9814

Trace Width of a Printed Circuit Board

This Demonstration shows a plot with contours indicating the minimum required trace width for a range of allowable temperatures and currents in a printed circuit board (PCB). The trace width is specified for an external PCB trace of a particular length. Other parameters are the copper depth, the FR-4 substrate depth, the size of the copper plane (usually ground) on the reverse side of the PCB, and the ambient temperature. The calculation is done using basic heat transfer techniques and assumes uniform temperature for the PCB trace.

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The author acknowledges Charles Mauney of Texas Instruments for his useful application note, Thermal Considerations for Surface Mount Layouts. This Demonstration assumes:
1. The board components are already sufficiently heat-sinked and the PCB trace has only to dissipate its own resistive heat. In particular, this assumption indicates a constant temperature across the whole PCB trace.
2. There is at least one large contiguous copper pad on the reverse side of the PCB trace.
3. The substrate conductivity is 0.25 W/(m K), which is valid for FR-4.
4. The copper resistivity is ohm meter at 20° C, with a temperature coefficient of 0.0068 per degree Celsius.
5. The air heat transfer coefficient is , which is a fairly conservative value for a nonturbulent environment.
6. Heat is dissipated only convectively and flows only through the top trace and bottom pad. The effects of lateral heat transfer and radiative heat transfer serve as a safety margin for the calculated numbers.
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