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May 1999

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Subject:
From:
Wade Oberle <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 13 May 1999 09:53:32 -0500
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Jim,
        I am not sure if I understood you correctly but here is my two
cents for what it is worth.  If there is copper under the area that is
being analyzed by SEM/EDX, you may be detecting the copper from that
source.  The SEM/EDX system uses an accelerating voltage.  Years ago
when I ran this type of system, we used an accelerating voltage of
either 10 Kv, 20 Kv, or 30 Kv.  The higher the Kv, the deeper the
penetration of the energy beam into the surface that is being analyzed.
It may be possible that the SEM/EDX is penetrating through the surface
layer of interest and into the layer underneath.

Wade Oberle
Manutronics, Inc - Division of Sanmina

        -----Original Message-----
        From:   Jim Marsico 516-595-5879 [SMTP:[log in to unmask]]
        Sent:   Thursday, May 13, 1999 7:26 AM
        To:     [log in to unmask]
        Subject:        [TN] BGA FAILURE ANALYSIS

        Good Day Technet:

        First, a little background...  We're in production using a 228
ceramic BGA for
        a military contract.  The base material of the component is gold
plated nickel,
        prior to solder ball attachment.  The gold plating is very thin,
around .25
        microns.  We've been experiencing approximately 5% (components,
that is, not
        solder joints) open circuits found at in-circuit testing (ICT).
Upon removal
        of the BGA (Air-Vac system) from the board, the specific ball
location where
        the open was identified at ICT shows no solder remaining on the
component,
        where all other locations has solder remaining.  What does
remain is a dull,
        gray, crystalline metallization.

        SEM/EDX analysis shows Ni/Sn and Cu/Sn.  It is presumed that the
Ni/Sn is the
        intermetallic layer created from the nickel base material and
the tin in the
        solder.  What puzzles me is the presence of the copper-tin
crystallites.  Since
        there is no copper in the solder or on the component, is it
possible that this
        copper has migrated all the way up from the board pad to the
component/solder
        interface?

        Thanks in advance...

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