PCB Routing Topologies Demystified
You’ve got your schematic and you know which pin should be connected where—it’s time to figure out how your components and traces will physically be laid out onto a board. Is it really as simple as connecting point ‘A’ to point ‘B’ until you’ve got a complete circuit layout that fits within the physical dimensions of your board?
The truth is that there’s more to PCB routing than ensuring continuity across your board. EMI/EMC considerations such as cross-talk, reflectance, and RF, to thermal considerations such as throttling, to power draw requirements for different components, are some of the many factors that go into designing PCB routing topologies. In this post we’ll cover the basics of routing topology in the context of PCB design.
Different types of routing topologies
PCB routing topologies are simply configurations for laying components and traces on a board. In many ways, a PCB can be thought of as a network—data must flow from one point to another, and it’s important to optimize routing topology to ensure signal integrity remains consistent throughout your board.
It’s perhaps for this reason that you’ll hear many PCB designers borrow terminology from network topology to describe the way traces and components are laid out on the board. Let’s take a look at the more common types of routing topologies:
By far the simplest type of routing topology is to simply make a direct connection between two points. The shorter the return path, the more efficient the circuit. The tradeoff is to figure out how to ensure every point in your board meets its corresponding match while accounting for differential pairs, terminations, EMI considerations, and physical dimensions and tolerances. It isn’t usually physically feasible for every point to get a direct line to where it needs to go, and so many topologies have evolved to address these different situations.
Daisy chaining is the act of linking multiple points together in series. In a linear daisy chain, the first point is connected to a second point, a third point is connected to the second point, and so on until the circuit is returned to ground. In a ring topology, the points are daisy chained into a ring. Daisy chaining is popular because it saves space.
“Fly by routing” is a popular type of linear daisy chaining used to link multiple DDR3 and DDR4 memory chips together, where terminating resistors or stubs are included on each unit to prevent reflection and improve signal integrity.
A star routing topology involves connecting multiple points to a central hub. In the context of PCB design this terminology is usually used to refer to the use of a central pad or via to link multiple points in a circuit to power or ground. This is useful when you don’t want any of the components impacting the power draw or signal as they would in a daisy chain. The idea is to provide a direct line for each point to power or ground.
The bus topology needs no introduction. It’s one of the most popular and widely used routing topologies in the electronics world, and involves connecting multiple points to a single power/signal line or component. Data, address, and control buses are just some examples of common architectures encountered in the PCB world.
General guidelines for PCB routing
There are a lot of factors that go into routing a PCB besides which topology you choose to use. Pad and trace thickness must be optimized for current carrying capacity, EMI/EMC considerations and manufacturability. Tolerances and clearances must be selected to prevent arcing and reduce the chance of shorts caused by solder runoff. That’s a lot of variables to juggle when routing your PCB designs.
Fortunately, modern PCB design tools make it significantly easier to manage all these routing considerations. Auto routers let you apply tolerances, clearances, and optimal cornering automatically as you link points together throughout your design.
Real-time signal analysis can help you further optimize your routing topology to eliminate reflections and improve signal integrity throughout your design. Eager to take your PCB routing topologies to the next level? Check out Cadence’s suite of PCB design and analysis tools today.