Reply to:   RE>assly:stencil aperture size for 20 mil pitch component

Nalin:

The other factor to consider is the copper coating on the PCB.  If it is HASL,
there is less paste volume required, since some solder is already present on
the pads.  If OSP coatings are being used, I would recommend a full 1-to-1
stencil opening to provide resultant solder to form a reliable solder joint.

IMO, I would NOT recommend HASL for SMT designs using 0.020"-pitch components.
 A FLAT pad surface is a key factor to maximizing assembly yields and
minimizing solder bridging or skewed components.

Bill Fabry
Truevision, Inc.
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--------------------------------------
Date: 11/21/96 9:03 AM
To: Bill Fabry
From: [log in to unmask]
From:  Karen Bozynski
*** Resending note of 11/21/96 09:17
       Process Development Engineering         (416) 448-6204
---------------------------------------------------------------
Subject: assly:stencil aperture size for 20 mil pitch component

There are additional parameters to consider in aperture design.
o  Your stencil thickness is your third dimension in determining
   solder volume, so a thicker stencil would require a smaller aperture
   (be careful of aspect ratio!), or you could get away with a larger
   aperture if a thinner stencil is used.

o  In addition to aspect ratio, the condition of the aperture walls
   will also determine your success in paste release.  You should find
   good results with laser cutting if your stencil vendor has
   precise alignment and control of the beam strength.

o  Your company's SMT process parameters for stencil alignment in
   the screening stage, module skew in placement, and reflow
   parameters will also dictate the optimal aperture for you.

Good luck!

Karen Bozynski                Internet: [log in to unmask]
Celestica Inc.                            Phone: (416) 448-6204
844 Don Mills Road, 34/178                  Fax: (416) 448-4736
North York, Ontario  M3C 1V7  CANADA