Dear Phil and others,
I would like to know what is the technical merit of the "tent" style
profile? As stated by Phil Zarrow "...And some, more complex assemblies
could still
>benefit from a soak to lower the gradient across the assembly...". Phil
previously stated that soaks are no longer needed because todays ovens have
such efficient thermal energy transfer. But is it possible to get perfect
thermal transfer? My personal opinion is that ease of profiling should be
the main goal. The best way to make profiling easy is to create an oven that
delivers the same temperature to the assembly as is programmed as the zone
setpoint, or as we call it "Equilibrium Heating".
It seems to me that there really is no technical merit. The benefit of the
"tent" style profile is increased throughput but are we acheiving this by
sacrificing temperature uniformity? I have a feeling that the reason tent
style profiles came about was to increase throughput. Someone needed to find
a way to produce more boards on an EXISTING assembly line without purchasing
a longer reflow system. One of our customers (Delco) started using the
"Tent" style profiles in 1990 to increase their throughput. In this
particular application it was justified because testing showed no loss in
energy transfer, however their board was small and had very little thermal
mass (I mean almost none).
It seems to me that no matter how good the thermal energy transfer of an
oven various thermal masses still exist on most assemblies. Especially with
the ever increasing population of circuit boards. Reflow ovens still need to
deal with these differing thermal masses to create Near Equilibrium Heating
to all spots on the board.
I would like to invite all solder paste manufacturers who are listening to
respond to the question:
"Other than increased throughput (which could be a sacrifice of temperature
uniformity) what is the technical merit of the "tent" style profile?"
Sincerely,
Brian Stumm
A Reflow Manufacturer
>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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