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June 2000

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From:
"Ingemar Hernefjord (EMW)" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 13 Jun 2000 11:30:46 +0200
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Steve,
good to hear your opinion about using 'glue' for powerFET mounts! Me too dislike that. We heatspread on CuMo or pure Mo also, and even use 84 series. Now, generally spoken, I'm a little worried about using adhesives at all where you have heat dissipation in the order of watts per chip. We are going to use EMMI, LCD and IR for (hopefully) verifying of the computerized Rth. OK. Back to Ag plated backsides, you may be right there about harmless silver intermetals and silver migration. If the silver is thin enough, the soldering occurs on the nickel, I will check the layer thickness. We use 80/20 AuSn for this FET, in principle all right, but I dislike  the solder wrinkles around the chip periphery, tells me something is wrong with the process, good 80/20 joints use to have the 'asymptotic' look, wrinkles use to be the result of moving chips or wrong metalurgy,  but nobody wants to dig in the matter just now, except for me, still suspicious..... thanks anyway, Steve, good to hav!
e a 'cousin' on the net, micro a little bit from PWB problems.

Ingemar

-----Original Message-----
From: Creswick [mailto:[log in to unmask]]
Sent: den 11 juni 2000 14:32
To: TechNet E-Mail Forum.; Ingemar Hernefjord (EMW)
Subject: Re: [TN] Chip backside metal


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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