Have you ever been lost? I mean really lost where you had absolutely no clue how to get to your destination? Well, as part of the initiation process for the Order of the Arrow, the National Honor Society of the Boy Scouts of America, I was selected for what is known as the Ordeal. This included being blindfolded, walked through the woods at night, and pushed down a hill. Where you landed is where you slept that night, and in the morning, you had to find the camp where the other initiates were. Obviously, you wake up a bit disoriented, but you have to find the best path to camp or you do not get to eat.
When designing your PCB layout, a similar dilemma exists for routing your traces. What is the best route to take to complete the connection? With many types of traces that can exist on a board and various concerns that may seem to be competing at times, the answer to this question is not always apparent. However, to complete your design, a path must be determined. Examining different trace situations can help us define the best PCB routing techniques and optimize the board layout.
The Best PCB Routing Techniques When…
When designing your PCB layout, there are many considerations that must be incorporated into your choices and decisions. A popular way to integrate these concerns is to adopt a design perspective that helps you organize your design choices. For example, if you design your based on board type, you may place a premium on signal integrity (SI) or power integrity (PI). Alternatively, if manufacturability is the most important consideration, then design for manufacturing (DFM) guidelines and tolerances may supercede other issues. Regardless of the perspective you choose, traces are typically viewed as a group and routing them is considered a collective exercise.
A collective approach is good for specifying a general set of rules or guidelines for routing the traces on your board. However, it does not encourage you to make the best routing choice for individual traces. Conversely, by viewing each trace separately, we can define the best PCB routing techniques to follow when a given situation exists, as listed below.
No trace width expansion is available
Trace width expansion is always restricted. This may be to ensure adequate solder masking exists, to reduce crosstalk between traces, or other reasons. If the trace needs to be modified for current capacity reasons, the best course of action is to increase copper weight.
There are obstacles along the path or trace
This is a common problem where either components or traces obstruct the desired path. Typically, changing component location is far too complex and it is easier to use vias and route under traces than move them. When routing around obstacles, it is best not to use sharp angles (≥ 45° is recommended). For routing under obstacles, you usually should choose the most direct or shortest route and least complex via type.
The signal is part of a differential pair
Differential pairs are a special class of traces. As such, they should be routed as if they were a single trace. This means that their lengths and widths should be the same, and the distance between them should remain constant. Therefore, any changes to one should also be performed to the other, if at all possible. If not possible, then you use a meander, which is an arbitrary trace path used to match the other trace’s length.
There are signals running on an adjacent layer
You should avoid running signals on adjacent layers, if possible. Instead, a ground plane should be placed between signal layers. However, if the signals must exist on adjacent layers, they should be routed orthogonal to each other to prevent coupling. This is true for adjacent layers in the stackup and for two-sided boards.
You have a ground signal
In most cases, your board will have one or more ground planes, where an entire layer or layers are dedicated to ground. If not, and your board contains multiple signal types such as analog or RF, digital or control, and power or DC, the ground traces should be kept separate. However, they can all be connected to a single point or system ground near the power supply return.
The list above is not exhaustive and does not cover every possible situation you may face for routing your traces. However, it does provide the best PCB routing techniques for the given situations, which are common when laying out your board.
In addition to the routing techniques presented above, there are additional general rules that you should follow when laying out a PCB. These include:
- Route traces as directly as possible and as short as possible.
- Route similar signal types together. For example, digital signals with digital signals and RF signals with RF signals.
- When routing ball grid arrays (BGAs) or for other vias, choose the least complex via option.
- For high-speed signals, there are special considerations such as material selection and impedance when determining trace routes and lengths.
- Always follow DFM guidelines as stipulated by your contract manufacturer (CM).
Routing traces is one of the most important tasks of the PCB design process. Routing traces well is a combination of utilizing the best PCB routing techniques, as presented above, and your skill in creating circuit boards. However, as with all aspects of board design, the best PCB development is achieved when you partner with a well-qualified CM. Tempo Automation is the industry leader for fast, high-quality prototype and low volume PCB manufacturing. We are committed to providing you with the best boards, fast and ensuring that you have the best development experience with a partner throughout the process.
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And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.
If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on the best PCB routing techniques based on a particular trace situation, contact us.