“Can the PCB handle it?”
It’s a question I’ve batted back and forth with my colleagues many times over the years. And it usually comes in the wake of an overly confident salesperson who blindly and hastily committed to having a new higher-current power supply in a few weeks, with the details spelled out on a bar napkin from the golf club where the product was hatched. By the time the pain rolls downhill to the engineering department faced with building the widget, the conversation quickly turns into: “We have to get twice the current out of the same box as before. Can the PCB handle it?”
Printed-circuit-board (PCB) design textbooks and IPC standards do a good job of discussing a PCB trace’s dc current-carrying capability up to about 30 A. However, little if any reference material exists beyond this threshold, either in current or frequency. We know to avoid critical current densities that cause the traces to fuse open or delaminate from the PCB. But how far away do we need to be from these values and where, if at all, are these values listed?
The easiest way to begin this discussion is with a couple of common-sense boundaries on allowable current density in plain copper-wire conductors. The first of these comes from the transformer design world. In this world, designing a transformer for minimal temperature rise with virtually no cooling will use current densities of 1000 circular mils per ampere (CM/A). This is a lot of cross section for very little current. For instance, a 10-AWG conductor would be used to conduct no more than (10,400 CM)/(1000 CM/A) or roughly 10 A. We know that this is exceedingly conservative.
Another boundary, called “fusing,” can also evaluate the sensible current-carrying capability of a PCB trace. At some lower current density, the conductor will have enough localized heating to melt and fuse open. We certainly don’t want this to happen on our PCB!
The 14th edition of the EE Handbook\[1]contains a chart on the fusing characteristics of copper on P 4-81.1 All of the tests that were performed used a one-inch-long conductor in free air. Clearly, these are very low current densities (CM/A) that shouldn’t be used on a PCB. This is where copper in the test specimen melts and fuses into the open state. The table need not be reproduced in this discussion, though is it a valid reference for the inquisitive engineer.
A PCB trace behaves a little differently. It’s flat with one side exposed to the air and the other laminated to the PCB material. However, as a common-sense boundary, the stated fusing current density for a 20-AWG wire (1020 CM cross section) at 10 seconds is about 45 A. At 0.1 seconds, it’s 450 A.
This is consistent with what we know of the I2t relationships of fusing. When there’s 45 A going through a 20-AWG wire, it corresponds to a current density of 22.2 CM/A. This is the current density that must be avoided. It may not fuse the trace open on a flat PCB with good exposure to air, but the trace will most assuredly overheat to discolor, peel, and degrade the surrounding laminate and traces on the PCB, and perhaps the components in the area.
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