Use tools like a via impedance calculator to accurately estimate capacitance, inductance, and impedance.
Managing via stubs is equally important to avoid signal attenuation caused by resonance. A via stub, the inactive portion of a via, forms a resonant circuit at specific frequencies. To mitigate this, ensure the signal’s maximum frequency is significantly lower than the stub’s resonant frequency or use backdrilling to eliminate stubs altogether.
When backdrilling, follow these DFM tips:
- The backdrill diameter should be slightly larger than the primary drill diameter, with a three mil tolerance.
- Maintain a minimum ten mil clearance between the backdrill and the trace or plane.
- A maximum stub length of ten mils can be retained without significantly affecting signal integrity.
Advanced backdrilling techniques now allow for stopping within one mil of the copper layer, ensuring precision without layer penetration. With careful design and implementation, via structures can be optimized for high-speed signal performance.
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