Ben,
The "Rules of thumb" can be found in IPC-7525.

BUT: I believe that in most instances the stencil material is full hard
stainless steel. When you remove a layer of that material over a large
portion of the stencil I expect irregular behavior of the remaining material
leading to local warping.
In such an instance I would suggest to use a 5mil stencil with a locally
added layer of 3mil material.

Regards,
Ahne Oosterhof.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Reuven Rokah
Sent: Thursday, July 14, 2011 01:11
To: [log in to unmask]
Subject: [Norton AntiSpam]Re: [TN] Stepped stencils - Is there a limit?

Check the stencil alloy, etching process and stencil frame.
The etching area and depth also important parameters.
Reuven

???? ??-iPhone ???

?-13 ???? 2011, ???? 17:41, "Gumpert, Ben" <[log in to unmask]> ???/?:

> Technetters,
> 
> Is there a rule of thumb regarding the amount of step that can be etched
into a stencil?
> 
> I've used stencils with 1 and 2 mil steps successfully, but recently I've
seen a problem with a 3 mil step (8 mil foil stepped down over most of the
printed area to a 5 mil thickness).
> 
> The problem I'm seeing is that the stencil tends to warp around apertures,
particularly in areas like fine pitch QFPs, where there are long apertures
close together, and the remaining material is a thin strip. This warp makes
the stencil act thicker and allow more paste to be deposited, leading to
bridges in the fine pitch QFPs.
> Anyone seen this problem before?
> 
> Thanks in advance,
> Ben
> 
> 
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