I was not going to comment but Ken and Bruce are on the right track and I'll
attempt to add a little more value a real life situations along the lines
that they have already stated.

In general, the over etching takes place on every conductor edge at about
the same rate.  Even when the parts are "normally etched" (not over etched)
you generally lose about 0.2-0.4 mils (I know I did not use metric!!!) on
each edge (0.4-0.8 mils total compared to the line width on the Artwork, i.e
a 4 mil line on the artwork would be 3.6 to 3.2 after etch).

I can't think of a customer that does not give some specific requirements
about the mnimum SMT pad size over and beyond IPC 6012.

But in reality, when you over etch the panel significantly the circuit lines
become defective before the SMT pads and therefore the pads become a mute
point. The only case this may not be true is a design with SMT pads only
without exteranl signal lines (not very likely).   So therefore, the issue
almost never comes up even without a seperate requirement for pads.

Mike


-----Original Message-----
From: Bruce Panke [mailto:[log in to unmask]]
Sent: Monday, March 12, 2001 10:51 AM
To: [log in to unmask]
Subject: Re: [IPC-600-6012] Over etch of component pads


I'm not sure if you've had someone from assembly comment yet, and I support
Ken's comment.

To take it a bit further - the end "use" of traces and SMT pads (square,
rectangular) are different.  SMT pads must have a tighter range as they
ultimately are the footprint and paste/solder volume determinant during
assembly - an attribute which a trace does not have.  To state the obvious,
the etcher can't tell the difference between a "trace" and a "pad", but
given the tolerances could be different, perhaps it's time stated tolerances
for pads differ from traces, especially where finer features are required
such as QFP's, and essentially all fine pitch.

Bruce Panke
Teradyne

----- Original Message -----
From: John Perry <mailto:[log in to unmask]>
To: [log in to unmask] <mailto:[log in to unmask]>
Sent: Friday, March 09, 2001 11:14 AM
Subject: [IPC-600-6012] Over etch of component pads

Good morning everyone,

Some users of the IPC-6012A and IPC-A-600F as well as myself have been
unable to find anything that describes allowances/restrictions for the over
etch of component pads.  I am not sure if there is an unwritten
understanding among the industry that when these documents refer to etching
characterisitics for conductive patterns, that the term "conductive
patterns" is inclusive for both conductors and surface mount component pads.

If the answer to the question is yes, the term is inclusive for component
pads, then I think we should add something to the specs to indicate that.

If the answer is no, then I'll need your help in determining where that
information should be listed and what are the industry recognized
requirements for them.


Best Regards,



John Perry
Technical Project Manager
IPC
2215 Sanders Road
Northbrook, Il 60062
1-847-790-5318 (P)
1-847-509-9798 (F)
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