The component mix on PCA (stencil thickness), stencil printer (blades
or pump), placement equipment(move the head, move the board),
tackiness of paste (how much is needed to keep parts in place during
process), method of reflow (how part behaves in oven) all effect the
stencil aperture. For OEM's, good stencil libraries are possible if
the company maintains an equipment set. Once CM's are involved, the
set varies widely. Knowing the pads are 1 to 1 with the stencil layer
allows for modification to the assemblers current best practice.

On 12/14/11, 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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