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					                                          Section 5:
                 Reflow Oven Heat Transfer
                                                 uniform product heating, as heat will
The Three Heat                                   conduct from a hot spot to a cold spot in the
Transfer Modes in                                product. Thus, if a product is difficult to
                                                 reflow, often a good solution is to reduce the
Reflow Soldering                                 oven heater setpoint temperatures and
                                                 conveyor speed. This allows more time for
There are three different heating modes          the conductive flow to occur and the product
involved with most SMT reflow processes:         will heat more uniformly.
conduction, convection, and infrared
radiation (IR). All three of these heating       Conduction hinders the process if an edge
modes occur naturally in our daily lives.        conveyor in contact with the board is cooler
Perhaps the easiest way to understand each       or hotter than the product. Conduction heat
of these heating modes is through example.       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
                                                 Infrared
     Hot                           Cold          Infrared Radiation (IR) occurs when two
     Spot          Heat            Spot          bodies of different temperatures are in sight
                                                 of each other. The best example of IR is the
       Heat Flows From Hot to Cold               heating of the earth by the sun. Dull, rough
      To Help Equalize Temperatures
                                                 surfaces absorb the sun°s rays better than
                                                 shiny, smooth surfaces. An object in the
   Figure 5-1. Example of conduction             direct sun light will become hotter than if in
   heat transfer.                                the shade.


Conduction                                                       Radiation
Conduction heat transfer occurs when two
solid masses of different temperatures are in                                        Sun
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                              Radiant Energy
the contact between the pan and the burner.                                 From The Sun
Conduction also occurs within the same                                     Heats The Earth
mass if a temperature differential exists
within the mass.
                                                         Figure 5-2. Example of infrared
Conduction can both help and hinder the                             radiation.
SMT reflow process. Conduction helps in

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   Solder Reflow Technology Handbook
IR in SMT applications works similarly.         Forced convection requires an external force
Fluxes, plastic components, and epoxy glass     that pushes or pulls the flow over the object.
laminate absorb IR very well. Shiny,            A common house fan is a good example.
reflowed solder will reflect the IR energy
away. Solder joints around small packages                         Convection
(such as chip resistors, capacitors, and
SOIC°s) are in sight of the IR energy and
heat very well. Solder joints around larger                                        Cold Object Is
                                                  Hot
                                                                                    Heated By
devices (such as PLCC°s) are shaded, and do        Air
                                                                                    Convection
not heat as well.                                 Flow
                                                                                   From The Hot
                                                                                      Air Flow

Convection
Convection heat transfer occurs when a fluid
(such as air, nitrogen, or water) passes over      Figure 5-3. Example of convection heat
an object (such as an SMT assembly). A             transfer.
cool breeze on a hot sunny day provides
convective cooling. Hot air from a hair         Typically, forced convection heating or
dryer provides convective heating.              cooling rates are higher than natural con-
                                                vection rates. Most reflow ovens today use
Convection heating or cooling requires          forced convection as the primary heat
contact of the flow with the solid part. Only   transfer mode.
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       Research International won the 1998 SMT Magazine
the cooler outdoors.                            Vision Award for advances in Soldering Equipment.


    Notes:




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                                                       Solder Reflow Technology Handbook
                                                                 Conductive heat transfer is proportional to
 Heat Transfer Equations                                         the cross sectional area (A) between point 1
                                                                 and point 2. In SMT reflow, a thicker board
 Conduction                                                      will provide greater populated uniformity
 In order to understand what parameters are                      than a thin board since the cross sectional
 important in conduction heat transfer, one                      area is greater.
 can consider the variables in the general
 conduction heat transfer equation.                              Conductive heat transfer is also dependent
                                                                 upon the distance between point 1 and point
                 Q = K * A * ( T1 - T2 ) / DX
                                                                 2 (DX). The greater the distance, the less
   Where:                                                        the heat transfer. Thus, a thicker board has a
    Q = Conduction heat transfer (W )                            better chance of maintaining a higher
    A = Cross sectional area (cm2 )                              temperature difference between the top and
    K = Thermal conductivity (W/cm - C)
    T1 = Temperature at point 1 ( C)                             bottom side of the board than a thin board.
    T2 = Temperature at point 2 ( C)
    DX = Thickness of material between                           IR - Infrared
          points 1 and 2 (cm)
                                                                 Infrared (IR) heat transfer occurs when two
 The thermal conductivity (K) is a measure                       objects at different temperatures are in sight
 of how well the object conducts heat.                           of each other. The heat is transferred by
 Insulators (such as epoxy glass) have very                      electromagnetic waves of 0.78 to 1000
 low thermal conductivity and permit very                        micron wavelengths. All objects emit some
 low amounts of conductive heat transfer.                        level of infrared energy.

 Conductors (such as copper) have a very                         The quantity of infrared energy emitted and
 high thermal conductivity and permit high                       the wavelength of the emission are both
 amounts of conductive heat transfer. Thus,                      dependent upon the absolute temperature of
 an SMT assembly with high amounts of                            the object. As the source temperature
 copper will heat more uniformly than one                        increases, the heat transfer output increases
 without much copper. Conduction through                         exponentially to the fourth power.
 the circuit board during the reflow process                     Increasing the source temperature results in
 improves heating uniformity, as the heat                        shorter wavelengths. Decreasing the source
 conducts from the hot spots to the cold                         temperature results in longer wavelengths.
 spots.
                                                                 In order to understand what parameters are
                                                                 important in infrared heating, one can
                         k = Therm a l
                             C onductivity
                                                                 consider the general equation for heat
                                                                 transfer between the heat source and the
     t
         1



                                                                 object being heated. The purpose is not to
             Surface 1                       Surface 2
                                                                 memorize the equation, but rather to point
                                                                 out the significance of what is involved.
                                                         t
                                                             2




                               D x




Figure 5-4. Conduction heat transfer model.




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 Solder Reflow Technology Handbook
The general equation for infrared heat             Control of infrared is generally done by
transfer is:                                       controlling the source temperature.
          Q/A = Fv * es * at * s * ( Ts4 - Tt4 )   Providing that the overall source emission
                                                   can be regulated, IR energy provides high
Where:                                             levels of repeatability.
 Q/A = Infrared heat transfer (W/cm2)
  Fv = Geometric view factor (0 - 1)
  es = Emmissivity of the source (0 - 1)           Convection
  at = Absorptivity of the target (0 - 1)          Convection heat transfer occurs when a fluid
  s = Stefan-Boltzmann Constant
         (5.67 x 10-12 W/cm2/ K4)
                                                   at a given temperature contacts a solid mass
  Ts = Source temperature ( K)                     at a different temperature. If the fluid is
  Tt = Target temperature ( K)                     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
                     IR Source                     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                 Q/A = H * (Ta - Tt )
      Resulting in a Lower View Factor
                                                   Where:
          Figure 5-5. IR view factor.               Q/A = Convection heat transfer per unit area (W/cm2)
                                                     H    = Convective film coefficient
                                                               (W/cm2- C)
The geometric view factor Fv is the fraction         Ta = Fluid temperature ( C)
                                                     Tt   = Target temperature ( C)
of energy that leaves the source that hits the
target. In SMT reflow, the oven chamber            The amount of heat transfer can be modified
designs yield very high view factors in the        by either changing the convective
range of 0.90 to 0.95. An important aspect         coefficient (H) or the temperature difference
of the view factor comes in product design.        (Ta - Tt ). Increasing the fluid temperature
If two very large components are in close          will increase the temperature difference, and
proximity to each other, the view factor to a      thus the amount of heat transfer will
solder joint between them is decreased,            increase proportionally.
which makes it more difficult to reflow.
                                                   The convective coefficient (H) can have a
The source emissivity (es ) and the target         significant impact on the heat transfer. The
absorptivity factors (at ) are in the range of     value of H is related to a number of
0.90 to 0.95 for most SMT applications.            variables, including velocity of the flow and
Solder paste is an excellent absorber of           angle of attack of the flow. Increasing the
infrared energy. Shiny gold components             flow velocity will increase the value of H.
may be difficult to heat, as they tend to be
reflective. Most often, however, the board         For example, when one is outside in the
material, solder paste, and the components         wind, a low velocity cool breeze on a warm
all absorb quite well.                             day will feel quite nice. However, a strong
                                                   wind on that same day may feel very cold.

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                                                         Solder Reflow Technology Handbook
That is because the higher velocity wind has
a higher convective coefficient, and thus
                                                          Heat Transfer Mode
removes heat more effectively from your                   Interaction and their
skin.
                                                          Effect on SMT Reflow
In SMT reflow applications, increasing the
velocity of the flow also improves the                    In SMT reflow applications, there is no such
amount of convection heat transfer. There                 thing as pure convection or pure IR. The
are practical limits to this, as too high                 only way to achieve a 100% convection
velocity may cause components to shift                    system would be to have gas flowing over
around.                                                   the product with no chamber walls inside the
                                                          reflow oven. Obviously the reflow oven is a
Parallel Flow                                             mechanical device with walls that absorb or
                                                          emit heat, so 100% convection is a physical
  Air                                                     impossbility.
                                              Boundary
Velocity
                                              Layer
  (V)
                At Surface, V=0               Thickness   To have a 100% IR reflow system, the
                                                          chamber would need to be operated in a
Perpendicular Flow                                        vacuum (no air fluid heat transfer). Most
     Surface               Air Velocity (V)               reflow processes currently occur in some
    Agitation
    Prevents
                                                          process gas whether air, nitrogen or vapor
    Boundary                                              phase techniques.
      Layer
     Buildup
                                                          However, reflow oven heat transfer designs
                                                          utilize a greater convection component,
    Figure 5-6. Fluid flow patterns.
                                                          while others utilize IR as the primary heat
                                                          source. For the sake of simplicity, this text
The flow direction also has a significant                 will refer to a convection dominate heat
impact on H. Convection heat transfer relies              transfer machine as a convection machine,
on contact between the flow and the object.               and an IR dominated machine as an IR
                                                          machine.
Parallel flow can result in a stagnation or
boundary layer in which the heat transfer is              In SMT reflow applications (with the
diminished. Perpendicular flow can break                  exception of vapor phase), convection,
up formation of a boundary layer and                      conduction, and IR all play a role in the
enhance the convective heat transfer.                     heating process. Referring to a system as
Perpendicular flow thus provides higher                   100% convection or 100% IR is an error in
heat transfer rates than parallel flow.                   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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Solder Reflow Technology Handbook