The origins of PCB Manhattan routing
Pros and cons of Manhattan routing on a PCB
How your PCB layout tools can help
PCB trace routing in the form of the New York City skyline
Although there are a lot of things that could come to mind when you mention New York, for me it is the sound of Frank Sinatra singing one of his most famous tunes. In 1979 Frank recorded his version of the theme from the movie “New York, New York,” and it quickly became one of his signature songs. Aside from this song along with thousands of other notable associations, do you know what else New York is associated with? If you said printed circuit board routing, you would be correct.
New York’s Manhattan borough is the most densely populated of the city’s five boroughs, and the crisscrossing pattern of its roads is laid out on a very regular grid. This has led to orthogonal routing on a circuit board to be commonly referred to as “Manhattan routing.” We’re going to take a closer look at PCB Manhattan routing, and how you can benefit from using this pattern in your own PCB layouts. So as the song says, let’s start spreadin’ the news…
Where did PCB Manhattan Routing Come From?
As we mentioned already, the term “Manhattan routing” is based on the regularly spaced roads in New York city that crisscross horizontally and vertically on a grid. Although we use the term to describe PCB routing, “Manhattan” is also associated with distance or length, and wiring as well.
Manhattan Distance or Length
The Manhattan distance is the shortest path that a wire can have when it is restricted to being routed orthogonally, or in the X and Y axis only. Manhattan distance becomes important when the two points being measured are not aligned in the same axis with each other. While the direct measurement of a diagonal path will be the shortest distance between the two points, the orthogonal path of following a grid pattern will be longer. Measuring Manhattan length is relatively easy by adding the X distance(s) to the Y distance(s) between the two points.
This is a method of laying out circuitry for an integrated circuit. The interconnects are aligned with a grid, and the connection points are perpendicularly creating a Manhattan pattern. This method of representing circuitry can be done either physically in the chip, in the documentation, or in both. There are also other terms for Manhattan wiring such as right-angle wiring, and they are all based on what is known as the taxicab geometry. This description again goes back to the idea of traveling in gridded orthogonal movements in order to get from one point to another in a city like Manhattan.
From these definitions, it is easy to see how the term “Manhattan routing” came to be. Let’s take a look next at what you can do with this type of routing pattern.
A Manhattan trace routing pattern on a printed circuit board
Strategies for Manhattan Routing on a PCB
In the picture above you can see an example of Manhattan routing. Traces on one layer traveling in an X-axis connect to a Y-axis trace on another layer using a via. The idea is to keep all horizontal traces on one layer, and all vertical traces on another. There are both pros and cons to routing your board like this though.
The Benefits of PCB Manhattan Routing:
Manhattan routing will help to keep your routing organized. Traces can be routed in groups to first go in one direction, then via them up or down to another layer, and switch directions.
You won’t block off routing channels as you would with point to point direct connections. With direct connections other trace routing is forced to take longer paths in order to go around the blockage.
Perpendicular routing between layers like this will help to prevent potential broadside coupling. This is crosstalk between traces running parallel to each other on adjacent layers.
This is the routing style used by a lot of auto-routers enabling you to use them to decrease your routing time.
Potential Problems with PCB Manhattan Routing:
Manhattan routing will force you to use more vias in your design. Vias add to the cost of manufacturing the design, and they can also increase the inductance of the traces and affect signal integrity.
The Manhattan length of a trace will be longer than a direct point to point connection, which can also affect signal integrity. Longer trace lengths could also change how the board layer stackup is configured.
Normally a circuit board ends up with a mixture of both Manhattan routing and direct point to point connections. A board may have a great deal of Manhattan routing on it for example, but the differential pairs and memory routing may still be more point to point in order to keep their routing on one layer. To help you keep your different net routing going in the directions that you want them too, make sure that you set up your rules and constraints for all of your routing criteria.
A CAD tool report showing the direct, Manhattan, and actual routed distance of a trace
How Your PCB Layout Tools can Help with Manhattan Routing
With a set of advanced PCB layout tools, you can set up a number of different design constraints to control the direction of your trace routing, plus much more. You will also have features in the tools that will assist you in your routing by reporting trace lengths and other useful data. In the picture above you can see how these tools have reported the direct distance, the Manhattan distance, and the actual etch distance of the selected trace routing. In this case, the actual trace routing is slightly less than the Manhattan distance due to its more direct routing pattern.
The PCB design system that you need for Manhattan routing and other advanced PCB layout tasks is from Cadence. OrCAD PCB Designer has the features and functionality that you need to configure your design rules and constraints before you route, as well as reporting back to you the status of your routing as you work.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.