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From:
Creswick <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Sun, 11 Jun 2000 07:32:20 -0500
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Ingemar,

I always believed the Ag layer to be essentially a "flash" to cheaply
(rather than gold) passivate the solderable Nickel layer, with the Cr/Ni-Cr
layer being the adhesion layer to the silicon.

Like a gold passivation, the silver should immediately go into solution and
allow the solder to wet to the underlying nickel layer.  Once alloyed, it
should create no more of a migration problem than silver bearing solders
like Sn62/Pb36/2Ag - should it???   Guess, I will have to re-check my
assumption that it is a 'flash' of silver.

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



At 03:49 PM 6/7/00 +0200, you wrote:
>Short q: Ag/Ni/Cr, what do you like that chip backsides for attaching power
FETs? Ag is 90%.  Unusual? IRC make such backside metalizing. Wonder
why...I'm somewhat concerned, Ag migration and strange intermetallics may
occur, depending on what solder or glue you use. Anyone mounting these
creeps? Steve?
>Ingemar Hernefjord
>Ericsson Microwave Systems
>
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