As a general rule of thumb, the following are essential fundamental factors for designing a high current board.
One of the primary factors determining how much current a trace can carry will be its width. Traces with higher width will carry more current. Generally, it is a rule of thumb to assume a trace width of 1mm per Amp of current took for a board with standard copper thickness. As you can notice, it soon becomes unreasonable wide when a trace has to carry 10s of amps, and such a path would take up a lot of space.
Copper thickness is the actual thickness of the traces that are on a PCB. The default copper thickness is usually 17.5 microns. Higher copper thicknesses can be 35 and 50microns. The higher the copper thickness, the less wide your trace has to be to carry the same current. More increased copper thickness does come at an additional cost but can help save space on your boards since, with higher viscosity, the trace width required is much less.
Manufacturer’s Technological Capabilities
As always, you can only design a board that your board manufacturer will be able to manufacture. In high current boards, the following two things are essential and should be looked at on your manufacturer’s technical capabilities page.
1. The copper thickness for outer layers – This would mean the two outermost layers on your board, top and bottom. The manufacturer will provide information on how thick(usually measured in microns or oz) they can make the copper on the outer layers.
2. Copper thickness of inner layers – This indicates the copper thickness limits on the inner layers, and often inner layers have to carry large currents, especially for packages such as BGA.
Besides these two factors, one must also look at what kind of temperature grade of board material the manufacturer can offer. Common ones are TG150, TG138, etc.
Finally, the one factor that will contribute to the longevity of the board and components is the heat that is produced by a high current track on a PCB. The power loss in watts can be calculated as the resistance of the trace multiplied by the square of the current flowing through it. In other words, P= I2R.
This figure ideally should be as low as possible, but even thick traces have a specific resistance, and when carrying large currents, the power loss increases significantly, and the trail heats up. Hot spots in the PCB can then lead to board warping, and layers peeling off and crack developing.
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