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April 2004

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DesignerCouncil <[log in to unmask]>
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"Brooks,Bill" <[log in to unmask]>
Date:
Mon, 26 Apr 2004 13:03:48 -0700
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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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