Stackups for High Power PCBs
By Parker Bondi
January 02, 2025
In this article I highlight a few properties of PCBs that are necessary to successfully design a reliable high power circuit board.
Typical Applications
High power is a subjective characteristic. Some obviously high power systems such as charging stations for EVs, projectors for movie theatres, or refrigeration systems on transportation trucks may require hundreds or even thousands of watts to run. A less obvious product where you need to implement these principles may be a stepper motor driver board with tens of watts and that is where this article will focus.
Defining High Power
A simple threshold for when to begin thinking about the principles of high power PCB design is when the width of the pads for a component on the standard 1/2oz (18um) plated to 1oz (35um) top layer of copper is not sufficient for the current draw.
Consider the example where we have created an H-Bridge motor driver using four SSM3K333R,LF N-Channel MOSFETs from Toshiba Semiconductor.
This tiny SOT-23 device has a Continuous Drain Current rating of 6A. A typical footprint and typical routing for a SOT-23 device is shown here with pad dimensions of 0.9mm long x 0.8mm wide and a trace that is as wide as the pad (0.8mm).
If the stackup for this device was the standard 1.6mm thick 2 layer PCB this 0.8mm trace would experience extreme thermal stress with a temperature rise of 165C as a result of the 6A of current. Even if your core material has a glass transition temperature of 170C the board may experience warping and delamination due to the repetitive nature of a circuit like the H-Bridge motor driver.
In order to create a reliable PCB the Temperature Rise should remain less than 40C. This is to avoid thermal expansion and contraction which would damage the board over time.
This would limit the current carrying capabilities of the PCB to 3.1A. A thicker copper layer with a total thickness of 3.5oz (106um) would support the 6A and 40C temperature requirement.
Conductor Properties
In common printed circuit boards the conductors are copper layers. All copper layers have a base copper weight and external layers also have a copper plating thickness.
Base copper weight is the thickness of the copper when it is prepared in laminated sheets. If this sheet is used as a layer in the middle of your PCB then the base copper weight would be the thickness of that copper layer. However, on outer layers manufacturers may also plate the PCB with more copper. This is how they put copper on the inside of the Vias. Base copper and plated copper are different structures of copper and thus they have different capabilities and different limitations.
Traces on the copper layers are created by a photolithography process where the manufacturer creates a photo-resist to protect where the traces will route while other areas are etched away.
There are many etching processes that manufacturers use, but all of them have limitations on the consistency of the width of the copper at W1 and W2 in the picture below.
Manufacturers know their process and will be able to provide a tolerance in which they are confident the etching process will effectively and reliably remove all copper between traces while maintaining trace width as long as the PCB is designed within their guidelines. Thicker copper layers require a larger copper-copper clearance. Manufacturers will often suggest these thicker copper options in layer pairs separated by a core dielectric.
Substrate Properties
Electrical energy in printed circuit boards propagates through the substrate between the copper layers. The copper traces act as waveguides to direct this energy to the load. The thickness of the substrate material also has a significant impact on the ability of the circuit board to carry high current. A thicker substrate allows the adjacent copper layers to carry more current.
Via Properties
A simplified approximation of a via is a copper plated cylindrical trace. It’s circumference is similar to the width of a trace and the via plating thickness is similar to the layer’s thickness. A larger circumference and thicker plating allows the via to carry more current. Vias are very effective at carrying current. For example, a standard 0.254mm diameter via can carry 2A of current. Additionally, a designer can simply use multiple vias in parallel to carry very high currents. In the vast majority of boards, the vias and the floor space they occupy is not the limiting factor for high power boards and designers will need to increase their conductor widths and plating thickness before beginning to worry about their vias. This understanding is sufficient for low power applications, however for an accurate calculation of the current carrying capability of a via a designer should use a tool that uses the method described in IPC-2152 which considers other via parameters shown below that impact the way the electrical energy flows through the substrate around the via.
Another Technique to Remember
Multiple layers can also be used to carry high currents to and from components. Designers can use a wide trace to go from the components pad straight into vias and use multiple layers of the PCB to carry the energy to the load. With this technique it is paramount that designers also use multiple return paths.
With this technique the external layer trace width may still limit the current, and so in addition designers could use a via-in-pad design technique. Designers should be aware that Vias in the component pads can lead to unreliable soldering and require specific manufacturing techniques to ensure high quality and reliable soldering of the components to the PCB.
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