Vapor phase vs. convection reflow in RoHS-compliant assembly
Figure 3. Vapor phase lead-free profile.
Figure 4. Vapor phase tin-lead profile.
and complexity to get the profile close so
that process development time is kept to a
minimum.
Conversely, VP reflow profiling can
be classified by process type to the point
Figure 2. Convection lead-free profile.
where there are fewer profiles to develop.
For instance, standard multi-layer 0.062”
lower peak reflow processing improved solder wetting and flow
thick PCBAs can follow the same lead-
based VP profile regardless of component
VP reflow requires a lower processing Visual inspection of micro sections
complexity. With advances in VP process
temperature. In convection or IR process, indicates that VP creates good solder joint
systems, monitoring of heat load during
temperatures can reach 245˚C to 265˚C performance. When larger thermal load
the soldering process allows the systems to
at the component level. VP temperatures components or clusters of components are
profile almost automatically. Ramp rates
stay at the boiling point of the fluid, present, time above liquidus (TAL) should
and soak times at peak temperature can be
typically established between 230˚C and be increased beyond the 60-90 seconds rec-
defined by the engineer and controlled by
240˚C. The lower temperatures make it ommended by solder paste manufacturers
the systems regardless of the product mix
possible to reduce the cost of the PCB by to accommodate thorough heat transfer.
during the process. In a true one-piece flow
using lower T /T material selections for Comparatively, in convection processing it
g d
on a prototype, it is much easier to get it
SMT assemblies. Savings of 10-15% and can be more difficult to ensure good joints
right the first time using VP processing.
even more could be seen on laminate costs on components with high thermal mass be-
The days of inadequate reflow tempera-
alone. VP also offers processing advantages cause achieving TAL in larger components
ture or over temperature on the first piece
with large mass components, such as con- may result in smaller components overheat-
are virtually eliminated by use of the VP
nectors, because the thermal equilibrium ing. There is no chance of overheating
systems.
is better. In convection reflow, particularly smaller or isolated components with VP
in higher temperature lead-free processes, because VP cannot heat a component
comparative data on lead-free and
correctly soldering large mass connectors higher than vapor temperature.
tin-lead solder joint creation
may overheat the rest of the PCBA.
Standard test boards (Figure 1) populated
reduction in profiling time
with common components, including
inert environment Another advantage of VP’s heat transfer
BGAs and QFPs, were built with vapor
The inert environment and consistency of characteristics and the uniformity at which
phase and convection soldering technology
heat transfer allow VP to be more forgiving it accomplishes heat transfer is that it
and tested at an EPIC facility. Reliability
with lead-free component terminations. makes it easier to understand the profiling
testing demonstrated that lead-free and
Less active no clean flux chemistries have relationship between PCBAs. In traditional
tin-lead joints produced by vapor phase to
proven to be adequate in soldering lead- profile development for a new PCBA,
be equally robust as those from convection
free terminations that demand high activ- sample assemblies with thermocouples are
reflow. The controllable, lower peak solder
ity fluxes in the convection reflow process. run through the reflow process numerous
temperature makes vapor phase ideal for
There is also cost savings due to lower times in order to get the right profile, with
soldering complex assemblies having sensi-
energy consumption. In addition to the manual inspection of the solder joints
tive lead-free SMT components.
elimination of nitrogen, electricity usage and flux residue used to determine if the
with VP is much lower. target profile is correct. Engineers develop

matrix charts on board size, layer count
www.globalsmt.net Global SMT & Packaging – March 2009 – 21
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