What is PCB Routing?

It's common to assume that routing PCB traces and vias on a PCB design is an easy operation. It would seem to be a very simple process to begin connecting components with copper once the board has been imported and the components have been arranged on the board. 

That may have been the case when low-speed TTL DIP components were used on straightforward circuit boards, but the design specifications of today are much more intricate. A PCB's traces may contain highly stringent design specifications intended to maintain signal integrity while routing.

Even though traces may have unique routing needs, you can set up and adhere to design rules for your traces with the aid of today's more sophisticated PCB routing approaches and capabilities. Depending on the signalling standard you're using and the necessary routing topology, you'll employ key routing approaches for your board. 

Don't worry if you're designing a PCB for the first time and you're ready for the routing stage; we'll show you how to accomplish it and tell you which routing specifications your PCB needs to meet.

Starting PCB Routing

So what exactly is PCB routing? Every PCB must have copper traces, which link components on the surface layer or internal layers. The suitable trace design you employ in your PCB will rely on various things, including what constitutes a "simple" device. In both low-speed and high-speed signalling standards, you'll discover a number of significant design routing criteria, such as:

• Trace carrying capacity, as high-current circuit boards may need big traces or even polygonal traces.
• Use of trace width on the board will assure manufactureability and have an impact on crosstalk.
• The breadth of any regulated impedance signals that must be established based on the PCB stackup
• Choosing the right PCB topology will affect how traces branch out and link to various components.
• Total losses along a trace, which establishes the longest trace length permitted
• Maximum skew parameters are set for protocols having source-synchronous clocks (SPI or I2C), parallel buses, differential pairs, and source-synchronous clocks.

It is your responsibility as a designer to strike a balance between all of these factors and choose which ones, out of the ones listed above, are most crucial for certain nets. 

For instance, whereas high current DC designs require large traces that are not necessarily impedance-specific, high speed designs depend on regulated impedance with differential pairs.

Let's look at some of the routing specifications for some of the simpler boards to get things started before moving on to more complex designs.

Simple PCB Trace Routing

You are normally free to choose a trace width that readily accommodates your component pins and leads if your design is not running at high speeds, is not thick enough to cause problems with crosstalk, and only needs to transport modest current.

 These designs allow for the usage of trace widths between 5 and 15 mils because they are tiny enough to be routed directly into pads on the majority of components. The diagram below shows a simple op-amp example with traces running between a low-speed IC, some resistors, and capacitors.

These kinds of simpler designs typically don't have to worry about impedance, common routing topologies, or high current. However, very few modern systems are so straightforward that no trace design or routing rule determination is necessary.

Routing Rules for Modern PCBs

Today's boards need some amount of trace design and routing principles to assure signal integrity, even those that merely use a basic MCU and low power stages. 

To ensure dependability and signal integrity, designers must define the trace shape requirements for their connections.

1. Find out how much current is needed in a particular trace; power circuits on PCBs can carry a lot of current.
2. Look in your component datasheets or your signalling standard to see if impedance management is required if current will be very low (less than 1 A).
3. If you need to use impedance control, figure out the trace width you need to reach your goal impedance.
4. To hit your impedance target if impedance control is needed. Also calculate the trace spacing needed if differential pairs are needed.
5. If impedance control is required, a single-ended or differential pair routing design will probably be used. Check your signalling standards to find out what your routing needs are, which may include things like a loss budget (which establishes total length), impedance specifications, and the permitted length mismatch in differential pairs or in a parallel bus.
6. Raise your knowledge of constructing traces for impedance-controlled routing & Study up on PCB trace design for high current.
7. You can establish design rules for particular nets in your design after determining any routing needs for your board. As you route traces, your routing tools will use these parameters to determine the trace width, which requires defining minimum or maximum trace widths in your design rules.

Impedance and Routing Topology

• You'll need to determine the impedance using one of several techniques when you need impedance control in your PCB layout. 
• There are formulas you can use to figure out the impedance in your design, or you can figure out the impedance you require in your design using more specialist software. 
• To guarantee that impedance objectives be attained, single-ended and differential pair impedance will require a specific geometry.
• The PCB design software's built-in calculator function is the quickest way to calculate impedance. This kind of functionality is not present in all PCB design applications, and those that do generate results with varying degrees of accuracy. 
• The best PCB design software will have an electromagnetic field solver that determines the necessary trace geometry automatically.
• These tools will determine the trace width and differential pair spacing necessary to get a target impedance using the dielectric constant and copper roughness data from your PCB.

An electromagnetic field solver from Simberian is integrated into Altium Designer's Layer Stack Manager and offers extremely precise impedance calculations at a desired frequency.

How traces are routed between component inputs and outputs, as well as how they are branched from one another to reach many components, are defined by trace routing topologies.

For instance, DDR routing employs a fly-by architecture, in which a single bus branches off to access numerous design components. In a different illustration, SPI employs a related bus topology with termination placed at the bus' load locations.

When a design calls for a single component to communicate with numerous loads through a single IO interface, other components may use point-to-point topology to get to multiple components.

Make sure you are familiar with the routing topology required by your signalling standards and whether impedance control is necessary for those traces.

What Is PCB Routing Trace?

You may easily route traces in your PCB layout by pointing and clicking on different parts of the board. Copper traces will be fixed at the user's chosen mouse click location along the route, eventually extending across the layout to the needed spot. 

When routing traces in your layout, the routing tools in your PCB editing application can automatically turn corners (often at a 45° angle) and install vias as you move traces between components in the PCB.

To avoid using too many vias or needing to add extra layers to solve the board, take some time to design a strategy for several routes before you begin routing PCB traces. Your PCB routing technique will depend on your PCB layout; if there are too many net crossings, it will be more challenging to route traces without making too many layer transitions.

Sometimes it's necessary to begin with the simplest routes first because they'll show you which ones require the most time and work to complete routing in the PCB layout.Some routes, like the escape from this BGA, might be extremely complicated.

This path ultimately terminates on the surface layer after travelling through two vias.

• Among the crucial PCB routing recommendations you ought to take into account are:
• In an effort to maintain regulated impedance traces on the same layer for a specific interface or signalling protocol
• Reduce via transitions on RF traces and high-speed protocols
• The best technique to avoid routing over plane splits and to maintain track of the return path in your PCB is to use uniform ground regions.
• Avoid making traces longer than necessary; try to keep them brief and direct.
• Don't be hesitant to employ polygons to build larger conductors for high current routing; these can be used to create conductors of any shape.

One issue that is closely related to PCB stackup design and routing is signal integrity. The arrangement of plane/GND/PWR layers with respect to your signal layers and routing is a major determinant of signal integrity, and routing over complete sections of GND is the best way to ensure your design will maintain signal integrity and have immunity to EMI (crosstalk, external RF noise, power supply noise, etc).

This straightforward advice and the aforementioned routing rules will help prevent or reduce many signal integrity issues and ensure your board continues to function.

The most sophisticated routing tools that can assist you in adhering to fundamental PCB routing rules are interactive. In other words, you can specify routes for a collection of signals using these semi-automated tools, and the routing tools will put the traces such that they automatically follow your design guidelines.

 The design rules for your nets and groups of nets are automatically verified as you develop your PCB layout when using this sort of routing. Many open source and freeware design tools require you to perform everything manually, but cutting-edge PCB design tools like Altium Designer can support your productivity while you complete your PCB layout and route traces throughout your board.

The interactive routing tools in Altium Designer offer intelligent, semi-automated PCB routing that adheres to your design guidelines and fundamental routing needs.

Using the full complement of PCB layout tools in Altium Designer® makes PCB routing much simpler. As you insert traces, the integrated design rules engine in Altium Designer automatically analyses your routing, allowing you to find and fix mistakes before you complete the board. 

Conclusion 

Additionally, each user of Altium Designer has access to a personal workspace in the Altium 365TM cloud platform where projects, component data, manufacturing data, and any other project documentation can be stored and communicated with collaborators.

We have only begun to scratch the surface of what Altium Designer on Altium 365 is capable of. Start your free trial of Altium Designer + Altium 365 now if you'd like to learn more about PCB routing.