Jan,

The FET requires 2 things: excellent thermal path and wire bonds. AuSn
provides a thin interface with minimal voiding without flux (although N2
enviornment or blanket is required). This is good for both requirements and
hence has always been the material of choice in this situation, especially
for high rel applications (I see you are a Defense contractor). It also has
the added benefit of not leaching Au (because it already is 80% Au) and
allows additional processing of other areas of the assembly at solder temps
without reflowing the original interface. Unfortunately, I'm not familar
with acronim "DBC" for substrate (European vs American?).  If you can better
define this material I might be able to make other suggestions for FET
attach.

One last note is that FETs can reach high delta T junction temp. This
combined with the operating temp can bring the interface dangerously in the
range of typical solder (SN63) reflow temps.  Hope this helps.

Bruce Misner

> ----------
> From:         Jan Merstrand (EMW)[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.;Jan Merstrand (EMW)
> Sent:         Thursday, March 30, 2000 4:11 AM
> To:   [log in to unmask]
> Subject:      [TN] Solder at lower temperature
>
> Hi guys and girls!!
>
> I have a question that I think many of you can answer.
> Today we solder FET chips with Au/Sn solder onto a DBC substrate with Au
> surface.
> The soldering area of the FET is Chromium/Nickel/Silver.
>
> I want to replace the Au/Sn solder with "something else".. I don't want to
> glue the chip to place, only solder.
> The FET working temperature reaches up to 120degC.
>
> Any ideas on what kind of solder to use?
> __________________________________________________
> Jan Merstrand
> Microwave MCM, Design/Production Support
> Ericsson Microwave Systems AB. Defence Manufacturing
> Office address: Bergfotsgatan 2, SE-431 84 Mölndal
> Phone: +46 31 747 0725, Fax: +46 31 747 3515
>
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