Gunter,
I am a little late in catching up with my technet correspondence. I am
studying the spreading of Sn on Cu and Au over at SUNY Binghamton
University. Sometimes it seems the more I study the less I know but let me
throw in my 2 cents.  I agree with your statements. I think that the
thickness of CuSn intermetallic will be greater with a limited volume of
solder. As you have mentioned, the IMC formation rate is limited by the
diffusion of Cu through the IMC and the removal of Cu from the surface of
the IMC into the Sn (dissolution). As the IMC gets thicker, the Cu atom
transport (atoms/sec) will be less. At the IMC/Sn interface, the transport
of Cu atoms from the IMC into the liquid will be governed by the Liquid flow
velocity if things are moving (faster velocity, faster dissolution). If the
liquid Sn is not moving, then the Cu concentration gradient in the liquid Sn
will control the IMC dissolution.

If there is a limited amount of Sn and you spend a long enough time at
elevated temperature, you can have an isothermal solidification condition.
This happens when enough Cu atoms make it through the IMC to convert all of
the Sn into an IMC. This behavior is fairly common in the brazing
literature. It may take a while because the slow solid state diffusion of
the Cu atoms through the IMC is controlling the rate that new IMC can be
formed. (Of course there is the wild card of Grain boundary diffusion where
faster transport occurs).

Also there will be less liquid Sn flow if you have low volume small solder
joints and a geometry like a BGA that does not encourage much flow. Again
resulting in a greater IMC thickness. If you have enough IMC formation at
the Liquid/Solid contact line your solder flow can stop.

Steph

Stephan Meschter               [log in to unmask]
Lockheed Martin Control Systems  Phone  :(607)770-2332
600 Main Street, MD R52F         FAX    :(607)770-2056
Johnson City, NY 13790-1888      MARCALL: 8 * 255-2332


> ----------
> From:         Guenter Grossmann[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.;Guenter Grossmann
> Sent:         Monday, February 08, 1999 5:15 AM
> To:   [log in to unmask]
> Subject:      [TN] Intermetallics
>
> Hi all
>
> I have a question concerning the growth of Intermetallics:
>
> Collin Lea states in his book, that the thickness on a copper specimen
> immersed into a large solder volume reaches an equilibrium which is 0.5um
> at 240deg to 0.9um at 345 deg. The dissolution rate of the intermetallic
> layers is equal to the growth of the intermetallics after this equilibrium
> is reached. However, this is in a large solder volume. Is the thickness of
> the intermetallics in solder joints with a limited volume also restricted
> to the equilibrium or does the intermetallic layers grow bigger since the
> solder will see an certain enrichment of copper?
>
> Best regards
>
> Guenter
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