Sonia, Are you using a solder preform. If so how thick is the preform.
Generally you should use a very thick preform with a very small diameter,
when the preform melts it will wet out on the substrates and sweep the
trapped air in front of it and you will obtain void free solder joints.
Gary

Gary Hillman
Service Support Specialties, Inc.

-----Original Message-----
From:   GARCIA BLANCO Sonia [SMTP:[email protected]]
Sent:   Thursday, September 15, 2005 5:34 PM
To:     'General MEMS discussion'
Subject:        [mems-talk] Indium solder voiding problem

Hi All,

Hopefully somebody can help me....

I am trying to bond two dies using indium soldering. The description of the
dies is as follows:
        Bottom die: Silicon/Ti/Cu/Ni/Au. The nickel layer is 0.7 microns
and
the gold layer 1.5 microns. Everything is electroplated.
        Top die: Silicon/Ti/Cu/Ni (2microns) /Indium (50microns and
30microns)

I am trying to bond both dies together bringing into contact the gold
(bottom die) with the indium (top die) and bringing them 30degrees C over
the melting temperature (156+30C). I have tried the following ideas:

        1. Perform an initial reflow just after electroplating of the
indium, using flux. Then, clean the sample very well to eliminate flux
residues, etch the possible indium oxide in HCl and perform a flip-chip
bonding bond without applying flux.
        2. Not perform an initial reflow and apply the flux during the
flip-chip bonding process.

In all the cases, I obtain bubbles inside my indium solder layer. I can
observe the bubbles (voids) when I break the bonds and observe the
remaining
indium in both the top and bottom substrates.

Anybody has any suggestions on how to remove this bubble problem?