word of caution (you've heard it b-4), as a general rule - if u design 
     imped. then u are saying to your vendor "build it this way", yes, I 
     think (as would most folks out here) u have then taken away the 
     vendors area or expert supporting role. finished bd. dim.'s fall into 
     that "must know area" for your vendor, however, it is never wise to 
     ass'u'me one knows everything (goes both ways, design & vendor), 
     working together is the most effective (cost & other) way to complete 
     the product.


______________________________ Reply Separator _________________________________
Subject: Design for Manufacture?
Author:  [log in to unmask] at SMTPLINK-HADCO
Date:    11/27/96 3:14 PM


In a previous discussion, I asked about designers calling out 
specific material thickness/stackup versus just calling out the 
desired finished board thickness. The general consensus was that 
unless there is a good reason for specific materials and thicknesses 
(for impedance control) it would be best to let the vendor decide 
the best stackup for a particular design.
     
In another discussion about impedance control, the question was 
whether the designer should take it upon himself to do all of the 
calculations, or to just try for appropriate trace widths and call 
out the desired impedance for those particular traces. The general 
consensus was that if the designer did all of the work and told the 
vendor exactly what to do and it came out wrong, the designer had 
only himself to blame. If he calls out only the desired impedance 
and lets the vendor "tune" his process for it, the vendor is to 
blame if it comes out wrong.
     
Well, I'm not trying to illustrate how we as designers should do as 
little as possible and leave responsibility with the vendor... and 
there sure has been a lot of talk about "Design for Manufactur- 
ability" and how we should do our best to learn what's going on 
instead of just "throwing it over the wall". Yet when you try to get 
educated, you are told to "leave it to the vendor".
     
What's a designer to do?
     
Just a semi-casual thought (not trying to start any wars or anything)
     
onward thru the fog,                                    Grasshopper
     
p.s. Have a nice holiday you guys, I'm outta here...
     
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