How to calculate typical signal trace clearance?

Question

Knowing nothing more than most of the signal traces on a project are 7.5mil and it's a high end audio product, i.e. it's not a high frequency or any 'advanced' application, what would be a safe minimal trace clearance between parallel-running signal traces, assuming it's safe to run the traces under scrutiny in parallel. I'm finishing up a PCB design project. I plan to hand it in to my boss tomorrow. He mentioned that the signal traces could have been set up with a 7.5mil clearance between them. Right now I have it at 12mil, but I want to minimize it as I hear the extra 5mil of clearance will not improve performance of the product in any way and I would like to conserve copper (less pollution, creation cost, I could make the ground and power puddles larger, etc.) I really need to learn EMFT! I like to be able to calculate and model things mathematically when I design, however my level of knowledge is not at the level to where I could go through the design at the schematic level, decipher the relevant info from the datasheets and answer this question on my own. Is there a PCB Design for Dummies? :)

Appreciate any advice. Thanks.

Answer 1

If it's high end audio I expect you have a spectrum up to a few hundreds of kHz. That's not particularly high in terms of the risk of capacitive coupling between traces, but there is a number of other factors.

1. impedance of the traces. A high impedance trace is more susceptible to coupling,
2. the energy in the coupling trace. For capacitive coupling the voltage is the relevant factor, for inductive coupling it's the current,
3. length of the traces. It's obvious that longer parallel traces have better coupling.

All this means that there's not a single simple formula for the required clearance. For the given frequencies I would choose 20 mils (0.5mm), maybe increase to 100 mils (2.5mm) for higher voltage/current traces next to high impedance ones.

Answer 2

The necessary clearance will vary between different traces, depending on the factors Steven mentions.
With Audio design there is often a high impedance, small signal front end, and a low impedance large signal output. For instance in a mic preamp you may have a gain of over 60dB with e.g. >1k ohm input impedance and <50 ohm output. Any traces from the two need to be kept well apart. The first PCB I made for a mic preamp turned into an excellent oscillator as I didn't pay enough attention to this.
Of course this applies to intermediate signals too but the risk/difference is likely the worst at input/output.
Also, if you use in/out transformers then be sure to keep them apart, as the same thing can happen with coupling of the flux. Input transformers are normally shielded to protect from things like this, but unless they have very good shielding and/or aligned just right then placing right next to each other is asking for trouble.

For a good book on Audio design I found Douglas Self's "Small Signal Audio Design" and "Power Amplifier Design Handbook" to be excellent. For PCB design in general often the books dealing with digital design (e.g. "High Speed Digital Design" by Johnson and Graham - old but still worth a look) often contain very useful information, especially about trace-trace coupling, EMI, planes, etc.