What is PCB Routing?

PCB routing is a very important step when designing PCBs. It usually builds on the preceding step, referred to a placement, which tells the location of each PCB component. PCB design comes with a quality similar to that of games, most especially when we talk about routing. Here, we are dealing with lines, colors, and shapes on a screen and the goal is to connect everything before the space is used up.

Seriously speaking, completing a PCB design routing using PCB router bits could prove difficult. Some people may see it as a game, but it is not. Rather, it requires much skill and patience from the designer. How can PCB designers be sure of success while routing? Just like all other things, the secret here is making use of the best tools. Here, we’ll be considering some of the challenges that designers usually face while routing, and the routing tools that can be of help.

Using PCB Routing to Connect the Nets

The conversion of schematic nets to physical traces on PCBs has for long been the layout engineers’ major responsibility. This was usually done manually, making designers go through a drawing and redrawing process of their circuitry at enlarged sizes on gridded mylar sheets.

When you are ready, the circuit drawings will be photo-reduced and covered using opaque tape so as to create the right tool for fabrication. For years now, the requirement for performance of new electronics had to force the utilization of board elements that are smaller, which could not be created accurately by making use of tape. Due to this reason, along with many others, cause the evolving of circuit board designs to a higher level – which is the CAD system.

There have been many benefits since these systems were introduced. First, from schematic data, designers can transition automatically to layout data. To do this, they don’t need to input the spreadsheets’ connectivity manually. Another benefit here is you can automatically check clearance tolerance found between the boards’ objects like trace to pad or trace to trace.

With CAD tools for PCB design, the work produced experienced an exponential growth rate. Things changed, and soon the electronics technology that continues to evolve started to demand more tools for PCB design, most especially in the routing area.

Challenges Designers Face During PCB Routing

The connection of traces during the layout of printed circuit boards could be fun. Pulling these traces and then hooking them to ensure the nets are completed could be cathartic. Every day, simple board designs, which allow for basic routing having minimal rules, are beginning to disappear.  

Design rules with high speed, tight requirements for manufacturing, as well as other constraints requiring much attention rather than just connecting dots have taken their place.

Below are some of the challenges designers face when handling PCB bga routing:

Shorter times for design: To ensure the competitive advantage is kept, the times required for HDI circuit board design should be trimmed back. Asides from this, board spins have to be scaled down. This will help in reducing prototyping expenses. By this, the designer will be more pressured to perform the routing correctly for the first time.

Analysis expectations: To perform the trace routing the right way, layout designers have to understand how their designs are configured before time. Asides from tools for your PCB cnc router, this also requires analysis and simulation tools.

Multiple design restrictions: There was a time where all the board’s nets had the same spacing rules and trace width, asides from ground and power, which needed to be wider. Different spacing rules and width for multiple nets have now replaced design rules.

Requirements for complex routing: So much routing on the high-speed boards of today will need some specific topologies and routing patterns. For example, DDR memory routing used in the past requires using the routing patterns of T-topology, while DDR4 and DDR3 require fly-by-termination topologies. More so, some impedance lines must be configured in order to match the stackup of the board layer, some differential pairs have to be routed together, as well as other requirements.

One way of managing all these routing challenges effectively is by getting yourself equipped with tools that serve advanced design requirements of circuit boards manufacturer.

Tools for PCB Routing

Routing a PCB successfully requires more than just features of advanced routing. Routing begins long before laying down a trace. Therefore, we’ll begin this list with some other functions your tools for PCB design should have.

Component placement checks and aids: To route a trace the right way begins with placing the components correctly. This entails putting the right spacing between those parts to be observed. Also, signal paths must be optimized and the board must pass manufacturability checks. Just like circuit simulators, ripping up your routing in order to correct problems associated with component placement could have a ripple effect on the whole design which you would want to avoid at all costs.

Circuit simulation: These tools will not just help get your design to the market earlier by revealing the problems with the design before building a prototype, they will also assist your routing. Most high-density, high-speed designs feature tightly-packed trace routing. This is important for impedance control, signal timing, as well as other reasons. Whenever you need to reroute and rip up incorrectly captured traces, however, the whole design can be thrown off-balance. By using tools for circuit simulation, from the beginning, you can use the right schematic data.

Analysis tools: You don’t have to wait to build the prototype before analyzing how effective your routing is. The best process for your design is to utilize embedded analysis tools, which can be found in the design system of your PCB to check your power and signal integrity as you route. This allows you to make changes to errors made while working, rather than go back to redesign the board.

Design constraints and rules: You don’t just have to control the trace spacings and widths for multiple areas and nets, however there are some design parameters that have to be managed too. Constraint managers will give you some control over the design’s physical and electrical attributes. With this, you’ll be able to set up length matching, trace lengths, differential pairs, as well as many other constraints and rules.

Having all these features, you will be ready for your PCB router machine. Below are some of the routing tools you’ll find very useful:

Slide routing: This grants the ability to clean traces quickly, grab trace segments, and pull it to your desired location. Even better, is its ability to move other objects such as traces and vials from the way while sliding.

Manual Interactive Routing: For sure, you’ll always have to hook some traces up manually. Your tools for design should allow this to be performed easily with different editing options to help in complimenting your routing.

Differential pair routing: Differential pairs have to be routed together ensuring that the spacing is consistent between the pair’s traces. These routers actually work in line with the differential pair rules, which are set up for the spacing and trace width values.

Fanout routing: This is also referred to as escape routing. With this automatic feature, you can pull out traces from high pin-count parts quickly and then connect them to vias.

Auto-routing: Auto-routing can take many forms. This ranges from single trace to the full batch type. Full batch means that the whole board will be routed.

Bus routing: This is another great and useful feature where some traces can be grabbed and then routed together.

Cleanup routing: Several tools that can clean up your board’s routing are available. Some cab mister your traces’ corner, while some others will get rid of unnecessary segments and jogs.

Trace tuning: The routers work in line with the constraints of the design. It also helps in increasing its length overall by adding some serpentine segments to your trace.

There are lots of other routing features to be used, and you can call them different names with respect to the tools used for the PCB design.

How to Makes These Tools Work

When a PCB design system has more routing features, then you’ll have more capabilities to get this job done. Usually, designers combine these tools to achieve the results they want to see in their design.

For example, you may wish to begin by making use of the Fanout editor in doing the escape routing. Then you may want to utilize the interactive router with your set up constraints to route the controlled impedance lines and differential pairs at the right spacing and width and in the right locations.

After this, you may decide to put in your main routing using different auto-routing features and then tune up your transmission lines (high speed) to the right lengths making use of the tuning features. Finally, you will make use of different combinations of cleanup tools to optimize the routing, which has already been done using the pcb depaneling router.

How to do Routing in PCB Design

Introduction

Routing is the process of defining copper traces on a printed circuit board (PCB) to connect components according to the circuit netlist. Good routing practices are crucial for a successful board layout. Routing determines the form factor and helps ensure proper signal integrity and electromagnetic compatibility.

This article will explore routing techniques, strategies, and design-for-manufacturing (DFM) guidelines for routing a PCB. We’ll look at doing routing in a PCB CAD tool step-by-step. Optimizing the routing layout is key to achieving a high-performing, error-free, and manufacturable board design.

PCB Routing Overview

Routing takes place after placement of components and involves:

• Connecting pins – Creating conductor traces between component pins on each net.
• Completing connections – Forming electrical connections across the entire PCB layer stackup using vias.
• Managing density – Distributing traces evenly across layers.
• Minimizing length – Keeping traces short, especially critical signals.
• Mitigating interference – Avoiding coupling between sensitive traces.
• Maintaining manufacturability – Meeting fabricator capabilities for line widths, spacing, etc.

Well executed routing is crucial for success when progressing from schematic to physical PCB layout.

PCB Routing Steps

Routing a board involves strategic steps and iteration:

Settings

• Configure routing layers, track sizes, via styles, etc.
• Set up design rules, spacing constraints, etc.

Priority and Critical Traces

• Route highest speed or most sensitive signals first.
• Use constrained routing to predefine paths if needed.

Power Distribution

• Define power plane layers and polygons.
• Route power traces.

Signal Routing

• Use autorouter for initial routing.
• Manually route priority traces.
• Route remaining connections.
• Optimize routing as needed.

Verification

• Review design rules and constraints.
• Perform signal integrity analysis.
• Check for manufacturability.

Following a strategic routing workflow avoids issues and rework.

PCB Routing Best Practices

Adhering to routing best practices will improve routing quality:

• Plan – Develop a routing strategy upfront considering current flow, priority nets, etc.
• Separate analog/digital – Keep analog and digital circuits isolated.
• Use parallel traces – Route differential pairs and bussed signals in parallel.
• Avoid 90° turns – Minimize sharp right-angle trace bends.
• Smooth curves – Favor gradual rounded corners for traces.
• Top-down routing – Connect the IC/component side first.
• Equal trace lengths – Match electrical length for critical signals like clocks.
• Symmetric layout – Make routing balanced and aesthetically pleasing.

These guidelines help maximize both function and manufacturability.

PCB Routing Styles

There are three general PCB routing styles:

Point-to-Point

• Traces directly between individual pins.
• Minimalist wiring approach.
• Can produce somewhat chaotic layout.

Node-to-Node

• Connects between terminals with some sharing.
• More structured than point-to-point.
• Still somewhat inefficient routing.

Bus Branch

• Main buses connect regions/nodes.
• Perpendicular branches tap off buses.
• Hierarchical, structured, and optimized.

Bus branch routing maximizes order and efficiency.

PCB Routing Considerations

Several factors influence routing:

Impedance Control

• Matched impedance for traces like clocks and differential signals.
• Requires width/space tuning.

Noise Mitigation

• Keep noisy traces away from sensitive nets.
• Sensitive clocks often need shielding traces.

Current Flow

• Wide power traces to handle expected current.
• Avoid bottlenecks and high current density.

Signal Integrity

• Review timing, overshoot, ringback, etc.
• Tune routing to ensure clean signals.

Manufacturability

• Constantly check design rules and fab specs.
• Adjust routing to fix violations.

Balancing these factors produces robust routing.

PCB Routing Layers and Vias

Careful vertical routing among layers is also crucial:

Routing Layers

• What layers are available – signal, ground, power, unused?
• Assign usage purpose – analog, digital, RF, etc.
• Distribute routing density.

Ground and Power Planes

• Solid regions provide low impedance and shielding.
• Split power regions to isolate noise if needed.

Via Styles

• Different via types – through hole, blind, buried, micro.
• Size vias appropriately for currents and manufacturability.

Minimizing Vias

• Limit vias especially on sensitive traces.
• Necessary for connections between layers.

Proper vertical interconnection is vital for performance.

Autorouting Techniques

Most PCB design systems provide autorouting capabilities:

When to Autoroute

• Starting point for initial routing.
• For lower priority nets.
• To gain ideas and strategies.

Autorouting Algorithms

• Maze routing.
• Line probe routing.
• Channel routing.

Strategy

• Route critical nets manually first.
• Funnel autorouter to remaining areas.
• Clean up results post-autorouting.

Autorouting assists efficient routing but manual work is still required.

inspecting and Verifying Routes

After routing, carefully inspect the board layout:

Visual Review

• Check for unaesthetic or uneven routing.
• Look for possible shorts or poor manufacturing practices.

Design Rule Check

• Validate all spacing and geometry constraints are met.
• Fix any reported violations.

Constraints Check

• Confirm compliance with electrical constraints set.
• Make adjustments as needed.

Signal Integrity Analysis

• Perform crosstalk, timing, and power integrity analysis.
• Tune traces as required to pass SI analysis.

Thorough inspection avoids issues during fabrication and testing.

Improving Routing Iteratively

Refine routing across multiple iterations:

• Optimize – Shorten, widen, space, and smooth traces.
• Adjust – Re-route poor traces causing violations or SI issues.
• Repurpose – Swap layer usage if certain layers are congested.
• Restructure – Significant changes to load balancing or architecture.
• Review DFM – Check manufacturing tolerances and adjust.

Repeating routing optimization is key until design goals are met.

PCB Routing Tools

Many software tools assist PCB routing:

• PCB editor – The CAD tool itself such as Altium, KiCad, Cadence Allegro, etc.
• Constraint managers – Control timing, spacing, topology, etc.
• Autorouters – Automate routing paths between connections.
• Signal integrity – Tools like HyperLynx analyze signal performance.
• DFM analysis – Verify manufacturability and tolerances.
• Simulation – Model effects of routing on analog and RF performance.

Leveraging routing assistance tools saves significant time and effort.

Conclusion

Well planned routing practices that leverage both automation and manual tuning are essential to implement a circuit design properly on a functional PCB. Following the strategies outlined in this guide will help new and experienced designers alike to produce clean, efficient, and error-free routing layouts ready for fabrication. Paying attention to routing is time well spent to avoid headaches resulting from suboptimal board layout.

Frequently Asked Questions

What are some typical routing mistakes or bad practices to avoid?

Avoiding sharp corners, uneven trace widths, length mismatch in differential pairs, cross-talk, routing too close to pads/vias, ignoring DFM constraints, and forgetting to assign net classes.

When should I route a design manually versus using auto-routing?

Critical nets should always be routed manually first. Auto-routing can quickly handle simpler connections but still requires manual clean up after.

How can I calculate the correct trace width for a given current?

There are trace width calculators that help determine the suitable trace width based on current and copper weight to meet temperature limits.

What are some ways to reduce crosstalk during routing?

Using grounded guard traces, assigned spacing constraints, routing aggressor signals on inner layers, avoiding parallel runs for long distances, and using lower dielectric constant materials.

How are differential pairs routed?

Differential pairs route as parallel traces very close together, matched in length. They have wider spacing to other traces but minimal spacing between the pair.