There was a time when routing traces on a printed circuit board was a very standardized operation, and large trace widths were common. Power and ground traces would be wider yet, of course, and you simply didn’t go any smaller because the board technologies didn’t need it and the manufacturing costs would have been too high anyway. But that was a while ago, and since then, the increases in PCB technology performance have changed the game. Fortunately, the PCB manufacturing techniques have changed since then as well.
Now, it is a matter of choosing which trace widths you need to support the technology that you are designing for. External layer traces may require different widths than internal, while controlled impedance routing requirements will specify yet even more trace width variations. You need to plan for all of this ahead of time with your contract manufacturer so that your final design can be manufactured without difficulty. To balance all of these needs together, you need to understand what the PCB trace width tolerances are before you start so that you can make the best choices for your design.
Optimizing for manufacturing includes understanding PCB trade width tolerance guidelines.
Problems with Using the Incorrect PCB Trace Width Tolerance
Designers who make the wrong choices on the trace widths for their routing may find that they have problems in one or all of three different areas of their printed circuit board. These areas are fabrication, performance, and assembly:
Fabrication
Consistently using too small of a trace width can cause expensive fabrication difficulties. When the metal of the trace is etched away, there may be more etched away than expected. The thinner the trace, the more susceptible it is to having too much metal removed. A 3 mil trace width has much less tolerance for losing metal than a 20 mil trace width would. This over-etch has to be closely controlled by the fabricator when small trace widths are being produced.
Problems with etching can also affect long, isolated traces, which may etch down more than traces that are in groups. Small trace widths are also dependent on the weight of copper that is used to fabricate that layer of the board. If the current-carrying needs of your routing require more copper weight (the thickness of the copper), then your fabricator will not be able to etch small trace widths for you on that same layer.
Performance
Using the incorrect trace width can have a huge impact on the electrical performance of your PCB. Controlled impedance traces must have their widths calculated for the board materials and adjacent ground or power planes in the layer stackup of the board.
Trace widths can also have an impact on the signal integrity performance [link to signal integrity blog post] of the board. If the traces are so thin that they get compromised during the fabrication of board, then the overall signal integrity of the board could be affected.
On the other side, traces that are too thin for high current loads can get hot and even burn through. The routing width of high current traces needs to be calculated for the cross-section of the trace (width and thickness) as well as the amount of current that the trace will be carrying. Internal power traces need to be wider than external because exposure to the air will provide more cooling to these traces than the dielectric layers do for internal traces.
Assembly
Trace widths that are too large can have an impact on how easily the board will solder during assembly. Wide traces that are used for power and ground purposes could act as a heat sink, causing uneven heating and leading to bad solder joints. This could also cause small surface mount passive components to stand on end in a condition called “tombstoning” during solder reflow.
Traces that are not the right size can also cause soldering problems under large components, such as BGAs or surface mount connectors. Problems like these can cause intermittent PCB performance problems and are more difficult to find with X-ray inspection equipment.
Clearly, the wrong trace widths can cause you problems in the manufacturing and operation of your circuit board. But there are things you can do to avoid these problems.
Trace Width Routing Recommendations
Trace width is controlled by the electrical performance needs of the circuit board balanced with the manufacturability of those widths in both fabrication and assembly. Here are some recommendations that can serve as a guideline:
- Not all fabricators have the same capabilities when it comes to how small a width of trace that they can reliably build. You need to know what their abilities are before they build your board.
- Many fabricators will be able to produce 3 mil trace widths, but you should restrict the amount of these traces as much as possible to avoid problems. Using a 3 mil trace width, however, can be a good solution for short distance escape routing under dense BGA components. Typically, the smallest preferred width though is 5 mils.
- Longer traces can be routed to 5 mil widths, but wider is better to avoid the etching problems that can happen to long traces that are apart from other traces.
- For power traces, a good rule of thumb is to calculate them at twice their expected maximum current. This will usually put them at 50 to 200 mils in DC circuitry, and anything larger than that should be run through a power plane.
- Remember to use appropriate thermal reliefs and smaller trace widths to connect small passive components (resistors and capacitors) to power planes. This will avoid potential heat sink and soldering problems for these parts.
- Adding layers to your PCB design to give yourself more routing room is one way to avoid using smaller trace widths. This needs to be carefully considered, however, as it can add cost on the fabrication side as well as introducing signal integrity problems.
- Decreasing your trace width could save fabrication costs by allowing you to reduce your layer count, but it will add to the difficulty of fabrication. Using trace widths below 3 mils can get tricky and expensive.
There is a lot to consider when making these choices for your PCB design. Fortunately, your CM should be a great resource readily available to you that can assist you.
Getting the Answers You Need: Resources for PCB Manufacturing
Your contract manufacturer has been building printed circuit boards to different performance and manufacturing requirements for many years. They are well versed in the materials used in the construction of PCBs, and how those materials should be configured to give you the best performance on your circuit board. They also have the experience and understanding to help you make the best design choices upfront, and then to bring your board successfully through the entire manufacturing process.
At VSE, we have an experienced engineering staff that will partner with you on your design and help you with all of your design questions. We also have extensive relationships with PCB fabrication vendors and know which vendor should be used for your specific board. Additionally, our manufacturing processes are complemented by both our highly trained technicians and our state-of-the-art facilities and equipment.
