How Do You Route Your Highly Constrained PCBs? (Part 1 of 2)

How routing is performed to meet the design intent of designers and engineers seems to be a topic of constant debate. Is manual routing better than automatic routing? Is designer-guided, computer-assisted (IOW auto-interactive) better? What’s the best way to produce the best board design?

To get a perspective from an industry routing expert, I asked David Price (president of DFM, a firm that specializes in providing interconnect routing services to PCB designers worldwide),  for his view on this topic. What follows is Part 1 of a 2-part routing blog series from Dave Price.

This always seems to me to be a straw-man argument, suggesting that there’s something particularly better about how the conductor on laminate was originated, as if how the cake was baked was more important than how it tasted. My experience is that we want to leverage the talent and skills of the designer and engineer to express their design intent as successfully as possible. In this case, it is really the destination that matters and we want to use all tools we can to get there.

The Allegro PCB design process is built upon the constraint-driven flow. Constraint Manager allows the design intent to be expressed as DRC rules that check, report, and confirm that the final routing meets all requirements for performance and manufacturability. At the end of the day, in a constraint-driven flow, if the design intent can be expressed as constraints, and the results satisfy these constraints, then how the routing is created should not matter—it’s a Black Box result.

Good routing needs to meet the constraints, but not all design intent can be expressed as specific constraints. Allegro provides three ways to express design intent in routing: performance and build constraints expressed quantitatively in Constraint Manager, planning and flow intent expressed qualitatively through the Interconnect Flow Planner (IFP), and design intent that can’t yet be expressed in constraints—routing judgments expressed elegantly and artistically in manual interactive routing.

Whether by autorouting, auto-interactive routing, or manual-interactive routing, the final routing wants to be good routing that meets design intent, is manufacturable, and does not impact yield in a negative manner. We want to produce final good routing by the quickest, most efficient, cost-effective means we can. I believe this is by using our best mix of these three approaches. The Allegro PCB Designer is built on getting the design done right the first time by offering a constraint-driven PCB design flow. Constraint Manager ensures that the final routing is good routing.

In this blog series, I will talk about different tools users can deploy from automatic routing to design planning tools to auto-interactive tools to plain and simple interactive etch editing tools. First, let me start with a powerful way to reduce the time to route your boards: autorouting with Allegro PCB Router (previously known as SPECCTRA). Next week, I will talk about how design planning can streamline and significantly reduce your time to get routing done on your boards. I will also talk about new auto-interactive capabilities that are introduced in Allegro PCB Editor and its options in SPB 16.6 Quarterly Incremental Releases (this capability will be shown in an upcoming webinar: Routing  Interfaces Quickly and Efficiently on PCBs (Part 2) on Sept 18.

Autorouting with the Allegro PCB Router (SPECCTRA)

The PCB router continues to evolve to meet contemporary routing and constraint challenges. PCB router works so well because it starts by connecting all signals (100% routed) and progresses to solve constraint conflicts. This algorithm eliminates the common complaint about other routers that remaining unconnects can’t be completed.

The Allegro PCB Router is very suited to all designs from small, less-constrained PCBs to very large, complex, highly-constrained PCBs—it’s very scalable. I start from the expectation that the Allegro PCB Router can route everything on any design until I can identify circuits and requirements that require auto-interactive or manual-interactive approaches. This is contrary to commonly held ideas that the router is used just for the simpler cleanup.

I use the router on the most demanding circuits, when it can solve these. That’s not always—but more often than not. The router capabilities continue to advance to meet today’s designs. It’s capable of true coupled-noise crosstalk control including both same layer and z-axis adjacent layer noise management, real-time relative timing, complex HDI routing, z-axis stub strategies, dynamic pin swapping, achieving challenging topologies, and sophisticated diffpair control. A version of the PCB Router is included in all Allegro and OrCAD PCB Editors, SI, and APD.

I most often route the entire board simultaneously. This makes autorouting different from interactive routing, which is by definition sequential—one circuit, then another. Sequential routing can still be done in the automatic autorouter when needed and appropriate. Key circuits that are especially demanding may need to be routed first. Examples might be DDR2 and DDR3, which I autoroute in the PCB router first. Real estate must be devoted to these so that additional routing accommodates itself to this routing. High-speed differential interfaces are often another example of route first.

Using the PCB router to improve your design process is not an either/or, all-or-nothing approach. There are many supplemental productivity improvements using the PCB router—fast assessments of fanout, placement, layer stackups, and routeability. Setting up and using the PCB router results in repeatability of standardized quality and quick-turn reroutes for ECOs.

I would like to hear from you on what you have tried with autorouters or other approaches to reduce the time to route dense boards (small or large).

Dave Price is president of DFM in Oakley, CA. DFM is a design and consulting firm providing interconnect solutions services to the OrCAD, Allegro, and general PCB engineering communities. DFM specializes in using and applying the Allegro PCB Router (known as SPECCTRA) to complex and highly-constrained designs using the Allegro constraint-driven flow. Dave is also a partner and product marketing director for BornToRoute in Jacksonville, FL, which provides products like MakeDo, MakeCAP, and MakeGood.