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August 2008

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Subject:
From:
Bill Kunkle <[log in to unmask]>
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Date:
Wed, 27 Aug 2008 10:44:57 -0400
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1 option would be 4 mils thick Ni electroformed, increase the QFN opening on
the toe to 10 mils,  7.8 x 10 with corner radii yields 0.55 area ratio.

How much space is there between the QFN and adjacent components?  If there
is .10", a 2 mil step would likely work with a metal squeegee.

Bill Kunkle
MET Assocs

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Kane, Amol (349)
Sent: Wednesday, August 27, 2008 8:13 AM
To: [log in to unmask]
Subject: [TN] Step Stencil Requirements


Dear Technetters,
I am looking for some information about using step stencils in production. I
have a 16 mil pitch QFN component with 7.8 mil square pads. This gives me
unacceptable area ratio using a 5 mil stencil. Looks like I am going to have
to use a 3 mil thick stencil with this component. I was thinking of using a
5 mil stencil with a 3 mil step.  There are chip components around the QFN
device. The board is Lead-Free with ENIG finish. Using a Type 4 solderpaste.

I was looking for inputs from folks who are using step stencils on the
squeegee types (metal vs. poly), changes in print parameters (speed,
pressure, Snap off etc,), board design considerations (keep out areas around
the component needing the step), resources I could go thru to obtain more
information, stencil houses who specialize in this process, and common
pitfalls.

Any inputs will be greatly appreciated.

Thanks,
Amol





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