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January 1998

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Subject:
From:
Charles Elliott <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 28 Jan 1998 11:30:06 -0500
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        Reply to:   RE>[TN] Use of Solder Mask Defined pads for BGA ?

Hi Johannes,
There is some debate about Solder Mask Defined (SMD) pads for BGAs.  Some
papers indicate that Non Solder Mask Defined (NSMD) pads increase the  fatigue
life of the solder joint (SMD pads cause stress concentration point in solder
joint).  However, it is interesting to note that many of the BGA component
suppliers actually use SMD pads on the BGA substrate.  So, although you can
choose SMD vs NSMD for your board, the supplier dictates the substrate design
(typically SMD).

To address your overprinting question (printing paste on solder mask), we have
been using overprinting for Ceramic BGAs successfully for the past couple of
years without problems.  The paste will pull back to the solder joint.  You
may want to verify on your particular assembly (X-sections, etc.).

Hope this helps.
Charles Elliott
Newbridge Networks

--------------------------------------

The pads of PBGA`s that we use are defined by solder mask  on  our
multilayer FR4 pcb`s
The diameter of stensil aperture is bigger than the pad diameter , so we
are actually printing
( no-clean ) paste on the soldermask.
Is this OK, or should it be avoided ? I presume that the adhesion between
stensil
aperture-paste is bigger than the other one between soldermask and paste,
resulting
to increased risk to solder balls, which could be fatal under PBGA ( pitch
.05" )

Stencil width is 0.15 mm (  6 Mils ), laser cut
Stensil  diameter .56 mm  ( 0.22" )
Pad  diameter    0.72 mm  ( 0.28" )  max
Solder Mask diameter 0.52 mm ( 0.20" )

Any opinions?

Brs, Johannes

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