Rule of Thumb in using Infinitely Thin conductor in EM simulation?

Question

Does anyone know when is it safe to use approximately thin conductor during EM simulation in Axiem?
How about the ratio of the dielectric thickness to the copper thickness?
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Answer 1

For conductor loss, both thin and thick metal will work pretty much the same.  The most obvious place where thickness matters is with coupling between metal on the same layer.  The penalty you pay for metal thickness is that AXIEM must solve for more unknowns, somewhere between 2 and 3x the amount.  
At some point the coupling due to the metal thickness has a big effect on the coupling.   It is hard to say an exact rule of thumb when to make the switch as it will matter how far away your ground plane is on the bottom and how thick the metal is.    What I suggest you do is setup a microstrip coupled line simulation using the technology you are using and then sweep the distance between the lines for both a thick and thin metal case and see when the two answers converge.  
A couple other points.  
1.  Check the option for using edge meshing when using thick metal.  For the most accurate answer and a good comparison, you will want to make sure you are edge meshing the thick metal.   
2.  I have not seen the ratio of dielectric thickness to copper thickness matter much, but you could run a similar experiment, just seeing if it matters.   The biggest issue here is making sure you get the distance between your ground plane and the trace right.  If you are using the boundary of the structure for the ground, this usually isn't a problem.  But if you have ground planes at other locations in your stackup, you will need to adjust dielectric thicknesses depending on if you are using thin or thick metal to make sure the thickness of the dielectric is correct.