The paste with the metal loading, tackiness of the flux, wether it is a
turret placement machine or a gantry machine, method of fabrication of
stencil and its thickness as well as component mix, inert or air reflow are
items that that affect the stencil design. Best left to the manufacturing
side, even more so if an EMS is used. 1:1 is the place to start and the user
of the stencil will have it modified as fits the process. Its nice to have
the old stencil as a starting point for the unique situations if it is
transferring from another location as well.

On Wed, Jan 7, 2009 at 9:04 AM, Pete Houwen <[log in to unmask]>wrote:

> Dave,
>
> Making those decisions from the PCB design or stencil manufacturing side is
> really backwards.  That's really up to assembly engineerring.  The 10% rule
> is
> just a starting point.  And multiple CMs might want their own shapes, based
> on
> their equipment/processses.
>
> We have talked about adding special stencils to our PADS library for years,
> but
> even with our own captive assembly, haven't come up with enough "standard"
> modifications to make it worthwhile.  We might do just a few that get
> modified
> the same every time.  The new mask adjust attributes Mentor added in v2007
> make it tempting for the PCB guys to engineer that, too, but that only
> allows
> for a same shape reduction/expansion.  They're probably most useful only
> for
> unique BGA, fine pitch, or via solder mask apertures.
>
> Even a control freak like myself doesn't want to take on that part of the
> job!
>
> Pete
>
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