Ben,

In the past I have been successful doing 10/6 and 10/4.  The main thing is getting adequate clearance around the step for the cleanout.  IPC7525 addresses some of the step stencil requirements.  You can use neoprene or polyurethane are common (70-90 durometer).

Photo Stencil can grow an Electroform which is very good, flat and I have never seen any waves like you can get from chem etching.   It has been a while since I used them but UTZ Engineering use to have a laminated stencil which they glued two pieces of stainless steel together creating a very sharp clean step.  It was much cheaper than the Electroform but there was one issue.  If you used an ultrasonic cleaner to clean the stencil the high frequency vibrations causes the stencil to delaminate.  I am not sure if they still offer it.

I suggest you check around with a few suppliers to see what they are offering.  There are even some techniques involving custom squeegee blades for step stencils now too.  I am on the APEX Technical Program Committee and there is going to be a couple good tracks on stencil printing, including incorporation of step stencils and even a couple papers of 3D PWBs and discussing the printing challenges printing those PWB with step stencils.

I hope this helps


Russell Nowland
Alcatel-Lucent
Advanced Manufacturing Engineer
Address: 14000 Quail Spring Parkway, Suite 100
Oklahoma City, OK 73134
email: [log in to unmask]
Desk: 405-302-1660
Cell: 405-203-0034
Fax: 405-302-1622
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gumpert, Ben
Sent: Wednesday, July 13, 2011 9:42 AM
To: [log in to unmask]
Subject: [TN] Stepped stencils - Is there a limit?

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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