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August 1999

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Subject:
From:
"Mcmaster, Michael" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 12 Aug 1999 15:09:32 -0700
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Phil

It sounds like you are using the DuPont via fill material.  If so, the
fill material can be plated with electrolytic copper.  I haven't seen a
design yet that left the via fill material exposed on the finished
board.  This would appear to eliminate your concerns with the silver,
though.

        ----------
        From:  Phil Bavaro [SMTP:[log in to unmask]]
        Sent:  Thursday, August 12, 1999 9:14 AM
        To:  [log in to unmask]
        Subject:  [TN] Filled vias

        Our design group just brought a new technology (to me at least)
out
        yesterday and I find it hard to believe that this is proven
technology in
        the industry.

        In order to transfer heat away from a part which generates quite
a lot of
        heat (upwards of 4 watts?), they have placed a matrix of vias
below it and
        then filled them all with silver filled epoxy, expecting us to
then screen
        solderpaste on top of this in order to connect the supposedly
solid barrel
        of the via with the belly of the component.  The assembly
consists of
        essentially a CCA mounted inside of small chassis and the only
cooling
        transfer is through the boards which sits on a boss in the
chassis.

        Is there published reliability data on solder connections made
by
        interfacing silver filled epoxy and eutectic solder?  If so,
please direct
        me to it. We do have a very small percentage of silver in our
alloy.

        The solderability of silver filled epoxy when exposed to
solderpaste on top
        of a nickel gold substrate does not leave me with a warm and
fuzzy feeling.
         I am confused how the intermettalics (I know, I know, which
one?) are to
        be formed, or even if the silver filled epoxy wets with
solderpaste under
        normal reflow temperatures.  If there are other concerns I need
to focus on
        as well, I'm open to all suggestions.  Someone mentioned
galvanic
        interactions......

        My hunch is that we will have large voids in the connections
below the part
        and this will reduce the heat transfer ability of the assembly
when
        compared with a solid solder filled via that wets all the way up
to the
        belly of the component.

        But then again I could be wrong..........please enlighten me.
        Phil Bavaro

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