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December 2011

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Subject:
From:
Jack Olson <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Jack Olson <[log in to unmask]>
Date:
Wed, 14 Dec 2011 09:31:45 -0600
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That's my experience, too....
As a designer or librarian, UNLESS you are responsible for creating your own paste screens (not the starting paste screen data file, the actual finished Paste Screen), it is probably better to make the paste screen apertures match the land sizes. 
You may do a perfect reduction on one part, but not on another. One supplier may have a specific preference, a different one may prefer something else depending on their process parameters.

From my (maybe limited) experience, suppliers would rather have a known starting place to work from, rather than have your best attempt at giving them what they need. 
As a matter of fact, several that I have spoken to say they discard our paste data and create their own starting place from the pads on the surface gerber layers and then adjust it, meaning any effort by us to guess what they want is literally a waste of time.

others may disagree, just trying to help,

Jack

.
On Wed, 14 Dec 2011 09:11:06 -0600, Pete <[log in to unmask]> wrote:

>Bob,
>
>I completely agree!  For efficiency's sake, that would be the best way.  But there are problems.  Few PCB CAD library designers have the knowledge to do that correctly, so the tweaking happens on every build anyway.   Sending the same assembly to different asssmbly vendors means that each may prefer their own answer to the stencil design.  Some footprints could be universally pasted, some may be process specific.  That gives you a library and gerebr files that may or may not be complete, and can change from design to design.  Tracking and communicating that leads to possible errors.
>
>I've had vendors in the past who wanted my gerbers to be modified when they get them.  For the most part, they have preferred to do it themselves. 
>
>Pete

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