FreeLists / si-list / [SI-LIST] Re: edge coupled coated microstrip differential impedance

["Andrew Ingraham" <a.ingraham@xxxxxxxx>]

Soldermask is supposed to have only a very minor effect on single microstrip
traces.  The effect would be somewhat greater for edge coupled differential
traces, but I don't know by how much.

Didn't the boards from your first vendor also use soldermask?

A while back, I went through an exercise where we tried to verify the
vendor's expectations with our field solver.  The board vendor either
adjusted or chose the dimensions to give us our asked-for target impedances
(single-ended stripline), then gave us the detailed physical parameters and
the resulting electrical parameters, including tolerances.

I tried to reach the same results using our 2D field solver, but
consistently missed by a surprising amount (not huge, maybe 10-20%).  I
included the fact that some of the dielectric layers were laminates of
different Rogers materials, with min & max Er as given to us, and tried a
variety of trace etching shapes in an attempt to approach the vendor's
results.  No such luck.

We came to the conclusion that the board vendor "knows more than we do"
(at least about their processing), and that whatever tools that board vendor
uses, they are better at getting the correct results, whether by means of a
predictable, proprietary "fudge factor," or something else.  We had a lot of
experience with that board shop and knew their work was good and we got the
right impedances, so we had to trust their figures, not ours.

Your mileage may vary.  I've been told by others that they are right on the
mark with their field solver predictions, so I don't know what was different
in our case.

In your case, maybe the dielectrics are significantly different and they
know they need to adjust things differently.  Or maybe the two board shops
use different tools, and one of them isn't very accurate.  The proof will be
in the test coupons.

Regards,
Andy


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