Hi Tim,
While your description is an adequate, but incomplete, working definition of
WETTING, your description of DEWETTING is not.
During wetting both intermetallic compounds (IMCs) Cu3Sn (thin layer near the
copper) and Cu6Sn5 )adjacent to the Cu3Sn layer nearer to the solder) are
formed. It is the Cu6Sn5 layer that grows in thickness as the result of
temperature and time. DEWETTING occurs when a solderable copper surface is
covered with a thin layer of solder (on top of layers of Cu3Sn and Cu6Sn5),
from which the Sn is consumed by the formation of more Cu6Sn5 with Cu
diffusing through the IMC layers. This creates a surface of exposed Cu6Sn5,
which when oxidized is not solderable, and Pb-rich solder phases, which also
become non-solderable with the formation of Pb-oxide.
While of course the formation of the IMC layer is temperature and
time-dependent, it is not a good idea to arbitrarily reduce peak reflow
temperatures or time at reflow temperatures (about 25C above Liquidus of the
involved solder) to less than 5 seconds anywhere on the assembly.
More product problems are created with inadequate wetting than with
de-wetting.
The solution is a thicker solder coat on the  Cu surface (take it easy at
HASL) or OSP.

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 973-543-2747
E-mail: [log in to unmask]