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April 2001

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Subject:
From:
"Ingemar Hernefjord (EMW)" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 6 Apr 2001 12:42:50 +0200
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Gidday,
I have something odd to discuss with you. We use Si powerFETs with 80AuSn. Backside of chips is 0.5 micron Ag over Ni and Cr. Have waded thru Wassink, Manko, AWS Soldering Handbook etc but found very little about this concept. Should work in principal but I'm a little worried about this:

a) the AuSn is so hard and brittle ,  GPa rather than MPa, the later which is typical for Sn/Pb alloys. I'm afraid there will be Au/Ag IMCs that will be even harder, and with even higher Youngs M.

b) the chip has typical CTE of some 2.6 ppm/K while AuSn har 15-20 and IMCs probably even more (IMCs use to have)

c) don't know about Ag dissolution into 80AuSn. If it dissolves slowly there is perhaps no problem, but if it dissolves fast we may have in fact soldering on Nickel instead. And Ni is known as poorly wetted . And tests show that we don't have very good wetting. The chips can't be removed without great force, but I'm not really happy. What will happen in long terms and what will switched power cycling do to the joint. The chip dissipates lots of Watts as being a power supply regulator.

d) the backside of the chip is VERY rough. The manufacturer has etched the back to an Ra-value that is some 5-10 microns. That can't be good when using solders with such a high surface tension. We solder in N2, but the exteriour of the joint lookslike face of Metusalem, wrinkles with other words. This tellsabout poor processing.

If someone has experience or thoughts about mounting Ag backsided semis with 80AuSn, you are welcome to add something that makes me less ambivalent.

Ingemar Hernefjord
Ericsson Microwave Systems

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