I may be mising something here but why not fill the vias with solder? this avoids the
following issues and will most likely achieve better thermal conductivity.
You should be aware of the following anyway:
1] When using silver filled adhesive to bond to  tin or tin alloys there is a
reaction at the interface. The reaction product has a different volume which can
break the joint, certainly electrical conductivity goes even if the mechanical bond
remains. I appreciate you are proposing to bond solder to a cured epoxy which is the
other way round, but I would not rate this a high reliability proposal, as the
failure mechanism is unlikely to be influenced by assembly order.
A lot of work has been done in this general area by companies developing Ag filled
adhesives as solder alternatives, and a deal of stuff has been published in their
efforts to promote their products so you may care to contact them for further
info/references.
2] Soldering to metal filled adhesives is not going to be too successful: by volume
the metal/polymer ratio in a typical epoxy is about 50/50. [To help picture this
this is about the same
as unreflowed solder paste].
To emphasise what may be obvious: Although the cured adhesive is hard and shiny like
a metal, this ratio does not change significantly on curing - unlike in reflowed
solderpaste. Thus the available metal area in the adhesive surface is going to be low
and anyway likely to be further reduced by being wetted by
epoxy.
3] Filling the vias with  the silver epoxy after soldering is not really an option
because of 1] and in any case epoxy does not adhere well to tin/tin alloys (whether
silver
filled or not) so you would get a low strength bond even before 1].






Mike Fenner
BSP, OX15 4JQ, England
T: +44 1295 722 992
M: +44 789 999 7715
F: +44 1295 720 937


-----Original Message-----
From: Phil Bavaro <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: 12 August 1999 22:20
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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