Potential problems with high current traces on a circuit board.
Design tips for routing power traces and using high current stitching vias.
Using your PCB design tools to help with power routing.
Via stitching can be used to help manage high current routing in circuit boards
One thing I learned from years of boating was to always respect the current of a river. Turning your back on it could lead to some unpleasant consequences, as the current will take you in a direction you don’t want to go if you are not prepared for it. In circuit board design, the same general rule of caution also applies. High current is a necessity of design, but it too can lead you into some unpleasant consequences if you don’t give it proper respect in your PCB layout.
Power and ground must be managed in your design and distributed correctly to the different components they are connected to. One technique used to manage high current in PCB layouts is stitching vias, which can help transfer the heat and energy of the current through the board. Here is some more information on using via stitching for high current traces in your next PCB design.
High Current Concerns on Circuit Boards
Many systems utilize a lot of power in their operation, and the circuit boards within these systems will need to conduct high current. However, if a board isn’t designed correctly for that level of current, it can fail either electrically or structurally. For example, a circuit board that doesn’t use enough metal to conduct the current through its power planes and traces may become too hot. This heat, if not distributed correctly, can affect the normal operation of components that are not designed for it. Eventually, the heat will create a domino scenario where more and more parts are affected, eventually resulting in the failure of the circuit board.
Another example of the potential negative effects of high current on a circuit board is the physical failure of the board’s structure. The materials used in the fabrication of the raw circuit board will tolerate a lot of heat, but only up to a certain level. FR-4, which is the standard material used for PCB fabrication, has a glass transition temperature (Tg) rating of 130 degrees Celsius. Beyond that point, its solid form will become unstable and may begin to melt. Even before that temperature is reached, however, the heat may end up burning through any thin metal traces on the board, creating an open circuit like a blown fuse.
To avoid these and other high current problems, care must be used in how these circuits are designed in PCB layout.
Short and direct power and analog trace routing between components
Electrical and Heat Considerations for High Current Circuits
High current can create a lot of noise in a circuit board, especially the current associated with a switch-mode power supply. The switching between on and off states will create EMI, which will increase in intensity as the rise time of the switching increases in speed. While this problem can be filtered, it also can be controlled with some of the following PCB layout techniques designed to reduce noise.
Components in a power supply circuit should be placed close enough for short and direct trace connections, while not breaking the following design for manufacturability (DFM) rules:
Power supply components should all be on the same side of the board to eliminate the need for intra-board routing.
High current components of the supply, like the inductor and the IC, should be as close as possible to each other for the shortest connection.
With the components placed together like this, the routing should be very direct within the power supply parts. You will want the traces as wide as possible to keep the inductance low and reduce the potential of EMI.
This strategy will give you better control over the electrical and thermal problems with high currents in power supply circuitry, but there is still the problem of routing high currents to other points in the board. The thermal problems in routing high current like this will require more metal, which may require routing on more than just one layer. Here is where using stitching vias in high current traces can help.
A power trace stitched with three vias to a trace on another layer
Via Stitching for High Current Traces in PCB Layout
When routing high current traces, it is always better to use as much metal as possible to reduce the heat and lower the inductance. However, many times there isn’t enough room on one layer of the board for power traces to be as wide as they need to be. The solution is to route the power traces on multiple layers of the board. By stitching the traces on the different layers together with vias, you will effectively multiply the current carrying capacity from what it was on just a single layer. In the picture above, you can see a power trace on an internal layer of the board that has three stitching vias in it to connect with a companion trace on an adjacent layer. This can free up space for other routing or additional components on the external layers of the board.
The caveat is that you still have a great amount of heat that can be created in high current traces. By stitching multiple power traces together with vias on internal layers, you will provide a way for more metal to share the heat, but it will still need to be dissipated. This can be done with thermal vias that conduct the heat to metal landings on the external layers of the board for cooling. The more cooling you can provide for your high current traces, the better your board will operate and the less likely it will suffer thermal damage.
Here are some other high current layout considerations to keep in mind as you are planning out your design:
PCB fabrication: If your board is going to be running very hot with high current, it may be best to explore other materials that can handle a higher operating temperature. Although these materials may be more costly, they may end up saving you expenses in the long run by avoiding thermal-related problems. You should also work together with your manufacturer to develop the best layer stackup configuration and power plane strategies for your high current board as well.
Board thickness: By increasing the thickness of the board you can increase the weight of the copper giving you a thicker trace. This may allow you to decrease the trace width, allowing for more routing and component placement room. As with any fabrication issues, these changes should be agreed upon with your manufacturer before you include them in your design.
Automated assembly: As we have seen, higher currents require more metal for electrical and thermal reasons. At the same time though, the same metal that is dissipating undesirable heat during operation may also create problems for PCB assembly. Large areas of metal can create thermal imbalances for smaller parts that can affect their soldering. To avoid this, make sure to use thermal reliefs when connecting parts directly to wide traces or large areas of metal.
Component placement: Parts that carry high currents and run hot should not be placed on the edge of the board if it can be avoided. By placing these parts more towards the center of the board, there is greater room for the heat to be naturally dissipated by the board.
In all of these design techniques, the best asset that you have working for you is the features and capabilities within your PCB design tools, which we will look at next.
The Constraint Manager within Allegro PCB Editor being used to set up design rules for power
Making the Best Use Of Your PCB Design Tools When Laying Out High Current Traces
Routing printed circuit boards often requires multiple trace widths and vias, depending on what is being routed. Signal traces require thin traces and small vias, while power and ground routing typically take the opposite. Sometimes, different power nets will require varying widths depending on the level of current that they are carrying. To help the layout designer with these challenges, PCB design tools typically have constraint management systems to set up trace widths and spacing rules.
In the picture above you can see a picture of the Constraint Manager in Cadence’s Allegro PCB Editor tools. With it, you can enter various widths for as many different nets as you need, along with their own unique clearances and preferred vias as well. Additionally, the constraint manager allows the user to set up net classes that specific nets can be assigned to. This gives you the ability to create a rule set for a group of similar nets so that you don’t have to alter each individual net. But, that is only the beginning of the capabilities that the constraint manager gives you.
With the constraint manager, you can set up rules for high-speed design topologies and trace lengths. You can also create rules for PCB assembly to govern the spacing between components or how solder mask should be applied to fine-pitch parts. There is even a section for setting electrical rules so that you are designing to a signal’s timing and delay values. With the versatility of the constraint manager, you will have the power you need to route with stitching vias for high current traces in your PCB design.
For more information on high current routing for power supply designs, check out this E-book.
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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