Usually, you specify a trace width as "---- +/- x mils". The reason for
that is that overetching is usually consistent by around 0.5 to 1 mil
irrespective of trace width. As a result, the percentage of trace width
tolerance gets worse as traces become thinner.
Unfortunately, that's a fact of life, and is the reason it's very hard
to get reasonable impedance tolerances on thin traces.
Andy K.
Sychip Inc
Office (972) 202 8852
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of
[log in to unmask]
Sent: Monday, April 26, 2004 1:05 PM
To: [log in to unmask]
Subject: Re: [DC] Etching Plating tolerance specs...
It's a great question, Kelly.
Seems like every way I've seen it, it's open to interpretation. Does a
plane area still have a "trace width"?
If the drawing says +/- 10%, does that mean the fabricator takes 10% of
the
smallest trace on the board and uses that number everywhere? Or does it
apply to each width?
Better yet, I've seen it as a trace/space tolerance. If a minimum trace
is
routed at minimum space anywhere, any deviation is a violation of one or
the other.
I know, I know... That really just didn't help at all....
"Dack.Kelly"
<[log in to unmask]> To:
[log in to unmask]
Sent by: DesignerCouncil cc:
<[log in to unmask]> Subject:
[DC] Etching Plating
tolerance
specs...
04/26/2004 12:58 PM
Please respond to
"(Designers Council
Forum)"; Please respond to
"Dack.Kelly"
Greetings,
An inconsistency in our fab specs regarding etching/plating tolerance
finds
me at a mental "cross-roads" regarding this subject.
The spec for some of our designs express the tolerance in the form of
"mils"
-- +/-.001, while other spec express the tolerance in the form of a
percentage -- +/-10%.
In one sense, on a design w/ .010 nominal lines and spacing, these two
specs
are the same. However, on a design w/ .005/.005 or .004/.004 lines and
spacing, using a tolerancing method of +/-.001 vs. 10% has drastically
different results.
I would like to see any feedback on the pro's and con's of each method.
What
method do you use and why?
-- Kelly
Kelly Dack
PCB Dsnr
IGT (International Game Technology)
Reno, NV
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