I’ve never been much of an artist. Whether it’s a lack of creativity or poor fine motor skills, I’ve only rarely been able to produce drawings and paintings I felt satisfied with by the time the pieces were complete. One of the things that greatly impressed me when taking a few directed drawing classes was how the illustrator was able to explain how they were visualizing features that they were drawing: a “C” for an ear or a “?” rotated 180° for a nose. Recognizing the broader shapes that contribute to the total work – in essence, being able to see the forest and the trees simultaneously – is fundamental. While this is something that has eluded me, I’ve found I have some ability to enact this process with circuit design.
In this schema, circuit designers have to construct the functionality of individual circuit blocks while building toward the overall operation of the board. The best method for circuit design is simulation tools that grant designers access to the logic of millions of components, allowing them to swap parts and test system response quickly. As an added advantage, most tools can easily translate circuits used in simulation models into schematic documents for PCB layout. Incorporating circuit analysis simulation into design processes can eliminate many common design and manufacturing issues, saving time on unnecessary revisions and manufacturing cycles.
Circuit Analysis Simulation Improves the Schematic
Schematic circuit simulation has been available for a long time in SPICE tools available from different vendors. Many PCB CAD tools will come with their version of SPICE, while others will connect to third-party SPICE applications. SPICE, also known as a simulation program with integrated circuit emphasis, allows engineers to model their circuits virtually to predict how they will behave once complete. These tools can simulate the most basic passive components up to sophisticated semiconductors packages and are invaluable for the PCB design process.
Unfortunately, SPICE models are not controlled sets; while there should be no issue outputting and importing models between different SPICE software, the logic of two supposedly identical components is not guaranteed. Even within one version’s library, outdated models can cause problems. Whatever the cause, incorrect or non-representative models may lead to runtime issues whose origins are difficult to detect. With this caveat, designers still benefit tremendously from simulation models; ensure care is taken when using older libraries or third-party SPICE logic unless from a reputable source.
SPICE tools save design engineers time and expense during the design cycle that would normally be devoted to building proofs-of-concept and prototypes. While simulations are not a perfect model of real-world performance, they are much more precise than rudimentary development tools like breadboards, where noise is a massive hindrance.
Additionally, many of the complex high-speed circuits in PCBs can’t be accurately tested on a bench without equipment totaling hundreds of thousands of dollars. These same circuits, however, can be simulated using SPICE tools, allowing the design to be validated far in advance of the layout. This workflow style has the added benefit of enabling design departments to iteratively resolve complex circuit problems instead of building multiple prototypes.
Potential Printed Circuit Board Problems without Circuit Simulation and Analysis
The use of SPICE tools can help with design problems during schematic capture, but to analyze actual circuit performance requires software tools that will interface with the PCB layout; the physical layout of the circuit still must be refined to optimize performance. Formerly, the circuit analysis for these refinements required time-consuming and expensive testing of prototype circuit boards. However, design engineers can now analyze in real-time within the design toolset while the board is still being laid out.
Impedance
Impedance analysis proceeds before layout to determine the trace widths for the layer stackup of a circuit board. Powerful field solvers calculate the impedance of high-speed transmission lines using a database of dielectric materials and provide designers and fabricators with values for trace width and spacing. Some CAD software also offers impedance checking during layout to confirm that the routed transmission line traces are maintaining their targeted impedance values. Impedance analysis combats reflection and other signal integrity problems.
High-speed timing
DDR (Double-Data Rate) and other high-speed routing topologies depend on accurate trace lengths to support the timing requirements of their circuits to prevent issues like a length mismatch for differential pairs. These trace lengths must be matched to prevent one line from being faster than another and arriving at different times to some critical circuitry.
Crosstalk
Traces routed parallel over long distances or spaced inadequately can inductively couple; the magnetic field created by current traveling down a trace can induce an unintended current on the victim line. This condition, known as crosstalk, can create signal integrity problems that will degrade the board’s performance. Crosstalk analysis is a useful detection tool for determining when traces need more space, a different routing path, shielding, or some other signal integrity solution.
Return paths
The clear return path of high-speed signals on the reference plane is another essential signal integrity check to minimize the amount of electromagnetic interference (EMI) the board generates. Typically, the biggest offender is split-plane routing, where a signal references different planes over its travel path. Return path analysis tools can inform layout designers immediately if their boards will potentially generate too much noise, preventing design teams from submitting boards unlikely to pass costly EMI/EMC testing.
Power integrity
When laying out a circuit board, small details in the design, such as a power pin not having a wide enough trace or large enough copper pour to source the current effectively, may be easy to miss. There may even be enough metal to pass the design rule check, but performance suffers under standard operating conditions. Power integrity analysis tools can give the designer immediate feedback during layout to determine which pins need a stronger connection to satisfy the power requirements.
Design rule checks
Although not considered an analysis tool per se, a designer should always check the layout’s basic design rules and constraints before sending it out for manufacturing. Circuit board fabricators and assemblers often find simple errors that should have been caught in design but weren’t because the rules were either not set up correctly or ignored altogether. Thankfully, the design rules in today’s CAD systems are very comprehensive and often include a 3D display for checking for total object clearance verification.
PCB Design Simulation and Analysis Tips for Better Results
A few simple updates to a layout process can help ensure that you don’t get caught off-guard by some common errors when your design goes to manufacturing. Consider incorporating these circuit analysis simulation best practices into your workflow:
- Use the design rules and constraints built into your PCB CAD tools.
- If your schematic capture program has its SPICE tools built into it, use them. Even minimal checking will help more than not checking at all.
- The same goes for layout analysis tools. Use whatever features are already in your CAD system if you have them.
- Consider third-party simulators or analysis tools if your PCB CAD system doesn’t have its own.
- Upgrade your PCB CAD system to a more comprehensive suite of tools that will give you the simulation and analysis you need.
- Consider working with some external design services.
At VSE, we have been working with design engineers like you for over forty years. We understand the challenges and can offer recommendations on CAD tools and services that can help. We also staff an in-house engineering department that can mitigate many of your design problems. Our team of engineers aims to help build electronics for our customers; with our valued manufacturing partners, we aim to deliver the most robust circuit board possible.
