On current carrying capacities of PCB traces

Abstract

Realizing the increasing importance of the PCB (printed circuit board) current carrying capacities in PC (personal computer) applications, this paper addresses the DC and AC current carrying capacity problems. The classic Fourier series method is used for both two and one-dimensional analyses. But for the simple case of a single conductor arbitrarily located along the PCB length, a direct analytical solution is given without using the Fourier series. It is found that the 1-D and 2-D solutions yield an almost identical result for a typical PC motherboard. Many inside understandings are gained by revealing the impact on the current carrying capacity of various parameters such as the time, conductor thickness and width, allowable temperature rise, heat transfer coefficient, PCB copper volume percentage, PCB length and thickness, as well as the number of traces and their separations. The time constant of a PCB in a typical PC application is in the order of 100 seconds. Thus, the PCB can hardly have time to thermally respond to the AC components of I/sup 2/ even for a frequency as low as 1 hertz. Thus for any practical purpose, the conductor traces can be sized using the mean square value of the current, just based on the DC analysis. The conductor trace location along the PCB thickness has very little impact on its current carrying capacity and there shouldn't be any derating factor for internal traces in their current carrying capacities. The IPC (1998) practice of derating the external trace current carrying capacity by 50% for the internal traces should be stopped. However, the location of a conductor trace along the PCB length does have impact. The trace located at the edge of a PCB will have a lower current carrying capacity than the trace at the center. The current carrying capacity is proportional to the square root of the trace thickness, and approximately proportional to the square root of the allowable temperature rise. But the dependencies of the PCB current carrying capacity to the trace width, heat transfer coefficient, copper volume percentage, PCB length and thickness, as well as the number traces and their separations are rather complicated, governed by the relevant equations derived in this paper. The PCB current carrying capacity design charts applicable to typical desktop PC applications are presented. They provide a tool to conservatively estimate the conductor size necessary to carrying the required current level. But more precisely predicting the current carrying capacity would require experiments that simulate the PC motherboard application conditions to determine the critical properties such as the heat transfer coefficient and the PCB equivalent thermal conductivity. This will be the future work.