Routing traces around on your PCB is like knitting a quilt. Not only can it take forever, but it requires very precise steps to ensure that all your stitches fit together seemlessly. If you get your stitches wrong, then everything can fall apart.
Just like we have automatic threading sewing machines that increase productivity, autorouters can do the same for your PCB. But be wary: not all autorouters have the same capabilities. The best PCB autorouter will do more than just route traces to match your schematic.
Manual vs. Automatic Routing
Routing is one of the most important PCB design activities. Proper routing techniques ensure that your board has some level of EMI immunity, is not susceptible to problems like ground bounce or ground loops, and looks like a true work of art. With simpler PCBs that use lower speed signals and do not include high pin count components, you can route your traces by hand in a reasonable amount of time.
Once you start working with high pin count components that require dense routing, doing everything by hand takes a significant amount of time. This also make it difficult to comply with your design rules; you’ll have to manually verify that nearby traces are spaced properly, that appreciable skew does not accumulate, and that traces behaving as transmission lines are impedance matched.
The situation becomes more complex when you start working with differential pairs. Differential signaling reduces crosstalk between signal and return traces, ensuring that your signals have high signal-to-noise ratio. A single pair of traces needs to be symmetric and properly spaced in order to ensure maximum crosstalk suppression. Your PCB design tools should make it easy to measure trace lengths on your board to ensure length matching and symmetry.
Working with signal nets requires that this level of precision be enforced across all interconnects in a net. Enforcing length matching during routing is especially important with high speed signals as this ensures your signals reach their destination without accumulating significant skew. Working with differential pairs in a signal net requires that all pairs have consistent spacing and symmetric meandering.
This is where automated routing tools become invaluable. The best PCB autorouters save you a significant amount of time during routing, especially when working with high pin count components. If you find that you need to move a component after you’ve placed your traces, a great autorouter will reroute your interconnects automatically.
Find the routing system that works with you
Pin Swapping With an Automatic Routing Tool
If you’ve ever designed a schematic for a PCB that includes a high pin count device like an FPGA, you’ve probably created your schematic without considering the board layout. This is understandable: there are already so many connections to keep track of in your schematic, and the schematic needs to be clear and legible.
Once you start laying out your components and traces, your board layout will look nothing like the order you enforced in your schematic, and you’ll likely have nets criss-crossing each other. The initial situation will likely be unroutable, and you’ll need to reroute all your connections between components. Since newer FPGAs and other components on BGAs use anywhere from 100 to 1000 connections, rerouting all of these connections by hand is a major task.
Once again, the autorouter saves the day. The best PCB autorouters have automated pin swapping capabilities, making it easy to rearrange your pinout to suit your needs. Laying out the rest of your components around one or more high pin count components can be difficult or even impossible without pin swapping. Automated pin swapping saves you from a near-complete redesign or re-routing by hand.
Routing around high pin count boards can be a total pain without an autorouter
Autorouting and Design Rules
Most PCB design software programs come with some sort of automated routing feature, but not all autorouters are created equal. The best autorouters easily interface with your design rule checking features, ensuring that your layout meets your critical design specifications.
The best PCB autorouters actually integrate your design rules into the layout process. Specifications like length tolerance, symmetric meandering, and trace spacing will be automatically considered during design, and your autorouter will place traces on your board that meet these critical design rules. This saves you time when verifying your design.
Working with differential pairs spread across signal nets carries and routing through vias carry their own design rules. Autorouting systems that are built around satisfying design rules don’t care what type of interconnect you are placing on the board. It’s all about ensuring that your routing strategy meets these critical rules while saving you routing time.
If routing interconnects between high pin count components manually doesn’t sound like your cup of tea, routing can be made simpler and faster with a great PCB design package like Altium Designer®. The ActiveRoute® tool and design rules engine in Altium can help ensure that your traces remain length-matched and meet your design specifications.
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