Hi Inge.
What die size?
I sincerely doubt you have Cadmium, can you be looking at spectral lines and
mistaking for Cd? 
If you do have Cd it must be contamination. I can't think anyone would apply
it as a surface preserver on die. IF present it could easily change wetting
ability of solder/solderability.
The voiding would easily be caused by surface contamination, on either die
solder or substrate.
IS your Au/Sn clean? have you contaminated it by "cleaning" it in house?
Does happen.
Has parts/substrate packaging changed, or are parts being re-packed before
use. eg put in poly bags or something. How about something off the wall like
machine maintainence - someone using illicit spray on conveyor that sort of
thing.

As a diagnostic try burnishing parts on a piece of un metallised circuit
tile. Al2O3 is good abrasive and substrates are very flat.
Then if necessary -(serially for each surface  -solvent degrease, DI wash ,
IPA de -water and then bake dry. 
I am curious why the wire bonding fails, you can have serious voids before
affecting that. Could there be something else unrelated coincidentally going
on?


Regards 

Mike 
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge
Sent: Thursday, October 09, 2008 5:00 PM
To: [log in to unmask]
Subject: [TN] Need soldering advice ASAP!

Hi all, especially the Indi-man,
We have soldered IRC's power MOSFET (100-250  W) dice for a decade without
problems. They use to be vacuum soldered to a solderable gold thickfilm
Al2O3 substrate with AuSn 50 micrometer preforms. Now there is a production
stop because of Al heavy wire wedge bonding problems. My analysis of this
case points at the presence of large voids in the solder joints  or maybe
partly dewetting (FineFocus is good but the resolution is not sufficient to
make me sure), something the US mechanical waves don't like. The die
backside is plated with something that I don't recognize: Cadmium over
Silver over Nickel. What the heck is that? Anyone who can figure out if AuSn
is the best preforms for that plating?
Thanks
/Inge 

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