Hi Steve,

>Regarding use of conductive adhesives - while it does seem rather silly to
>attach (a generally high-power device) with poor thermal and electrical
>conductivity polymers, we have done it (rarely) for those occasions that did
>not require sustained current handling capability.  I believe they were more
>of a Commercial application than Military, however.  Definitely not Space or
>Implantable applications.  At least the silver presents a more chemically
>bondable surface than does pure gold, for example.
>
>We even used some of the silver-glass materials for a Mil application, but
>the die were only ~3mm square.  Die were mounted on moly-tabs (spreaders) to
>facilitate re-work, if required.  The tabs were adhesively bonded with
>traditional Ablebond 84-1 series adhesive - now that the thermal capacity
>and 'contact' area was increased by the tab.
>
>Steve Creswick

While materials like 84-1LMI do have a thermal resistance of more than 10
times
that of typical solder alloys, we have found that this does not neccessarily
impact significantly on the overall thermal resistance as a thinner bond can
be acheived and the die bond thermal resistance is also usually only a small
part of the overall thermal resistance of path the heat takes from junction
to ambient. In some trials a few years ago we found the average thermal
resistance of a chip bonded with 84-1LMI was slightly worse than for SnPb
solder, but the spread for solder was greater, with the worst soldered sample
actually being worse than the worst adhesive bonded sample!

David Whalley

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