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Which tools are best for SMT rework—conduction or convection?
hot air and will not
melt the connector
on replacement.
The solder on
the PCB is easily
tinned with a
solder iron before
alignment of
connector under
microscope. The
corner pins are
tacked with the
iron, flux is applied
and top hand-held
hot air is used to
solder reflow of the
connector pins.
Method 2: Hand
Figure 6. QFP removal.
Option 1—
This rework process is new as two
Alternatively, the connector can be hand
technologies exist in one component. QFP
Figure 5. Fine pitch 0.5 mm connectors
soldered with the new UFTC solder
is an old technology and can be removed
cartridges that can reflow solder pin-to-pin
with conduction tools or convection tools
temperatures from hand-held hot air tools,
or by using a drag soldering technique.
and hand soldered back.
because operators cannot shield them
Some bridges may occur, which can be
This method of thermal QFP rework
removed later with solder braid or hoof tips,
will only reflow the QFP leads; however, the
which are designed to remove solder from
FusionQuad pack is similar to the TO-220
reworking small fine pitch sMT
joints by wiping outwards at 180° to pins.
with a connection under the middle of the
If density does not allow this “wipe out”
method, then fine solder braid can be used.
The additional pads in the middle are
Plastic connectors are often reworked
Cut the braid with cutters to a pointed
the same as the LGA (land grid array), with
using hand-held hot air tools with air
end and dip in flux and iron, touching the
a ground pad in the middle. The best way to
temperatures high enough to melt the
braid a little higher than contact with pin.
remove these FusionQuad packages is to use
plastic body.
This will pull solder up the wick by capillary
top and bottom convection heating. Due
The solder on the connector pins
to the complexity of the part, solder paste
will not reflow from the top hot air alone
The UltraFine Series solder cartridges
must be applied directly to the center of the
without melting. In removal, this is usually
have been specifically designed with small
component only during the rework process.
not an issue as the connector is discarded
hoof and chisel tip geometries to solder
after removal. This means the replacement
these very fine leads. The first way is to pre-
cannot usually be done with the same tool
tin the pads with solder, and reflow the pin
Convection is the only way to reflow all
and heating technology or the connector
individually, thus limiting bridges. The pads
the connections at once to allow removal.
will be distorted.
can be reflowed with an 0.8 mm UFTC
Removal is easy and can be done with
drag tip, which solders multiple pins at a
a small rework system, or on a rework
time using a drag soldering technique.
machine that will place and align for
Method 1 : Hot air
Option 2—If the pads are flat, the part
replacement. Vision top to bottom is
The connector can be replaced with hot air
can be aligned, placed onto the pads and
required for the part to be replaced.
if a preheater is used to aid heating from
tacked into place. Using a very small hoof
The removal process profile must reflow
below. This lowers the source temperature
tip solder can be applied to each pin.
the outer pins as well as the inner pad. The
needed to achieve reflow from the top hot
This method is more time consuming but
best way to achieve the desired results is to
air tool.
considered easier by some operators. Note:
drill the PCB for thermocouple attachment
For example, if the PCB temperature
Gel flux works well in these applications to
during profile development.
is taken to 80°C and measured with a
stop bridges.
NOTE: The PCB must be cleaned with
thermocouple, the source temperature
solder wick after removal to level all solder
of air from below is likely to be no more
reworking FusionQuad™ pack
connection points. (Inner and outer lead
than 200°C and will take 1-2 minutes
These packages from Amkor®, which
to reach 80°C at the PCB top surface.
combine an LGA (land grid array) and QFP
This temperature will not damage any
(quad flat pack) in one device, offer low cost
components on the bottom of PCB.
miniaturization from 100-300 I/O, with
Once the PCB pads are cleaned of solder
More sophisticated preheaters are
enhanced QFP electrical performance for
using a solder iron and solder braid and
available that drive the PCB to temperatures
higher frequencies. With excellent thermal
wiped clean with flux remover, placement
automatically and provide feedback as to the
dissipation, without increased costs, they
can proceed.
temperature automatically, making it a fully
have been designed originally for the disc
As with LGA rework, the part is
automatic process before top heat is applied.
drive industry but are now migrating into
soldered to the PCB and has a small gap
Using this method allows much lower top
many other products.
from part to PCB (called standoff height) of
18 – Global SMT & Packaging – October 2009
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