The old expression is, “give them an inch and they will take a mile.” I may not have completely understood that expression when I was younger, but it sure has come into focus now that my teenagers have started asking me for money. On the bright side, there is a lot more room in my wallet these days for things other than cash.
It is not unusual for people to always want more, especially when laying out a printed circuit board. We want more room for placing components, we want smaller traces to make our routing easier, and we want fewer design restrictions so we can easily hook up the connections. Going down this particular path of least resistance, however, usually results in some big problems with our PCB layout. There are reasons why these restrictions are in place, both from a board performance perspective as well as manufacturing. Let’s take a look at some of the common reasons behind PCB trace width and spacing restrictions, and how by working within these restrictions, you can design a better board.
Performance Considerations for PCB Trace Width and Spacing Restrictions
When trace routing is used to distribute power and ground throughout the components on your circuit board, the traces must be large enough for the current they are carrying. To do this, the trace can be wider, thicker, or both. Whereas trace width is determined by your CAD tool’s routing rules, the thickness of the trace is determined by the amount of copper weight that is used to build that layer of the circuit board. A half ounce of copper spread evenly over a square foot of circuit board will be 0.7 mils thick, one ounce will be 1.4 mils, and so on. To route the correct width trace for power traces then, you need to calculate the current along with the copper weight of the layer. Which layer of the board you are routing on is also important because external layers have better heat transfer than internal layers as the air dissipates the heat better than the internal dielectric layers.
There are also restrictions on traces used for high speed signals such as controlled impedance lines. Using an impedance calculator, you can enter the board width, type of materials, and layer stackup, in order to determine the trace width needed to hold a specific impedance value, such as a 50 ohm line. High speed design routing will also require certain spacing restrictions as well in order to control noise that could be picked up from adjacent lines that are too close. Serpentine routing patterns for traces that have to be tuned to precise lengths also require specific spacing, which is usually at a ratio of three times the trace width.
Trace Width and Spacing Rules for Manufacturing
So far, we have looked at how trace width and spacing restrictions affect the performance of the board electrically, but there are also manufacturing concerns as well. Traces that are too close to each other or to other metal features of the board could potentially develop shorts during fabrication. Each fabricator will have its own minimum trace width that they will build, but 3 mils is a common minimum spacing value. Copper weight also must be factored in here as well. The higher the weight, the larger the minimum spacing is needed by the fabricator to build the board. Although a higher copper weight is more desirable for routing that carries high levels of current, it can pose a problem for fabricating narrow signal trace widths. Another routing concern is acid traps, which can occur when trace angles are too sharp.
Here are some other manufacturing considerations for trace width and spacing restrictions:
- Traces running between the pads under small surface mount components may have the minimum trace to metal spacing, but can still cause manufacturing problems. The part may not sit squarely resulting in tombstoning, or form a solder bridge with a trace and cause damage if the part is lifted for re-work.
- Metal that is too close to the edge of the board could present depanelization problems. Additionally, exposed copper could come into contact with other conductive elements and cause a short on the board.
- Traces that are too wide on the surface mount pads could form solder bridges with adjacent pads or traces. These could cause excess current or shorts.
- It is also important that drill hole sizes match the traces to which they connect. Although vias run through the board they serve the same function as traces that run along the surface. In many cases, they are extensions of them.
DFM for High-Speed Digital PCBs
Go to the Experts: Ask Your CM About Trace Width and Spacing Restrictions
The good news is that you have an excellent resource in your contract manufacturer to help you through these types of trace width and spacing restrictions. Your CM has many years of experience building boards of all kinds of different technologies, and they already have the answers you will need for your specific project. The best thing that you can do is to engage early on in your design with your CM to determine the proper trace widths and spacing that your PCB design will need.
|Tempo‘s Custom PCB Manufacturing Service
At Tempo Automation, we are ready to work together with you from day 1 of the PCB manufacturing process. We have the experience and the resources that you need to answer your design specific questions in order to make sure that your design is manufactured to the highest levels of quality.
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.