HOW TO
REWORK A PCB WITH A PREHEATER & A HOT AIR PENCIL
With multi
layer PCB's and the increased density of surrounding components near
the SMD's, there is a heat sink effect that draws heat away from the
pads. The incorrect way of overcoming this problem is by increasing
the applied temperature, resulting in an increased potential for
damage to the PCB board and components. When soldering in these
dense areas, bottom side preheat has many advantages that will
overcome this problem.
SETTING UP THE PCB FOR RESOLDERING THE SMD......... Place the PCB
in a horizontal fixture that allows the preheater to be placed under
the PCB rework area at the correct height. The preheater will blow
focused hot air under the PCB. The heater is set at the correct
temperature to maintain the bottom of the PCB at a constant
300°F.(150C). At this safe low temperature the risk of thermal
damage has been eliminated, including the bottom side components
directly under the heat. The heat will be transferred through the
board, to the pin / pads and to the SMD. This safe temperature is
maintained on the board throughout the rework process.
PREPARING FOR THE SOLDER PROCESS: Check pads on PCB board for
coplanarity (equal height) and clean tinned pad surfaces with a good
cleaning solvent. Using a syringe apply a thin bead of solder paste
63/37 w/flux along all of the pads. The amount of paste dispensed
will depend on the size of the nozzle. It will take some practice to
judge just how much paste to apply for different size SMD's.
Carefully place the SMD directly into the paste on the pads. Use
magnification to insure accurate pin / pad placement on all sides.
Once the SMD is set in place, be careful not to move it.
USING A HOT AIR PENCIL AND BOTTOM HEAT: Apply bottom heat and
wait until solder paste starts to activate. This will be visible
when the flux in the solder paste starts to smoke. Now set the air
pencil for low air velocity and a temperature of 500°F(260C). With
the solder pencil slowly move from pin to pin and apply heat. Watch
as each individual pin / pad solder joint as it is formed and then
move on to the next. If a solder bridge is formed, apply more paste
flux and heat again. If this does not work, use solder braid while
heating with the hot air pencil. Clean all connections with an acid
brush and flux cleaner. Check with magnification for accuracy. Place
preheater on cool air and allow rework site to slowly go back to
room temperature. This method works quite well with fine
pitchcomponents.
We
at Chip Quik, Inc. are committed to bringing you quality rework
products. All products that we sell are tested and evaluated in our
own lab.
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NEW REWORK CHALLENGE FOR THE ENGINEER & TECHNICIAN
SMD Rework has become a greater challenge since the
introduction of the latest PCB's. The new boards have multi
layers and an increased density of surrounding components near
the SMD's resulting in a heat sink effect that draws away the
applied heat from the solder pads. This makes the required
heat delivery to the solder joint even more difficult to
maintain. The incorrect way of overcoming this problem is by
increasing the applied temperature. Higher temperatures result
in a greater potential for damage to the PCB and adjacent
components. There is a better method.
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REWORK ON A PCB WITH A BOTTOM
PREHEATER
 SMD rework does offer an
alternative choice other than the traditional solder iron.
When applying heat to a solder joint, a certain amount of time
is required to transfer the applied heat to the thermal load
(solder joint). This time is determined by the efficiency of
the heating device and the thermal conduction of heat to the
solder joint. A quality solder iron is the most efficient
device because the heat makes direct contact with the solder
joint. A safe alternative method is the use of hot air for PCB
bottom preheat. Hot air convection does not have as good heat
transfer as direct contact but does offer some advantages,
especially for higher volume rework and thermal processing.
THERMAL PROCESSING: When soldering a PCB, a certain amount
of time is required to bring the temperature of the joint from
ambient (room) temperature to reflow. The safe rework
challenge is to apply the controlled heat only on the joint
for as little time as possible. Longer heating periods will
allow damaging heat to be transferred to undesired areas,
causing board delamination and damage to adjacent and
underside components. As you can see the control of the
temperature in a specific location for a controlled amount of
time will determine the quality of the rework and reduce the
potential for inflicted damage. This controlled temperature
process is called thermal profiling.
BOTTOM SIDE PCB PREHEAT Bottom heating the PCB under the
site where the rework is to take place will allow the
temperature to be gradually raised to a safe point prior to
reworking the SMD. This is done by placing the PCB in a
horizontal bracket and applying focused hot air from a
preheater directly under the site to be reworked. The distance
below the PCB and the focused heat must be controlled to
deliver a safe temperature just below reflow. For traditional
boards the temperature is 300° F. (150C. By focusing hot air
under the PCB, the heat will gradually transfer through the
board to the pads and to the SMD component to be reworked.
This gradual rising temperature slope of controlled hot air
will prevent damage to some of the more sensitive components.
Now whatever rework that is done on the top side of the board
will be enhanced. Soldering, de- soldering and solder removal
will be safer, faster and more efficient.
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USING CHIP QUIK® WITH PREHEAT FOR SAFE SMD REMOVAL
 Due to the low melting
temperature of Chip Quik® removal alloy (136°F.)(58C) an SMD
can be removed using bottom heat only. With the PCB bottom
heated to a safe bottom temperature of 300° F(150C), apply
flux to all leads of the SMD. Now apply Chip Quik low
temperature alloy to all the pins. The removal alloy will
easily become molten because all the pin / pad connections are
now preheated to 300° F. The applied CHIP QUIK® alloy will
melt and fuse into each individual pin /pad connection
resulting in a new low temperature alloy with increased
thermal mass. While in this molten state the SMD can easily be
removed.
CLEAN UP............ Clean the pads with a swab dipped in
flux while the preheat is still being appied. This clean up
method is much better and safer than with solder braid. You
are now ready to install the new SMD.
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***COMPLETE PREHEAT HOT AIR SYSTEM*** Part #
SMDSYSTEM3..... $475.00 free shipping in USA
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COMPLETE HOT AIR SYSTEM for $475.00
AN ALTERNATIVE TO THE SOLDERING IRON AT AN AFFORDABLE PRICE
-------------CHIP
QUIK ON-LINE STORE-------------- »
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