The basis of electronic design is the creation of a circuit followed by testing that circuit to see if it performs as intended. I have memories of my father armed with a soldering iron and a multitester building all manner of electronic projects and then testing them to see if they worked. And while his love for electronic design certainly sparked the same interest in me, today’s PCB designs call for a more advanced level of circuit validation.
Many software tools are available for circuit testing, including simulation and analysis programs that work with your PCB CAD system. But as commonplace as these tools have become, many designers still aren’t using them. This reluctance can have a serious downside in terms of wasted design time, added expense, and the potential for poor electrical performance or outright failure of the circuit board. Here we’ll continue to look at common PCB issues and examine some of the printed circuit board problems that circuit simulation and analysis can help you avoid.
The Downside of Not Using Circuit Simulation and Analysis in 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, or “simulation program with integrated circuit emphasis,” allows engineers to model their circuits virtually to predict how they will behave once completed. These tools will simulate the most basic passive components up to sophisticated semiconductors and are invaluable for the PCB design engineer.
SPICE tools depend on their models, and here is where some problems arise. The quality of these models can vary, and the models themselves can vary between different versions of SPICE. In some cases, an out-of-date model or one that doesn’t fit the version of SPICE being used can generate false results when running the simulator.
SPICE tools can save design engineers time and expense during the design cycle that would normally be devoted to building breadboards and prototypes. Today, many of the complex high-speed circuits in PCBs can’t be accurately tested on a test bench without equipment costing hundreds of thousands of dollars. However, these same circuits can be simulated using SPICE tools, allowing the design to be validated before the layout. This workflow style has the added benefit of enabling design departments to iteratively resolve complex circuit problems instead of building multiple prototypes.
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. We’ll look more into those tools next.
Potential Printed Circuit Board Problems without Circuit Simulation and Analysis
While SPICE circuit simulators can tell you whether or not the circuit concept will work, the actual physical layout of the circuit still needs to be refined for the circuit board’s maximum performance. The circuit analysis for these refinements also used to require time-consuming and expensive testing of prototype circuit boards. However, design engineers can analyze from within the design tools while the board is still being laid out.
Impedance analysis must be done before layout to determine the trace widths for the layer stackup of a circuit board. To control the impedance of high-speed transmission lines, it is essential to carefully calculate the board dielectric materials and width and the width and spacing of the traces. Some CAD tools also offer impedance checking during layout to confirm that the transmission line traces being routed are maintaining their target impedance values. Impedance analysis is important for combatting reflection and other signal integrity problems.
DDR (Double-Data Rate) and other high-speed routing topologies depend on accurate trace lengths to support the timing requirements of their circuits for the best performance of the board. Many of these trace lengths also need to be matched to prevent one line from being faster than another, and the CAD tools will analyze the routing and report any problems.
When a strong signal overpowers a weaker signal due to its proximity, it can result in the weaker signal mimicking its behavior instead of accomplishing its task. This condition is known as crosstalk, and it can create signal integrity problems that will degrade the board’s performance. Crosstalk analysis is an essential layout tool that designers should be acquainted with and regularly use.
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 is generating. Return path analysis tools can tell layout designers immediately if their boards will potentially generate too much noise.
When laying out a circuit board, it can be very easy to miss small details in the design, such as a power pin not having enough metal for a robust connection. Although there may be enough metal to pass the design rule check, in reality, there isn’t enough to support the amount of voltage that the pin will conduct. Power integrity analysis tools can give the designer immediate feedback during layout and determine which pins need a stronger connection to satisfy the power requirements.
Design rule checks
Although not considered an “analysis” tool, 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 not used at all. Thankfully the design rules in today’s CAD systems are very comprehensive and often include a 3D display and checking for a complete object clearance verification.
PCB Design Simulation and Analysis Tips for Better Results
Here are some tips that can help ensure that you don’t get caught by some of these errors when your design goes to manufacturing:
- 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 that you need.
- Consider working with some external design services.
At VSE, we have been working with design engineers like you for over thirty years. We understand the challenges, and we can offer recommendations on CAD tools and services that can help. We also staff an in-house engineering department that can mitigate many of the design problems you face. Our goal is to help you design the most robust circuit board possible to get the expected electrical performance when we build it for you.