| has been around for many
years in many different variations. It uses an inert liquid
that when heated, creates a very stable uniform heat transfer
medium in the form of vapor. This heat transfer medium replaces
heat energy very quickly. It transfers heat to the product
by condensing this heated vapor to the product with little
regard to mass. Another property of vapor phase is the maximum
temperature at reflow. The maximum temperature is tied to
the boiling point of the fluid. This means vapor phase won't
damage parts with overheating. The condensation effect of
the vapor also allows the vapor to transfer heat to all surfaces
of a product evenly without the effects of "shadowing",
"deflection", or "reflection" . These
facts combine to make vapor phase soldering one of the most
solid and repeatable reflow and heat transfer processes.
The earliest units used simple methods to heat the liquid
which created the vapor. Later, preheat was added as a process
enhancement. This first type of preheat generally utilized
a secondary vapor with a lower fixed temperature. The product
was first lowered on an elevator system into the preheat or
"secondary" vapor which hovered above the primary
vapor. After a certain temp was attained in the secondary
vapor, the product was then lowered into the "primary"
vapor for reflow. Cooling coils have generally been used as
the containment apparatus for the vapor blanket. As the generated
vapor is much heavier than air, it remains in the bottom of
the tank. The cooling coils then condense any excess vapor
to maintain a set height of the vapor blanket. Condensed fluid
returns to the boiling liquid.
Current design of most vapor phase units have removed the
"secondary" vapor in favor of an IR or Convected
type of preheat. This design change took place to provide
better control of the preheat thermal curve, as well as to
remove the secondary vapor from the process. This secondary
vapor was often Freon in the early days and was discovered
to cause ozone depletion.
The vapor blanket or the layer of saturated vapor within
the machine is at an even temperature throughout. There are
no side to center or front to back variations in the vapor
blanket temperature. Large mass differentials on the board
itself are of little consequence to the process because they
extend the reflow time only slightly. Small parts will reflow
somewhat sooner than large parts, but it has been found that
for reflow times of up to a minute, extremely large mass differentials
can be handled. This is due to the large reservoir of heat
available and relatively rapid heating capabilities of condensation
heating. This process is so stable that a minimum amount of
thermal profiling is required.
Most vapor phase units utilize some sort of conveyor system
to transport the product to be heated though the critical
stages of the oven: Preheat, Reflow, Flash Off, and Cooldown.
You can generally find these ovens in either "Batch"
type or "Inline" type, they can use palletized conveyors,
belted conveyors, transport system conveyors, or a mix of
several types. We now have a process that takes the board
or assembly through a nearly ideal reflow curve. This process
or more appropriately, this combination of processes provides
for stability and uniformity and yet is extremely flexible
and cost effective, while maintaining the highest quality
and reliability.
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