This sounds like uncompensated etchback.  Would it be possible for you to
ask your board supplier to perform "etch compensation" on the production
artwork?  Most if not all vendors are agreeable to doing this, since they
have a good feel as to how much copper is lost (widthwise) during etching.
Then you would probably see a much smaller variance in trace/pad widths.
Then you could specify a tighter tolerance on pad widths if so desired.
Just a thought...
Rgds,
Roger M. Stoops,  CID   [log in to unmask]
TRIMBLE NAVIGATION LLC
Engineering and Construction Division
5475 Kellenburger Rd.
Dayton, OH   45424-1099      USA
Telephone:  937 233 8921      800 538 7800      ext. 288
Facsimile:  937 233 7511

http://www.trimble.com


 -----Original Message-----
From:   d. terstegge [mailto:[log in to unmask]]
Sent:   Monday, February 04, 2002 5:33 AM
To:     [log in to unmask]
Subject:        [TN] Tolerance on padwidth

Hi Technet,

IPC-6012 mentions "when not specified on the master drawing the minimum
conductor width shall be 80% of the the conductor pattern supplied in the
procurement documentation".
This is fine for the tracks, but it may cause problems with the solderpads.
Quite often I see that the 12 mil designed fine-pitch pads have a width of
only 9.6 mil on the actual products, leaving a marginal process window for
assembly.
I can live with 11 mil instead of 12 mil, but 9.6 mil is too small for my
taste. The questions:
1)   Is there an additional specification for the etching tolerance of the
solderpads ?
2)   If we put additional requirements on the master drawing, what would be
an acceptable value ?

As always I'm very interested to hear your comments,

Daan Terstegge
SMT Centre
Thales Communications
Unclassified mail
Personal Website: http://www.smtinfo.net

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