How to Achieve a Robust PCB Design Workflow for Signal Integrity?

Signal Integrity (SI) signifies the signal’s ability to propagate along PCB traces without distortion. Signal integrity is about the quality of the signal passing through a   transmission line. In this article, Tim Wang Lee acknowledges some of the designers’ pain points regarding robust PCB design workflow and signal integrity. 

About the presenter:
Tim Wang Lee, signal integrity application scientist, Keysight Technologies.
Q1: How to achieve a robust PCB design?
Tim Wang Lee: To achieve a robust PCB design workflow consider the following points:

Understand possible root causes of signal integrity issues.
Create a virtual prototype using Electronic Design Automation (EDA) software.
Rely on a dependable manufacturer for layout, fabrication, and assembly and that will be Sierra Circuits .

Q2: What is the difference between traditional design and the robust design approach?
Tim: In a traditional design, we fabricate the prototype once we create a design and layout based on our vendor guidance. In the case of a robust design approach, we create a virtual prototype, simulate and validate the prototype after creating the design.
Q3: What are the advantages of having a virtual prototype?
Tim: A virtual prototype helps us to troubleshoot and understand the design in an efficient manner. Investing a little more time in creating a virtual prototype gives you a lot in return.  It also helps us to achieve better signal integrity. The advantages are summarized in the below image.

The effort is a little higher, but the ability of troubleshooting and performing what-if-analysis increases.
Q4: What is the robust design workflow for signal integrity?
Tim: The signal integrity workflow is as follows:

Rules of thumbs estimations – this addresses signal integrity issues
Create a virtual prototype
3D extraction
Perform what-if-analysis

The workflow diagram is as shown below:

Q5: What are the rules of thumbs to be followed?
Tim: The rules of thumbs are as follows:

8-7-3 rule: 

8dB loss at Nyquist, Eye with 75% UI jitter, 30% height

0.1-0.2 dB/inch/GHz:

 0.1-0.2 dB of loss per inch per GHz for transmission line

Single-ended 50 ohms (FR4)

Microstrip impedance: Width/Height = 2 – width of the trace is twice the substrate height.
Stripline impedance: Width/Height = 1 – width of the trace is the same as min of the substrate heights.

Differential 100 ohms (FR4):

Spacing  = 3W uncoupled ; Differential impedance = 2 times single-ended impedance.
Move the traces closer, this decreases differential impedance.

    Stub resonance (GHz) for FR4 is 1.5/stub length (inches)...