Hi Paul -
By soak (vs. non-soak) solder paste profiles, I believe you are referring to
the "traditional" ramp -soak-spike profile as opposed to the "tent profile".
The soak profile came from the earlier radiant and IR/Convection ovens that
were, by comparison to current Convection Dominant (Forced Convection) ovens
and Vapor Phase, were far less efficient in terms of heat transfer.  This
combined with the relatively poor thermal conductivity of plastic substrates
(glass epoxy, FR-4) as compared to ceramic, meant we had to have a soak period
(I call it Preflow) for thermal-mechanical reason - to allow the cooler
portions of the assembly to catch up to the warmer areas (typically the
corners as this was where the most surface area was).  The paste
manufacturers, for the most part, took notice of this and  formulated  their
solder pastes so that the flux could endure the soak period.  How much
depended upon the formulation and the type of flux.  RMAs (including no-
cleans) could endure, sometimes, as much as 2 minutes while most OAs wanted
shorter durations (due to the solvents used).  It is very important that the
maximum soak duration called out by the solder paste manufacturer (hopefully
on the data sheet  - maybe) be noted by the user.  So what you have was a case
of the cart leading the horse.
With the newer, more powerful Convection Dominant ovens, some applications do
not require a soak and time could thus be trimmed off the reflow cycle.  This
also has the advantage of reducing the time above the glass transition
temperature of the substrate as well as the entire duration of exposure of the
assembly to the higher temperature excursion.  I suspect from conversations
with solder paste manufacturers that most paste formulations do not have a
minimum Preflow soak duration - but the user should check with their solder
paste manufacturer to be sure.  And some, more complex assemblies could still
benefit from a soak to lower the gradient across the assembly.
My friend Rob Rowland has a discussion of this in his column this month and
last in SMT Magazine.  I have also mentioned it in one of my Circuit Assembly
columns some time back.  Rob and I had discussion on this many years ago when
he was with Iomega and did a lot of pioneer work in the "tent profile".
Again, with the current generation of more thermal transfer efficient ovens,
in many applications - it is a concept whose time has come.
Regards,

Phil Zarrow
ITM, Inc.
Durham, NH
www.ITM-SMT.com

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