Section 5:
Reflow Oven Heat Transfer
The Three Heat
Transfer Modes in
Reflow Soldering
There are three different heating modes
involved with most SMT reflow processes:
conduction, convection, and infrared
radiation (IR). All three of these heating
modes occur naturally in our daily lives.
Perhaps the easiest way to understand each
of these heating modes is through example.
uniform product heating, as heat will
conduct from a hot spot to a cold spot in the
product. Thus, if a product is difficult to
reflow, often a good solution is to reduce the
oven heater setpoint temperatures and
conveyor speed. This allows more time for
the conductive flow to occur and the product
will heat more uniformly.
Conduction hinders the process if an edge
conveyor in contact with the board is cooler
or hotter than the product. Conduction heat
transfer can result in a hot or cool spot along
the edge of the product, preventing uniform
solder joints on the outer fringes of the PCB.
Conduction
Hot
Spot
Heat
Flow
Cold
Spot
Heat Flows From Hot to Cold
To Help Equalize Temperatures
Figure 5-1. Example of conduction
heat transfer.
Conduction
Conduction heat transfer occurs when two
solid masses of different temperatures are in
contact with each other. A good example is
when a pan is placed on an electric burner.
Most of the heat is transferred to the pan by
the contact between the pan and the burner.
Conduction also occurs within the same
mass if a temperature differential exists
within the mass.
Conduction can both help and hinder the
SMT reflow process. Conduction helps in
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Infrared
Infrared Radiation (IR) occurs when two
bodies of different temperatures are in sight
of each other. The best example of IR is the
heating of the earth by the sun. Dull, rough
surfaces absorb the sun’s rays better than
shiny, smooth surfaces. An object in the
direct sun light will become hotter than if in
the shade.
Radiation
Sun
Radiant Energy
From The Sun
Heats The Earth
Figure 5-2. Example of infrared
radiation.
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IR in SMT applications works similarly.
Fluxes, plastic components, and epoxy glass
laminate absorb IR very well. Shiny,
reflowed solder will reflect the IR energy
away. Solder joints around small packages
(such as chip resistors, capacitors, and
SOIC’s) are in sight of the IR energy and
heat very well. Solder joints around larger
devices (such as PLCC’s) are shaded, and do
not heat as well.
Convection
Convection heat transfer occurs when a fluid
(such as air, nitrogen, or water) passes over
an object (such as an SMT assembly). A
cool breeze on a hot sunny day provides
convective cooling. Hot air from a hair
dryer provides convective heating.
Convection heating or cooling requires
contact of the flow with the solid part. Only
the layer of the flow that is in contact with
the part is actually transferring heat.
Convection may be classified as natural or
forced.
Natural convection occurs when no flow is
being forced over the object. Temperature
differences between the object and the
environment create the convective heat
transfer. Perhaps the best example is the
chimney effect, where a strong convective
current rises away from the hot embers to
the cooler outdoors.
Forced convection requires an external force
that pushes or pulls the flow over the object.
A common house fan is a good example.
Convection
Hot
Air
Flow
Cold Object Is
Heated By
Convection
From The Hot
Air Flow
Figure 5-3. Example of convection heat
transfer.
Typically, forced convection heating or
cooling rates are higher than natural convection rates. Most reflow ovens today use
forced convection as the primary heat
transfer mode.
Research International won the 1998 SMT Magazine
Vision Award for advances in Soldering Equipment.
Notes:
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Heat Transfer Equations
Conduction
In order to understand what parameters are
important in conduction heat transfer, one
can consider the variables in the general
conduction heat transfer equation.
Q = K * A * ( T1 - T2 ) / ∆X
Where:
Q = Conduction heat transfer (W )
A = Cross sectional area (cm2 )
K = Thermal conductivity (W/cm -°C)
T 1 = Temperature at point 1 (°C)
T 2 = Temperature at point 2 (°C)
∆X = Thickness of material between
points 1 and 2 (cm)
The thermal conductivity (K) is a measure
of how well the object conducts heat.
Insulators (such as epoxy glass) have very
low thermal conductivity and permit very
low amounts of conductive heat transfer.
Conductors (such as copper) have a very
high thermal conductivity and permit high
amounts of conductive heat transfer. Thus,
an SMT assembly with high amounts of
copper will heat more uniformly than one
without much copper. Conduction through
the circuit board during the reflow process
improves heating uniformity, as the heat
conducts from the hot spots to the cold
spots.
k = Thermal
Conductivity
t
1
Surface 1
Surface 2
t
Conductive heat transfer is proportional to
the cross sectional area (A) between point 1
and point 2. In SMT reflow, a thicker board
will provide greater populated uniformity
than a thin board since the cross sectional
area is greater.
Conductive heat transfer is also dependent
upon the distance between point 1 and point
2 (∆X). The greater the distance, the less
the heat transfer. Thus, a thicker board has a
better chance of maintaining a higher
temperature difference between the top and
bottom side of the board than a thin board.
IR - Infrared
Infrared (IR) heat transfer occurs when two
objects at different temperatures are in sight
of each other. The heat is transferred by
electromagnetic waves of 0.78 to 1000
micron wavelengths. All objects emit some
level of infrared energy.
The quantity of infrared energy emitted and
the wavelength of the emission are both
dependent upon the absolute temperature of
the object. As the source temperature
increases, the heat transfer output increases
exponentially to the fourth power.
Increasing the source temperature results in
shorter wavelengths. Decreasing the source
temperature results in longer wavelengths.
In order to understand what parameters are
important in infrared heating, one can
consider the general equation for heat
transfer between the heat source and the
object being heated. The purpose is not to
memorize the equation, but rather to point
out the significance of what is involved.
2
∆ x
Figure 5-4. Conduction heat transfer model.
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The general equation for infrared heat
transfer is:
Q/A = Fv * ε s * αt ∗ σ * ( Ts4 - Tt 4 )
Where:
Q/A =
Fv =
εs =
αt =
σ =
Ts
Tt
Infrared heat transfer (W/cm2)
Geometric view factor (0 - 1)
Emmissivity of the source (0 - 1)
Absorptivity of the target (0 - 1)
Stefan-Boltzmann Constant
(5.67 x 10-12 W/cm2/°K 4)
= Source temperature (°K)
= Target temperature (°K)
IR Source
Control of infrared is generally done by
controlling the source temperature.
Providing that the overall source emission
can be regulated, IR energy provides high
levels of repeatability.
Convection
Convection heat transfer occurs when a fluid
at a given temperature contacts a solid mass
at a different temperature. If the fluid is
hotter than the mass, the mass will be
heated. If the fluid is cooler than the mass,
the mass will be cooled. Perhaps the easiest
way to understand convection is to look at
the convection equation and note the
significance of each variable.
The following is the general equation for
convection heating or cooling:
This Area is Shaded by Adjacent Components
Resulting in a Lower View Factor
Figure 5-5. IR view factor.
The geometric view factor Fv is the fraction
of energy that leaves the source that hits the
target. In SMT reflow, the oven chamber
designs yield very high view factors in the
range of 0.90 to 0.95. An important aspect
of the view factor comes in product design.
If two very large components are in close
proximity to each other, the view factor to a
solder joint between them is decreased,
which makes it more difficult to reflow.
The source emissivity (ε s ) and the target
absorptivity factors (α t ) are in the range of
0.90 to 0.95 for most SMT applications.
Solder paste is an excellent absorber of
infrared energy. Shiny gold components
may be difficult to heat, as they tend to be
reflective. Most often, however, the board
material, solder paste, and the components
all absorb quite well.
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Q/A = H * (Ta - Tt )
Where:
Q/A = Convection heat transfer per unit area (W/cm2)
H
= Convective film coefficient
(W/cm2-°C)
T a = Fluid temperature (°C)
T t = Target temperature (°C)
The amount of heat transfer can be modified
by either changing the convective
coefficient (H) or the temperature difference
(Ta - Tt ). Increasing the fluid temperature
will increase the temperature difference, and
thus the amount of heat transfer will
increase proportionally.
The convective coefficient (H) can have a
significant impact on the heat transfer. The
value of H is related to a number of
variables, including velocity of the flow and
angle of attack of the flow. Increasing the
flow velocity will increase the value of H.
For example, when one is outside in the
wind, a low velocity cool breeze on a warm
day will feel quite nice. However, a strong
wind on that same day may feel very cold.
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That is because the higher velocity wind has
a higher convective coefficient, and thus
removes heat more effectively from your
skin.
In SMT reflow applications, increasing the
velocity of the flow also improves the
amount of convection heat transfer. There
are practical limits to this, as too high
velocity may cause components to shift
around.
Parallel Flow
Air
Velocity
(V)
At Surface, V=0
Boundary
Layer
Thickness
Perpendicular Flow
Surface
Agitation
Prevents
Boundary
Layer
Buildup
Air Velocity (V)
Figure 5-6. Fluid flow patterns.
The flow direction also has a significant
impact on H. Convection heat transfer relies
on contact between the flow and the object.
Parallel flow can result in a stagnation or
boundary layer in which the heat transfer is
diminished. Perpendicular flow can break
up formation of a boundary layer and
enhance the convective heat transfer.
Perpendicular flow thus provides higher
heat transfer rates than parallel flow.
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Heat Transfer Mode
Interaction and their
Effect on SMT Reflow
In SMT reflow applications, there is no such
thing as pure convection or pure IR. The
only way to achieve a 100% convection
system would be to have gas flowing over
the product with no chamber walls inside the
reflow oven. Obviously the reflow oven is a
mechanical device with walls that absorb or
emit heat, so 100% convection is a physical
impossbility.
To have a 100% IR reflow system, the
chamber would need to be operated in a
vacuum (no air fluid heat transfer). Most
reflow processes currently occur in some
process gas whether air, nitrogen or vapor
phase techniques.
However, reflow oven heat transfer designs
utilize a greater convection component,
while others utilize IR as the primary heat
source. For the sake of simplicity, this text
will refer to a convection dominate heat
transfer machine as a convection machine,
and an IR dominated machine as an IR
machine.
In SMT reflow applications (with the
exception of vapor phase), convection,
conduction, and IR all play a role in the
heating process. Referring to a system as
100% convection or 100% IR is an error in
terminology. An oven may be dominated by
convection or IR, but the other heat transfer
is always present. IR dominant systems
usually range in the convection/IR ratio of
60/40 to 40/60. Convection dominant
systems have ratios in the range of 70/30 to
90/10.
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