- Are there guidelines as far as trace length vs frequency? I assume that ~3 inch traces are fine with 20MHz (15 meters), but what is the general case?
At my work, the guideline is, if the electrical length of a trace is longer then 1/10 wavelength, you need to treat it as a transmission line. At a minimum, this means you must terminate with a resistor matched to the impedance of the line. How do you figure out what resistor value to use? You estimate what the impedance will be during design, and then you adjust the value to minimize ringing during DVT.
Now, there is some subtlety here about the true meaning of 1/10 wavelength. For a sinewave, this is straightforward. For a square wave, which is the sum of many sines, you must use highest frequency component as your estimator. As you sharpen the corners of the square with a faster slew rate, you increase the frequency of the fastest sine competent.
What this means is, for a digital signal, drive strength directly affects the electrical length of the line. Higher drive strength can easily turn a line that does not ring into one that does.
I learned this the hard way when a supplier made an "improvement" to a digital buffer without telling us. This change increased the slew rate, which caused ring so bad that the receiving chip started to latchup. A board we produced that had been working fine for years suddenly started randomly locking up.
It appears that I should solve for odd characteristic impedances of 50 Ohms each. Is this true?
Yes, the differential-mode impedance is equal to twice the odd-mode impedance (at least for symmetric geometries).
My board house can do 5/5 mil trace/space. Is there any reason not to use these values?
If you can fit a design with larger gaps and wider traces, then the differential impedance will be less sensitive to over-etching or under-etching during pcb fabrication.
How much of an effect will soldermask have on the circuit? Should I keep the differential traces uncovered?
Is a fairly small effect. But if you can, it's worth checking the geometry with a simulator that can account for the effect. Your fab shop may be able to do this for you, if they aren't a totally low-cost shop.
Leaving the traces unmasked is also an option. But remember to leave a sliver of solder mask around each component pad to act as a solder dam.
If you are using a higher-end fab shop, you can also simply specify that you want 100-ohm differential traces, and let them adjust the trace width according to their knowledge of the material characteristics, solder mask thickness, trace thickness, etc.
Best Answer
Solder mask is some sort of thermoset polymer. Solvents for that sort of stuff are nasty. Look for a mechanical solution instead. Steel tools can scratch the mask off without damaging much the copper.