Does it really matter if your 6 mil line is 5.9 mils? Ultimately aren't
the main considerations that the target impedance is met, that the line
width is sufficient to carry the required current, votage drop, or
whatever? The bottom line when writing a specification is that the spec
is necessary to the manufacture, assembly, and function of the circuit.
If there is a reason to specify +/- % or mils then do it. If there is no
reason then why would you even specify a thickness tolerance? If you
write a spec somebody is going to try to build to it and somebody is
going to be inspecting for it. That adds cost, sometimes substantially.
Does anybody even know what thickness range is acceptable on their
designs? How do you know?
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of
Brooks,Bill
Sent: Monday, April 26, 2004 3:04 PM
To: [log in to unmask]
Subject: Re: [DC] Etching Plating tolerance specs...
Hi Kelly,
The 10% rule should really be applied to traces under a certain size...
and the +/-.001 inch tolerance should apply to larger traces that would
be significantly out of tolerance if 10% was applied to them. The need
to use a percentage tolerance comes from trying to use a fixed tolerance
like +/-.001 on a trace that is too small like .006 in. width. The .001
change is width becomes very significant for these smaller width traces
and that's why the adopted 10% tolerance came into play where .001
inches would be greater than 10%. Neither method stands alone as it can
not describe accurately the tolerance scheme being employed.
On the other hand, I believe, a combination of the 2 can accurately
describe the end results in a way that can be measured and verified. I
believe a uniform note could be created that could encompass all
variable situations encountered and give good repeatable measurable
results.
i.e. -
Note: All etched and/or plated features shall be held to within 10% of
cad generated artwork originals, or +/-.001 inches which ever is the
lesser variation in geometry.
Does something like that come close to describing what is desired?
Bill Brooks
PCB Design Engineer , C.I.D., C.I.I.
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
-----------------------------
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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