Dominic,

The info in IPC7525 is a good staring point.  There are several factors to take into account when doing your DFM on the board layout.  The first being your stencil design.  Another is the direction of travel of the squeegees.  The hard one to get from your designers is keeping paste height compatible parts in close proximity of each other.  By this I mean do not put a .4mm pitch device or 0402 next to something like a butt-joint power supply where you needs lots of paste height and volume.  Enough about the stuff you cannot control at this point.

First of all you need the squeegees to evacuate as much of the paste as possible.  You also need the area immediately adjacent to the aperture to be clean.  If the squeegee blade is leaving paste residues on the stencil then you will have the inconsistent paste thickness.  So one thing to take into account is squeegee direction.  The area parallel to the squeegee will clean out fairly well.  The areas parallel to the travel of the squeegee will not and that is where you need the keep out.  You can play with your squeegee pressure and improve your clean-out.  If squeegee pressure alone does not work you may need to change to a rubber/neoprene squeegee blade.  They will form into the step better.  I am assuming your steps are on the paste side of the stencil.  There are some people doing contact side steps but I feel they are best left for creating pockets for label and such.  One of my EMS's tried using a contact-side step on the BGA and it did not work very well.

I have also heard of some blade suppliers creating a custom squeegee blade in an attempt to match the step areas in the stencil.  I have never tried anything like that and I would think setup and consistency might be difficult.

The short answer is you have to clean out and around the step to get a good print.  You can do it by having a generous clearance around the edges parallel to the direction of travel or try some different blades.  If you use the rubber you have to be careful of you will scoop-out paste for your large apertures.

Good luck


RUSSELL NOWLAND
ALCATEL-LUCENT
REPAIR ENGINEERING SUPPORT
14000 Quail Spring Parkway, Suite 100
Oklahoma City, OK 73134
T: +1 405 302 1660
F: +1 405 302 1622
M: +1 405 203 0034
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dominic Boudreau
Sent: Thursday, November 10, 2011 11:57 AM
To: [log in to unmask]
Subject: [TN] Step Stencils design

Hello Technetters, I would like to request your help regarding step stencils; I never worked before with these of stencil so all recommendations would be really appreciated.  We have a step-up stencil that has 3 thicknesses: 8, 6 and 5 mils. Most of the stencil is at 6 mils and there are some areas that are at 8 and at 5 mils. The problem that we have is that the paste thickness that we obtain on the  5 and 6 mils areas is around  8 mils... The stencil has step-down from 8 to 6 mils and from 6 to 5 mils . Here are my questions:

1) Which keep-out out distance we should have. is the rule of thumb of IPC-7525  correct ( 35 mils for each 1 mil step thickness)?.
2) Which type of stencil is recommended , electroform?
3) Do we need a special type of squeegee blades?
4) Presently we use step-up stencil, should we use step-down stencil?

Thank you
 Dominic Boudreau

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