Dewetting is a thermodynamic reaction between copper (Cu) foil and tin
(Sn) from the eutectic lead/tin (PbSn) solder.

WETTING REACTION
When the PbSn solder melts the Sn diffuses into the Cu.  Intermetallic
compounds of CuSn are created between the copper and the lead/tin
solder.

These intermetallics allow solder to "WET" the copper.

DEWETTING REACTION
However -
if the soldering temperatures are too high and/or
if the solder is held above the melting temperature too long
then either Sn3Cu6 or Sn6Cu3 intermetallics (I forget the actual ratio)
are created between the copper and the solder.

These intermetallics will cause the solder to "DEWETT" from the copper.
(solder will bead-up on copper like water on a waxed car)


CONCLUSION
In order to reduce or eliminate dewetting phenomenon, use a lower
soldering temperature or reduce the amount of time the solder is held
above the melting point.

By lowering the peak soldering temperature and/or reducing the amount of
time above the melting point,
less of the Sn3Cu6 (actual ratio is not important to us) compound will
grow between the copper and the lead/tin solder joint.

Regards -
Tim Tezak
ACT Manufacturing
Hudson, MA
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From: Poh Kong Hui
To: [log in to unmask]
Subject: [TECHNET] (TechNet): Assy: Dewetting