Download Reflow Oven Convection Methods

Section 6:
Reflow Oven Convection Methods
Research International DeltaFlo 10LN Low Nitrogen Convection Oven
Convection Oven
Technologies
While convection is a relatively simple
technology, there are several technical
aspects to consider. This section discusses
some of the key differences including:
•
Forced convection versus natural
convection
•
Turbulent flow versus laminar flow
•
Fresh gas input versus recirculated gas
input
•
Defined convection flow path versus
uncontrolled flow path
•
Methods of forcing convection gas flow
Natural convection occurs when the air (or
nitrogen) surrounding an object is at a
different temperature than the object. The
density differences between the hot and cool
air causes a physical motion to occur, which
assists in heat transfer. Forced convection
Page 27
occurs when an independent device (such as
a fan) forces the air over the product.
Typically the convective coefficient is
greater with forced convection than with
natural convection, which results in
improved heating of the product. Early
infrared/convection ovens relied on natural
convection to help improve heating
uniformity. Forced convection ovens
usually have better heating uniformity than
the infrared/convection ovens.
Forced convection can be either laminar or
turbulent. Laminar flow is characterized by
very stable flow patterns. Convection heat
transfer requires contact between the air and
the object. (See Figure 6-1.)
Thus, a stable gas jet allows more air to
come in contact with the object. Because of
this contact, laminar forced convection can
have heating efficiencies of greater than 30
percent. Turbulent flow is characterized by
vortices, eddy currents, and high heating
rates. Thermal efficiencies are typically
only 10 to 15 percent.
Research International
Solder Reflow Technology Handbook
The non-defined flow path ovens (Figure 63) do not direct the flow from zone to zone.
Gas eventually reaches the exhaust and
leaves the oven.
Turbulent Flow
• Air Jets are Unstable.
• Most of the Air Flow Does Not
Contact Object
• Low Heating Efficiencies
(5 - 15% )
• High Heat Transfer Rates
These ovens work satisfactorily thermally,
however they usually have oven flux
cleaning issues associated with high-volume
PCB production.
Laminar Flow
Air Velocity (V)
• Air Jets Remain Stable.
• Most of the Air Flow Contacts
the Object
• High Efficiencies (25 - 35%)
• Medium Heat Transfer Rates
Fresh Air Input
Exhaust
Defined vs Non-Defined Reflow Oven
Flow Patterns
There are two methods of directing the flow
within convection ovens. The defined flow
path oven (Figure 6-2) moves the gas from
zone to zone. It does not allow the same gas
to become trapped in a given zone, which
could become excessively contaminated
Exhaust
Figure 6-3. Example of a non-defined flow
pattern – reflow process gas is recirculated
within each zone. Flux is not removed from
the zone and builds-up in the heater cavity.
Figure 6-1. Forced convection flow patterns.
with fluxes. These ovens will typically stay
cleaner by preventing flux buildup.
PREHEAT
DRYOUT ZONES
1
2
3
4
LAMINAR FLOW CONVECTION TO BOARD
REFLOW
5
6
COOLING SECTION
7
FRESH GAS IN REFLOW ZONES
Figure 6-2. Example of a defined flow pattern – process gas is recirculated to the
previous zone. Flux is removed via exhaust collectors. Fresh process gas keeps oven
clean (free of flux residue). Also see Figure 5-4.
Research International
Solder Reflow Technology Handbook
Page 28
How is the reflow process gas
recirculated to the previous zones?
Heater configuration continuously
cleans oven convection flow jets.
1
2
3
4
5
6
Highest pressure here
forces recirculated gas to
be constantly purged.
Fresh input gas into every bottom zone insures clean process
atmosphere and versatile bottom/top temperature control.
Figure 6-4. Positive pressure in the reflow zone purges flux laden gas to the exhausts.
This design feature creates the defined flow pattern within the reflow oven.
Recirculated Gas: Too Much
Recirculation Means Flux Cleaning
Air introduced into a heating zone may
either be fresh or recirculated. Fresh gas
input requires heating the air from ambient
to the zone temperature. In comparision,
recirculated gas input requires only heating
the air a small amount, since the gas is
already “hot”. Thus, an advantage of
recirculated flow is a lower power
requirement to heat the zone.
Recirculated flow has the disadvantage of
picking up contamination each time the air
is re-used. No-clean fluxes prevent solids
from building up on the PCB. These solids
end up in the reflow process gas and will
collect in the heater cavity if the oven is not
designed properly. The result is a dirty
process atmosphere or machine downtime
due to the need for oven flux cleaning.
Notes:
Page 29
Research International
Solder Reflow Technology Handbook
Ways to Generate
Forced Convection in
Reflow Ovens
There are three methods of generating
forced convection in reflow ovens – fans,
compressors and flow amplifiers.
Fans
Fans are an inexpensive and reliable method
of moving high volumes of air. Fans
typically have low pressure generating
capability, so exit velocities may be limited.
However for most board profiles this should
not be a problem. Most convection reflow
ovens use fan forced convection.
meters. Compressed sources have high
pressures, allowing high velocities if
desired. An air compressor or compressed
nitrogen supply is required.
Compressed gas reflow is used when the
application requires an extremely accurate
thermal profile with filtered process gas.
Flow Amplifiers
The third common method used to create
convective flow is the flow amplifier. The
flow amplifier uses a compressed source to
induce flow. In a flow amplifier,
compressed gas flows through the inlet into
an annular chamber. The gas is then
throttled through a restriction at high
velocity. The compressed gas flow towards
the outlet adhering to the outside wall. A
low pressure area is created in the center,
which induces flow.
Figure 5-1. Forced convection flow patterns.
Inlet Gas(Compressed)
Flow is restricted
with small ring
nozzle causing
cold spot.
Exit
Flow
Entrained
Air
Convection fan in a preheat zone. Note that the fan motor
is outside the heater chamber for improved fan reliability.
Compressed gas flow
through annulus
Figure 6-5. Schematic of a flow amplifier.
Compressed Gas Sources
Compressed input gas sources (such as
compressed air or nitrogen) are also used to
create forced convection. The pressure must
be regulated in order to achieve constant
flow. Compressed sources have the
advantages of providing a controlled
atmosphere, which is free of particulate
contaminants.
If compressed air is used, the humidity can
be controlled using an inline dryer. Flow
velocity is easily controlled with flow
Research International
Solder Reflow Technology Handbook
The advantage of a flow amplifier is that
flow volumes are similar to a fan without
any of the fan life issues. However, a major
drawback to the flow amplifier is that the
flow amplifiers tend to clog with fluxes.
The ring nozzle causes the gas to expand
and cool. The cooldown of the gases causes
flux to precipitate out of the flow. There is
also a thermal efficiency loss with the gas
cooldown.
Page 30