Has the prospect of doing something new ever overwhelmed you so much that at first it seemed impossible to do? It happened to me a while back when I built a new tool shed. I had never constructed a building before and trying to figure out all that needed to be done with the piles of lumber in front of me threw me into a panic. Fortunately I had some good instructions to follow and some reliable help to back me up. I started with step one, and kept right on going until I was finished.
Routing traces on your first complex printed circuit board design can evoke a similar panic. Oh sure, you’ve gone to school and practiced on a few training designs. You may have even helped out here and there on with some routing, or done some routing clean-up. But when you’ve been handed the reins of your first complex project requiring some dense routing, that’s when the fun begins. You may experience the same thing as many PCB designers before you have, that overwhelming sense of “how am I ever going to get all of these traces to fit on this board?”
Take a deep breath, you’re going to be OK. Although we can’t reach out of this article and route your board for you, we can help you with descriptions of some of the typical routing scenarios that you may run into. Don’t panic, it all starts with the first step and then gets better from there as you understand PCB routing angles.
PCB Routing Angles: Nets, Guides, Rats, and Routes
If you haven’t had much time working with PCB layout before, you may be wondering what designers are talking about when they talk about routing a board. Here’s a miniature glossary that may help:
Nets: When logical pins are connected together in a schematic it will form a net. When the schematic is pushed into the PCB layout, the same connections exist but now they are between the physical pins. Therefore the connectivity between pins in a schematic and between pins in a layout are both referred to as “nets.”
Guides: When you place the component footprints on the board, the nets will be represented by thin lines that stretch from their associated pins. These lines are often referred to as “guides.”
Rats: The unrouted guide lines that stretch between all the pins on the layout can be so numerous and jumbled up together that some designers will refer to them as a “rats nest.” Because of this, the individual guides are sometimes called “rats.”
Routes: Turning guides or rats into metal traces is the process in PCB layout known as “trace routing,” or simply “routing.” As you create the metal traces in your CAD tool, the thin guides will eventually all be replaced by neat patterns of routed traces.
To summarize, the schematic nets pushed into a PCB layout are represented by guides that will look like a twisted maze of lines like a rats nest. These guides will be routed into traces so that the logical nets in the schematic will now be actual connected traces of metal on the PCB layout.
The traces will run between all the connected pins on the circuit board, and can move up and down the layers of the board through drill holes called vias. In some instances large areas of metal are needed for nets that conduct power and ground, and those areas of metal are referred to as planes. Because traces, vias, and planes all conduct electrical signals, they are all considered to be intelligent net objects in a PCB layout.
Managing copper traces on a printed circuit board can be challenging
Getting Started on PCB Trace Routing
When you first start to route all of those guides, it can look very intimidating. One of the best things that you can do before you route is to have arranged all of your component footprints so that your guides are as short and direct as possible. This will allow you to better see and plan where your routing needs to go on the layout.
Your PCB layout tools will have a lot of different features in them to help with trace routing. One of the first things that it will offer is the ability to turn a grid on or off. There is nothing wrong with routing without a grid, but it will make it more difficult to keep the routing straight and it may make edits to the routing much more difficult. Another feature that will help is setting up the design rules so that your trace widths and clearances between traces are governed by the design tools. The grid and the design rules will be extremely helpful to you in routing neat organized patterns that you can easily edit later on if needed.
Creating PCB Routing Angles from Guides
There are a lot of different types and styles of routing that are used in PCB layout. You may use some or all of these techniques as you design your board:
Point to point: These are short routes that are easy to see and create, such as the connection between an IC pin and a resistor pin right next to it.
Multi-pin nets: Nets that connect multiple pins may be scattered throughout the board. The best thing is to place these parts as close together as possible, but that isn’t always possible. To route these nets you may have to go around obstacles, and up and down through the different layers of the board using vias.
Angled Routing: Usually trace routing is orthogonal (horizontal and vertical), and corners are mitered at a 45 degree angle. There are many exceptions to this, but this is the normal practice. Your PCB design tools will have routing settings that allow any angle routing or orthogonal routing.
Curved Routing: Some specialized designs such as high-speed or RF circuitry may require curved routing, but those are exceptions to the standard practice of orthogonal routing. PCB design tools often have specialized routing routines within them to handle curved routing.
Bus Routing: When there are multiple nets that are associated together, such as eight nets of a memory circuit, those eight nets are referred to as buses. Usually these buses need to be routed cleanly together like the multiple lanes of a freeway.
Measured Lengths and Matched Lengths: Some traces need to adhere to a specific length for the most optimal signal performance on the finished circuit board. There are also nets that must match each other in lengths as well. PCB layout tools have specific functions within them to handle this kind of routing so that your trace lengths stay within a specific tolerance range that you control.
Differential Pair Routing: Some signals must be run side-by-side with their matching net in pairs. PCB layout tools have specialized routing features that will automatically route these pairs together as if you are routing a single trace.
Don’t settle for anything less than what the printed circuit board needs
You’re On the Right Path Now, Keep On Routing
When you are routing, avoid leaving trace stubs dangling off of a pin or a junction of traces. These stubs can act as antennas and cause poor signal performance on the board. Also keep an eye out for traces that you have accidentally duplicated. You may have started routing one direction, changed your mind, and now that you are going the other way neglected to remove the original trace. This can also happen if you remove a trace, but forget about the via that it was connected to. Then this via is left dangling causing more signal and manufacturing problems.
You’ve got this, you’re going to do a great job routing your PCB layout. Another way to help ensure that your layout is routed well is to use the most advanced design software as possible. OrCAD is a PCB design tool set that has a long history of helping designers like you. OrCAD has the features and functionality that we’ve talked about here that you need to successfully route your board.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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