Is voltage important for calculating PCB trace width? If not, why?

Question

As per the IPC 2221A the formula for calculating PCB trace width follow the process given below;

First, the area is calculated:

Area[mils²] = (Current[Amps]/(k*(Temp_Rise[°C])^b))^(1/c)

Then, the width is calculated:

Width[mils] = Area[mils²]/(Thickness[oz]*1.378[mils/oz])

For IPC-2221 internal layers: k = 0.024, b = 0.44, c = 0.725

For IPC-2221 external layers: k = 0.048, b = 0.44, c = 0.725

Where k, b, and c are constants resulting from curve fitting to the IPC-2221 curves.

Why is voltage not considered in this formula?

Answer 1

Voltage is considered for spacing between conductors not for the width of conductors.

In IPC2221A you can find it in "Electrical clearance".

Answer 2

The only voltage that matters when calculating PCB trace width is the voltage drop in the trace itself, since the heat in the trace will be according to P=U*I.

This voltage drop will be according to U=R*I, where R is calculated from the trace dimensions. So the voltage (-drop in the trace) is indeed considered, only in a slightly different form than what you might recognize.

Answer 3

The only point voltage would enter consideration is at very high voltages (multiple kV), where corona discharge is a concern. In that case, a wider conductor (and also gentler, usually mitered or rounded, corners) is desirable.

As a practical example, here's a high voltage trace found on a Sony Trinitron monitor deflection board. The highlighted traces/components form the dynamic focus path; bias is a maximum of only 1kV. Note they opted for silkscreen covering, which may improve insulation, but neither soldermask nor silkscreen are reliable and approved as insulation (at least not to these voltages). And that seems to be borne out by the conspicuous dust deposits on these traces.

At these voltages, corona is negligible, and I don't think they made any particular considerations for width or shape -- they're routed in the same style as other traces. It is odd that the traces are so fat coming off the flyback transformer (bottom-left), but this isn't maintained along the whole path so clearly it wasn't required. Do take note of the conspicuous use of slots (board rout) to increase creepage distance to other traces/components, particularly around the cluster of resistors/diode (top-right).

(This was a CRT monitor, with around 30kV to the tube; all the dangerously-high voltages were handled by insulated cable and special connectors direct to the tube or sockets. The highest voltages after that, are the 1kV peak (horizontal deflection), 1kV DC (this focus circuit), and the 100-200V for general power distribution and video output. So, despite the highest voltage at the tube, almost everything in circuit is fairly pedestrian. This is another good lesson: only use what you have to. If you need high voltages, handle them as little as possible, as carefully as necessary (i.e. heavy insulated wire and connectors); push as much functionality as you can, to low voltages where it's easy to handle.)

Other commercial examples: CCFL lamp inverter circuits, laser printer electrostatic supplies. There aren't many applications for very high voltages, in bare circuits without potting, I think. There are industrial applications of course, but these are less likely to be seen (unless you happen to be working in power electronics). So, afraid I don't have other examples to hand, but this should be a good starting point on where to look.

In short, it's no accident IPC makes no concern with respect to voltage; it's not something you have to worry about in almost any circuit.

Answer 4

Voltage doesn’t matter to trace width, because voltage doesn’t travel down the trace.

Voltage is a difference between two points. It doesn’t go through or along a wire. It is also called Potential Difference, because it is a difference.

It doesn't exist at a single point, so what value would you use? You can look at current at particular point of a trace and say there are N electrons flowing in up this trace. You can’t look at the point and say "what is the voltage?", you have to look at the full length of the trace.

Answer 5

Current is what does the heating, and voltage drives the current. So from that POV current is the fundamental quantity.