Introduction
Those who may be looking for a signal integrity (SI)/power integrity (PI) simulation software package should have an idea of what functionality and other characteristics they can expect from such a tool during both pre-layout design and post-layout design. This article briefly describes the most important post-layout elements when selecting a SI/PI software simulation package. The most important pre-layout design simulation software elements were addressed in a previous article (Reference 1).
Post-layout vs. Pre-layout SI/PI Simulation
For post-layout design, you run simulation to validate performance after the design is complete and you have prototypes in hand. This step occurs after pre-layout activities have been successfully completed. Post-layout is used to find post-layout design problems that may have been missed during the pre-layout design phase or to verify that any revisions made to the design have not introduced any SI or PI problems that didn’t exist before the design change.
For pre-layout design, you run simulation and sweeps to help plan key design elements and to define routing constraints prior to building actual prototypes. Pre-layout simulation helps you design nets with good SI and design power distribution networks (PDNs) with good PI.
Post-layout SI/PI Design Simulation
Post-layout SI/PI design simulation is all about finding post-layout SI/PI design problems, i.e., when you have first prototypes in hand and are bringing up boards for the first time or later on in the product development cycle, such as during verification or functional testing where you might discover signal integrity, power integrity, crosstalk or timing related issues.
The post-layout SI/PI simulation software you use should be able to simulate “what if” design variations that help you determine design changes that improve both SI and PI. At a minimum, the SI/PI simulation software used should be able to help you find, fix, measure, evaluate, and re-design as required in the following simulation areas:
- Measure bypass quality for single-ended signal vias.
- Measure interaction between single-ended signal vias and the PDN.
- Measure PDN noise from IC power draw (find PDN locations that require better decoupling).
- Measure PDN impedance (Z) at key locations on the board (find minimum number of capacitors needed to meet target PDN Z).
- Measure DC power loss and current density (find metal areas and stitching vias with excessive current).
- Evaluate the electrical behavior of interconnect and signal vias in the frequency domain (measure return loss and insertion loss for signal nets).
- Measure bit error rate (evaluate bathtub curves and BER for a SERDES channel).
- Evaluate the eye diagram for the channel.
- Measure Z for trace segments and signal vias.
- Measure timing for signal nets (use results to complete a timing budget).
- Measure signal quality characteristics for one or more nets.
- Measure crosstalk between signal nets (caused by coupling from switching aggressor nets).
- Find nets with incorrect timing (screen the entire interface or find nets with wrong flight times).
- Find nets with poor signal quality (excessive overshoot, ringback, non-monotonicity, etc.).
- Find nets with excessive crosstalk.
Other Important Attributes of Post-Layout SI/PI Simulation Software
The SI/PI simulation software should have the flexibility that allows you to screen an entire design for problems, simulate only a group of critical nets, or only verify design revisions have not introduced problems on critical nets.
As noted above, the SI/PI simulation software should also provide detailed and up-to-date documentation that carefully describes how to perform various simulations. It should be relatively easy to learn and use. It won’t get used if it’s challenging to learn and use, and your designs will suffer.
Finally, post-layout SI/PI simulation software should be reasonably priced. If priced too high, it will be hard getting management’s approval to purchase it. If priced too low, it may not be of high quality or provide all the features needed to perform effect post-layout SI/PI simulation.
References and Further Reading
- Pre-layout Signal Integrity/Power Integrity Simulation Software Expectations, In Compliance Magazine, (July 2021).
- HyperLynx® SI/PI User Guide, Software Version 9.4.1
