News ......

IPC-7525 B revision,  has been recently issued.

Reagrds
Gabriele

-----Messaggio originale-----
Da: TechNet [mailto:[log in to unmask]] Per conto di Nowland, Russell Howard
(Russell)
Inviato: giovedì 10 novembre 2011 20.09
A: [log in to unmask]
Oggetto: Re: [TN] Step Stencils design

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
[log in to unmask]
-----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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