High Current PCB Design Guidelines for Reliability

Key Takeaways

• Selecting the right materials for PCB fabrication and increasing copper layer thickness are essential to handle high currents effectively.

• Proper component placement away from board edges and closer to the center aids in natural heat dissipation.

• Via stitching enables PCBs to handle more current without overheating, significantly reducing the risk of circuit damage.

Via stitching can be used to help manage high-current routing in circuit boards

In order to create reliable boards, high current PCB design guidelines are essential. Specifically, one important technique used to manage high currents in PCB layouts is stitching vias, which allows for the transfer of the thermal and electrical flux density associated with the current more efficiently 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.

High Current PCB Design Guidelines Summary

High-Current Concerns on Circuit Boards

Many systems require high power in their operations, and the circuit boards within these systems will need to conduct high-currents without failure. If high current PCB design guidelines are not followed, the board will not be able to handle the current needs, and 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 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 glass transition temperature 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 Guidelines for High Current PCB Design 

High currents can create a lot of noise in a circuit board, especially currents associated with switch-mode power supplies. The large swings in current associated with the switch going from on to off states will create EMI, which increases in intensity as the rise time of the switching increases in speed (i.e., in highly efficient CMOS power supply components). Filtering can help, but astute designers can use design for manufacturability (DFM) layout techniques to minimize noise at the source:

1. Components in a power supply circuit should be placed close enough for short and direct trace connections.
2. Power supply components should all be on the same side of the board to ease routing and improve performance. 
3. 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.
4. Design traces as wide as possible to keep the inductance low and reduce the potential of EMI. 

This strategy will allow for greater control over the electrical and thermal problems with high currents in power supply circuitry. However, there is still the problem of routing large currents to other board sections. Typically, designers cannot 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, discussed below..

This strategy will allow for greater control over the electrical and thermal problems with high currents in power supply circuitry. However, there is still the problem of routing large currents to other board sections. Typically, designers cannot 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, discussed below.

A power trace stitched with three vias to a trace on another layer

Why Use Via Stitching for High-Current Traces?

Via Stitching for High-Current Traces in PCB Layout

An important high current pcb design guideline, is the use of via-stitching. In routing high-current traces, it is best to use as much metal as possible to increase heat capacity and lower 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 board layers. Daisy-chaining traces in this fashion effectively add the current carrying capacity across the layers utilized and may open up additional routing space or component placement on the outer layers.

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. 

Design Tools for High Current Guideline Adherence

Design tools are how designers implement layout changes, meaning the more comprehensive the design tool, the less time is required to accomplish tasks. For example, power and ground traces typically require much larger widths than signals due to the increased current. Universal rulesets would make routing difficult for either, not to mention any signals or areas of the board that require their own rules. For this reason, different power nets may require varying widths depending on the amount of current they are carrying. For this reason, to ensure adherence to high current PCB design guidelines, PCB design tools contain features like  OrCAD X Constraint Manager that allow 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 X PCB Designer.