If you work in the electronics industry or in research, there’s a chance that simulations are a regular part of life. Simpler systems can be designed with intuition and simulated after designs are completed, but more advanced systems running at high frequencies or very high data rates need qualification before and after a PCB layout is complete. Simulation software has to take a more prominent role in PCB design for many advanced systems.
Unfortunately, many simulation tools are not so intuitive for most designers as they were not built for use by PCB design software users. The tide is turning and these systems are getting much better in terms of usability, but their use within the design process is what makes simulation tools so powerful.
Simulation-driven design for electronics starts by creating an interface between your design tools, data management system, and simulation applications. Today’s professional electronics design teams are multifunctional with experience spanning across electrical, mechanical, thermal, and reliability disciplines. Design teams need systems that help them quickly share physical design data, export simulation models, and run design evaluation simulations.
The simulation-driven design process for PCBs spans three broad areas and follows a particular process:
The process is iterative, which is why I draw arrows pointing back to previous steps. Any problem identified in circuit simulation results can force you to go back into the schematics and modify yout circuit designs. At the PCB simulation stage, the results may indicate required modifications in the circuitry, PCB layout, or both. This can be the case with EMI simulations, SI/PI, and thermal simulations; all of these results may indicate required changes in your circuits, which can then force you to make changes in the PCB layout.
SPICE users know all about circuit simulations. A huge range of important behavior can be examined and evaluated in SPICE simulations, both in the time domain and in the frequency domain. SPICE simulations are the mainstay of circuit designers, and the basic
All of these tasks can be performed in SPICE simulations as long as model definitions for components are available. Any of the above listed areas could take up space for its own article, so I won't go into those points here.
Systems that require digital signal integrity or RF signal simulations at the circuit or schematic level tend to be much more advanced and they require an equivalent circuit model or linear network defining the behavior of that structure. Simulations with these structures in your circuits use network parameters, typically ABCD parameters or another linear network parameter set that allows simple cascading between linear components.
Linear network models for these structures can be extracted with a simulation program like Keysight PathWave ADS, Simbeor, Ansys, or CST Microwave. An example extracted linear network determined from a package design on a PCB is shown below.
Extracted linear network for two component packages and their interconnect on a PCB. Image courtesy of Simberian.
Once this linear network or equivalent extracted circuit is brought into your circuit simulator, you can simulate the performance of your intended interconnect design. This type of simulation process is a standard approach used to understand behavior of interconnects on fast digital buses, such as DDR, PCIe, MIPI standards, etc. Once the intended design is qualified, you can use this in your PCB layout along with your other components.
Circuit simulations assume a perfect world, where a circuit is isolated from all other circuits. Real effects involving noise, signal coupling/crosstalk, and radiated/conducted emissions need to be determined at the PCB level once the layout is finished.
Once the PCB layout is complete, board-level simulations are performed to evaluate important interconnects, power, EMI/EMC, and temperature. Not all parts of the PCB need to be simulated; the most important circuits that were examined in circuit simulations should also be examined in post-layout simulations where possible. For example, this could include:
The list of specific board-level simulations that could be performed is quite long, but the goals are always the same: ensure that the placement of parts and interconnect designs in the PCB layout does not affect the performance of your system as it was qualified in circuit simulations. It's important to compare the pre-layout and post-layout results to ensure the device is not greatly affected by the elements placed into the PCB layout.
Much of the time, this relates to signal integrity, and this area of design typically receives major focus. Here are can example simulation results that you can generate from your Altium PCB layout:
Eye diagram comparisons, S-parameter simulations, and impedance calculations are some of the main tools used to qualify interconnects for digital signal integrity.
Other areas where post-layout simulations are very important at the PCB level are power integrity and thermal integrity, both of which relate to functionality and reliability, respectively. However, things can change yet again once the design is put into its completed assmbly and intended enclosure/packaging. This is where collaboration with another group of simulation engineers is needed to qualify mechanical and thermal performance, as well as potential for EMC failure.
Once the board is fully assembled, the thermal demands can change somewhat, and the board may need to be qualified mechanically to ensure its reliability, ability to withstand shocks, or ability to withstand vibration. These are just a few examples of the mechanical points to examine in a completed assembly.
These aspects of qualifying an assembly are performed using more advanced field solvers and they do not directly involve your PCB design and layout software. The PCB design tool you use should provide a compatible mechanical or electro-mechanical export file that can be used in your field solver application.
For airflow simulations, a CFD-thermal co-simulation application is needed, and this is typically performed by a multiphysics specialist. An example involving the completed assmbly, complete with