>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
>
The 2 different approaches are not totaly opposed to each other. I think
Designers need the knowledge about the manufacturing process but must be
very carefull not to tie the vendors hands. We can only know the
theoretical, not the exact process in each individual shop. The vendor
himself must be allowed to make adjustments for his particular process.
Document the "END" product, do NOT document how the vendor gets there. 

Regards
Fred Pescitelli
Phoenix Designs
1285 Turner Rd.
Lilburn, GA   30247
[log in to unmask]
770-923-3465

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