Hello Phil-
Sounds like you've got a tough one here.
In trying to visualize the proposed set up, I had a few questions:
1. Is the heat dissipating component a PQFP? A metal or Kovar case, or
plastic?
2. As mounted, does this component come in contact with the PWB, or is there
a gap under the component body? About how big a gap?
3. Just a thought: so-called thermal vias under hotter running ICs are
fairly common; they are usually just PTH vias, essentially solid plugs of
Cu, running the entire thickness of the board. Silver filled epoxy is a
matrix of epoxy and silver particles [or possibly silver plated particles?].
It might be possible that Cu vias are lower thermal resistance paths than
typical, as-applied and as-cured conductive epoxy plugs! Also, beware of
unevenly distributed particles in conductive epoxies; insufficient mixing
could cause drastically reduced performance.
Thermal conductivity values for Cu vias and for "ideal" [and also, perhaps,
"typical"] conductive epoxy plugs should be compared, in order to help
decide a prudent approach.
4. Cu vias, or epoxy plugs, will be part of a total thermal efficiency based
on how they interface [mechanically] to the bottom of the hot component.
Combinations of both types of vias, combined with interface materials such
as solder [the option you've already described], or thermal grease, or a
thermal interface gasket, or just mechanical contact, should be accounted
for when comparing the cooling efficiency of various options. A poor
interface to any type of via will torpedo the whole cooling scheme!
5. Speaking of poor thermal interfaces, I'm skeptical of solder wetting to
epoxy, though I have no experience with this combo.
Regards,
Michael Alderete
Aerojet
Phil Bavaro wrote...
------------------------------
Date: Thu, 12 Aug 1999 09:14:25 -0700
From: Phil Bavaro <[log in to unmask]>
Subject: 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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