Which Net Requires Shielding: Clock Net or Signal Net, and Why?

Reason:

If we do shieding to clock net, will it be increase rise time and fall time for the clock signal by coupling cap?

Yes, You are absolutly right, shielding the clock net can increase the rise time and fall time of the clock signal.

When you add shielding (usually grounded wires) around the clock net, it increases the coupling capacitance between the clock net and the shield. This additional capacitance slows down the signal transitions, leading to longer rise and fall times. While shielding helps reduce noise and crosstalk, it can also introduce more load on the clock net, which in turn can degrade the signal’s switching speed.

To balance this, designers may need to carefully size the buffers or drivers in the clock tree to account for the extra capacitance. It means during circuit design only we need to consider the capacitance value due to shiled net.

Thanks for answer, but i have anothor question, Why do we give priority to clock shielding, even though it increases coupling capacitance for the clock net?

We prioritize clock shielding even though it increases coupling capacitance because:

Clock Signal Integrity: The clock is the most critical signal in a circuit, and even small noise or interference can lead to timing failures. Shielding ensures the clock signal remains clean and free from crosstalk.

Minimizing Skew and Jitter: Shielding helps reduce clock skew and jitter, maintaining consistent timing across the chip.

Prevent Crosstalk and Noise: The clock signal is highly sensitive to noise and crosstalk because it drives many critical elements. If the clock gets distorted due to interference, it can lead to timing violations like clock skew, jitter, and incorrect data capture.

Although shielding may slightly increase coupling capacitance, the benefits of improved clock reliability and performance outweigh the downsides, making it a priority in design.