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February 2005

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Subject:
From:
"Bill.Birch" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Bill.Birch
Date:
Wed, 9 Feb 2005 11:46:47 -0500
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        Hi Peter,

        To some extent the aspect ratio determines the "rules";  Once you
get above 4 to 1 the influence of non-functional pads starts to play a role,
especially when layer counts and/or higher resin contents constructions are
used.  In an ideal situation there would be "no pads" in the barrel
structure, this would permit maximum compliance/compression during the
thermal excursion.  In reality we have random "locking" locations that focus
the stress toward the bending moment, creating a stress riser.

        Basically, the higher the aspect ratio (or volume of resin) the more
compliance is required.  In order to maximize the strength of the barrel we
need to "re-distribute" the stress somewhere away from the focal point
(usually the centre of the construction) by removing non-functional pads
from the central zone.  Our experience has shown us that distributing the
stresses equally across the central zone of the barrel achieves the highest
reliability, specifically when good quality plating, materials and
processing conditions are involved.

        The majority of premature failures that we investigate include
individual connections between the internal pad located in the central zone
of the via, the barrel cracks are predominantly on the B stage (pre-preg)
side of the foil.  The pad creates the anchor point, which becomes more
influential as the foil thickness increases.

        A standard design rule that we recommend would locate non-functional
pads at the signal or plane layer locations closest to 1/3rd of the distance
from the top and bottom of the structure (total two non-functional pads,
this is in additional to the functional pads.  This approach divides the
barrel into 3 "stress" sections;  The upper and lower sections receive
virtually no tensile or compressive stress (most of the stress' are heading
to the surface, between the vias), the location of two constraining
connections permits a more even distribution of stress around the central
area of the barrel.

        For a worse case condition a secondary rule could apply as follows:
If you have a 16 layer board and the signal layer is connected only on layer
9, locate a non-functional pad on layer 8 (to balance the centre stress
distribution), plus apply the 1/3rd rule.

        I hope this has helped to explain the logic.  Obviously, the rules
are relative to the product complexity and construction, I would be very
interested to hear the results of any other testing that has been completed.

        Best Regards,

        Bill.

-----Original Message-----
From: Pete menuez [mailto:[log in to unmask]]
Sent: Wednesday, February 09, 2005 9:03 AM
To: [log in to unmask]
Subject: [TN] Removal of Non-functional pads


Can someone explain the advantage of removing non-functional pads on
innerlayers? We are contemplating adding non-functional pads to our designs
but want to understand the impact if we do.

Thanks

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