How high-current traces negatively impact board reliability and service life.
Thermal and electrical concerns as a motivation for via stitching.
Further design tips to accommodate high-current traces.
Via stitching can be used to help manage high-current routing in circuit boards
High current is a necessity of design, but it can lead to some unpleasant consequences in a PCB layout if improperly harnessed. Power and ground must be correctly managed and distributed in a design for the multiple and varying component connections.
One technique used to manage high currents in PCB layouts is stitching vias, which improve the ability of the system design to transfer the thermal and electrical flux density associated with the current through the board. Analysis tools can also be combined with via stitching for high-current traces to determine how effectively power is being delivered to and transferred from planes.
Why Use Via Stitching for High-Current Traces?
Design of power planes will have to accommodate different net through-hole vias and traces routed on these layers out of necessity. Any routing features reduce the current-carrying area and may result in poor performance or intermittent functionality.
Thermal routing is symmetric to electrical routing: excessive necking of copper carrying large amounts of current can result in poor transportation of heat from its generating source. This can result in the early aging of board material and components before eventual device failure.
Via stitching for high-current traces can also provide shielding by being spaced close enough around offending traces (a process known as via fencing) to reduce in-plane coupling. Additionally, this shielding can be extended through the board with a shielding can.
High-Current Concerns on Circuit Boards
Many systems demand high power in their operations, and the circuit boards within these systems will need to conduct high-currents without failure. If a board’s conductors lack the robustness to handle the current needs, it can fail either electrically or mechanically. 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. If thermal routing is poor, the sustained heat can stress the normal operation of components, depending on their thermal rating. Left unchecked, the heat will lead to premature aging and eventual 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 standard materials used in bare board fabrication will tolerate heat without changing underlying material properties up to its glass transition (Tg) temperature. FR4, which is the standard material used for PCB fabrication, has a Tg rating of 130℃; at this temperature, the rigid form gives way to a more ductile state that rapidly degrades its structural integrity. However, failure can occur before the Tg temperature is reached due to the continual buildup of stressors and the potential for catastrophic failure in other board materials. Avoiding these and other high-current problems, therefore, requires a careful approach to PCB layout.
Short and direct power and analog trace routing between components
Electrical and Heat Considerations for High-Current Circuits
High currents can create a lot of noise in a circuit board, especially the current associated with a switch-mode power supply. The large swings in current associated with the switch between on and off states will create EMI, which will increase in intensity as the rise time of the switching increases in speed (i.e., in highly efficient CMOS power supply components). Filtering will be necessary, but astute designers can use design for manufacturability (DFM) layout techniques to minimize noise at the source:
Components in a power supply circuit should be placed close enough for short and direct trace connections.
Power supply components should all be on the same side of the board to ease routing and improve performance.
High-current components of the supply, like the inductor and the IC, should be as close as possible to each other to minimize impedance and power dissipated to the environment.
Design traces as wide as possible to keep the inductance low and reduce the potential of EMI.
This strategy will afford greater control over the electrical and thermal problems with high currents in power supply circuitry, but there is still the problem of routing large currents to other areas of the board. Typically, designers will be unable to modify the thickness of an individual trace (this is fixed by layer at the stackup); instead, designers can add additional volume to the trace by routing it intentionally across multiple layers. Here is where stitching vias spring into action.
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 increase the heat capacity and lower the inductance. When there isn’t enough room on one layer of the board for power traces to be as wide as necessary, designers can tie together traces across different layers with a via array. The solution is to route the power traces on multiple layers of the board. Daisy-chaining traces in this fashion effectively adds the current carrying capacity across the layers utilized and may open up additional routing space or component placement on the outer layers.
A power trace stitched with three vias to a trace on another layer
While stitched traces provide greater resilience to high currents, there is still a need to dissipate the high heat build-up. Thermal vias enable thermal routing to the outer layers, where a greater surface area aids dissipation through convection and radiation mechanisms.
Here are other high-current layout considerations to keep in mind during layout:
- 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
Using PCB Design Tools for High-Current Trace Layouts
Design tools are the medium with which designers implement layout changes, which means the more comprehensive the design tool solution, the less time is required to accomplish tasks. As form follows function, board features may change in shape and size, depending on their role in design. For example, power and ground traces typically require much larger widths than signals due to the increased current. Universal rulesets would frustrate routing for either, not to mention any signals or areas of the board that require their own rules; sometimes different power nets will require varying widths depending on the amount of current they are carrying. To help the layout designer with these challenges, PCB design tools contain design rule features like the Constraint Manager that allows designers to customize their rule check system.
Routing with stitching vias for high-current traces requires a smart ruleset that covers both general and specific cases. The Constraint Manager rule construction for high-speed eases this process during layout. Feature settings also extend to PCB manufacturing such as component spacing and fine-pitch solder mask rules, or electrical parameters like timing and delay. Cadence’s PCB Design and Analysis tools support users with customizable DFM and fully integrate with OrCAD PCB Designer.
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