Where I live, traffic is a constant nuisance to be planned around. If at all possible, it’s better to make your weekday trips before, after, or between the standard morning and evening commute. Some of this is due to population effects, and people living in dense urban areas should expect some gridlock due to over-capacity. In fact, there’s a highway I refuse to drive because it lacks lanes, thus ensuring that even a moderate amount of traffic results in significant wait times.
Clogged roadways are similar to a PCB that does not account for the high amount of charge trying to pass through a far too narrow trace. A too-narrow trace for a particular current is likely to act as gridlock by starving components reliant on that power net of the current flow necessary for operation. Such a problem becomes far more sinister than runtime errors. Excessive heat can lead to board warpage, delamination, and other board failure modes that are likely to lead to scrapping. High current PCB practices need to be employed to keep a board in operation but functioning in lockstep with its design intent to avoid any issues during prototyping or field use.
High Current PCB Design Starts With Traces
It’s important to realize how the dimensions of the trace will affect its handling of large amounts of amperage to properly account for the increased thermal load of high current designs. To start, consider the cross-sectional area of a standard trace. Viewed head-on, this can be crudely viewed as the window for current to flow through. As the ampere is defined with the elementary charge per second, the only way to increase the amperage is to increase the charge flowing through an area (assuming the same amount of time has elapsed). However, doing so comes with a physical cost — the more electrons flowing through an area, the more likely collisions will increase on a random walk by an electron through the cross-section. Like an ideal gas, electrons colliding will result in heat, and when this happens at significant magnitudes associated with high current, the heat becomes appreciable. The heat gain can be substantial enough to irreversibly damage the board if the traces are not properly sized.
It is necessary to change the in-plane width of the trace or increase its thickness with thicker copper foil cores to increase this cross-sectional area. Both have pros and cons, but typically a hybrid method is employed. This is because thickness can be easily increased as the interior board space is usually not disturbed by a thicker copper-clad laminate. While width in the plane of the board should be increased as well (and this is probably a more straightforward first course of action if the current needs are not much greater than standard operating values), this can be a challenge in densely populated boards.
Layout and Manufacturing Decisions Affect High Current PCB Design
Designing around high current will take into account multiple layout tools and guidelines that ensure proper power delivery without compromising the integrity of the board or any sensitive circuitry. Below is a non-exhaustive list outlining some of the considerations for power delivery in a high current design:
- Inner vs. Outer Layers: Inner traces have the explicit advantage of not competing for layer space with components and their associated keep-out regions. Generally, high power traces and copper regions will inhabit the inner layers of the board. However, it is essential to note that the same width copper feature on an inner layer will have a lower current carrying capacity than one on the outer layer due to atmospheric conductivity.
- Tinning/Finish: A relatively inexpensive way to squeeze out more performance of the copper features is by covering the exposed copper with a layer of tin or soldering alloy. Therefore, the effective thickness of the copper feature is increased. Additionally, the tin or tin alloys have a greater resistivity value than copper, further improving the current-carrying capability.
- Via Stitching: For power delivery and thermal dissipation, tying high current polygon features together somewhat serves as a crude sum of the current and heat sinking parameters of the individuals. While necessary for thermal pads underneath IC packages, the technique can also be used to tie power traces together when a single layer is unsatisfactory.
The Role of Impedance in High Current PCB Design
Incorporating design aspects to minimize impedance will ensure minimal power drop along high current sections. Recall that power dissipated through resistive heating is P = I2R – besides minimizing current, the only way to reduce the power loss is by improving circuit design to reduce the impedance.
Impedance will be equal parts passive and active design, as characteristic values of traces will result from the stack up and the height and width of the traces. Fortunately, maximizing current carrying capacity, such as widening and thickening power traces, also reduces impedance. This process should intuitively make sense, as increasing the ability of current to flow would naturally inhibit the ability to resist this action.
As mentioned, the layout designer can actively combat impedance. Here are a few things to keep in mind while routing:
- Keep trace lengths short and avoid turns as much as possible.
- Make sure trace spacing rules are robust enough in DRC to handle wider power traces.
- Avoid long sections of parallel-running traces, especially if those are traces with high frequencies, fast switching speeds, or have other signal characteristics that could lead to coupling.
VSE Can Handle Your High Current PCB Designs and More
For a high current board build, many aspects and rules a designer may normally take for granted in a “standard” build must be modified to encompass the greater physical capabilities of a high current design. As with any board, designers, fabricators, and assemblers must be in lockstep to create a final board that meets the challenges of high current operations. Here at VSE, engineers build electronics for our customers. We’ll collaborate with you and our valued partners with care and professionalism to realize the design intent of your board and ensure an exceptional product.
