Sorry there is no simple answer - you will have to make some assumptions, or mark up your print to use specific imaging technology.
First, trace shape depends on the etch resist used - photoresist or metallic electroplate. If the fab process uses photoresist for the tent-and-etch process, the trace shape will be trapezoid, just as with inner layers.
If the etch resist is metallic, the copper cross section depends on the thickness of the photoresist used, and unfortunately the resulting copper shape may not be consistent across the same board. Typically, the photoresist does not fully contain the plating and results in a cross section "mushroom". The most complicated is when the pattern plated copper first mushrooms over the top of the resist, and then is covered by a metallic resist like tin electroplate. A few shops may still use tin-lead electroplate, The most complicated would be nickel metallic etch resist that is stripped off - fortunately not common.
The photoresist is stripped away before etching the starting copper foil on the surface. Etchant attacks the sides of the traces, but with spray etching, to a lesser degree than etching "downward" on the starting foil. Metallic etch resists cause a small amount of "galvanic attack" at the junction of the metal and the copper. Nickel or nickel/gold junction with copper casuse a lot of galvanic attack, and much undercut. Tin or tin/lead has more galvanic attack than inert tenting photoresist, but much less than nickel. It is never wise to fabricate so that etchant causes slivers of metal to crack off the edges of the circuit trace.
Finally, the copper thickness on a board may vary with current density - thicker at board edges and isolated traces - where it may mushroom. Where the traces are close together and near the center of the board, there will be the trapezoids you describe. Starting with thin foil - half ounce or less - and using thick photoresist will make for more consistent copper cross sections.
Denny Fritz
-----Original Message-----
From: Dan Skweres <[log in to unmask]>
To: TechNet <[log in to unmask]>
Sent: Thu, Oct 28, 2010 11:04 am
Subject: Re: [TN] profile (section) of outside layer trace
Mark,
hen we calculate impedance we typically have the foot of the trace wider
han the top.The mushrooming of the trace actually depletes or breaks off
hen tin stripping or resist stripping.
an S.
---- Original Message -----
rom: "Mark Larson" <[log in to unmask]>
o: <[log in to unmask]>
ent: Thursday, October 28, 2010 10:48 AM
ubject: Re: [TN] profile (section) of outside layer trace
ortobello? or ... I hope not morel
he reason I am asking is for impedance calculation, and I guess the
ushroom shape is another reason to go stripline, better control
prediction) of the finished line shape
-----Original Message-----
rom: Joyce Koo [mailto:[log in to unmask]]
ent: Thursday, October 28, 2010 9:14 AM
o: [log in to unmask]; Mark Larson
ubject: Re: [TN] profile (section) of outside layer trace
Mushroom.
-------------------------
ent using BlackBerry
---- Original Message -----
rom: Mark Larson [mailto:[log in to unmask]]
ent: Thursday, October 28, 2010 10:01 AM
o: [log in to unmask] <[log in to unmask]>
ubject: [TN] profile (section) of outside layer trace
during etching a trapezoid shape is created right? with long side of
rapezoid up (away from dielectric)? but after plating?
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