Small clearance in RF applications - 5-20MHz

[Garyl]

I am designing a ham radio receiver PCB for the prototyping batch.
PCB is two sided, top is mostly ground, and the bot is the main tracks pattern.
PCB will be made by the chinese pcb factory.
My question is, is the clearance (spacing between tracks and tracks or copper pour) size important, and how big the clearance should be?
I suspect that the tracks with low clearance may add a stray capacitance to the circuit, but I don't know how much it matters at the used frequencies (4MHz up to 20MHz or soo, maybe even higher).

 

[SLK001]

All manufacturers will have clearance standards that they can meet and you must design for. You need to contact them to get these values. Are you designing in 50Ω? Are you using 50Ω traces? If so, what kind of system are you using (µstrip, coplanar waveguide)? When I design RF circuits, the RF path is kept at my system impedance (usually 50Ω). To minimize the space needed, I usually employ a coplanar waveguide (CPW) for the RF path. With this CPW system, the trace-to-trace capacitance is accounted for in the design of the traces.

For non-RF parts of the circuit, just use the manufacturer's design guidelines for your minimum trace-to-trace clearance and trace-to-ground clearance. It is my experience that you really have to have a densely packed circuit board to need to use the manufacture's minimums. Therefore, use something easy for them to produce, like 15-20mil lines minimum and 20mil spacing. For non-RF circuitry, the stray capacitance is trivial and need not be worried about. Especially at your frequencies.

 

[vfone]

Generally the spacing limit, without any extra charge, is 6 mil (speaking about Chinese manufacturers). This spacing should be fine for your application.
Some PCB manufacturers charge even few times more just for 1 mil less spacing.

 

[biff44]

at those low frequencies, i would not worry about it too much. make it big enough gap so they do not short out if there is condensing moisture, and you are good to go. At least 0.020" gap i would think