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Which tools are best for SMT rework—conduction or convection?
removed with a soldering hoof style tip. This Figure 2 shows a PLCC with a pitch of
is often not required with larger pitches 1.27 mm drag soldered after a bead of
when the volume of paste is controlled to a solder has been applied. This is done after
consistent thin line. pad has been cleaned with solder wick to
remove old solder. The component is then
Replacing very fine SOICs easy to align onto flat pads and tacked into
Conduction method—Ultra-fine SOICs, place on the corners with solder iron. The
which commonly have 0.4 mm and 0.5 blade is used to drag solder into the J-lead
mm pitches, require a very fine solder tip of the PLCC.
and pin-by-pin soldering for replacement. The J-lead acts as a spring for
Some small drag soldering tips have component protection against G-force and  
been developed for this, but they require vibration. This component needs to have
Figure 3. T0-220 power transistor.
a different flux technique and higher an outer fillet called a toe and a difficult-to-
operator skill level. achieve inner fillet called a heel. Without
Convection method—These ultra-fine adequate heating, the heel fillet is not
parts can also be placed on a rework system always visible and may need a second solder
that can align, place and reflow the solder application.
with hot air. This has to be carried out
on a rework system with vision and x- and Method 2: Hot air and solder iron
y-movement of package or PCB, to align tinning method
leads to pads while in heating mode. The next method is to apply flux, then tin
the pads with a solder tip such as a hoof
reworking plccs tip design. Place the part on top of that
PLCC (plastic leaded chip carrier) pads and tack the corners in place with a
components are robust packages and can solder iron. Lastly, use hot air to reflow the
be reworked with four-sided conduction part back. Note: Gel flux is used with this
tips or with convection heating. It is method as it helps hold the part in place
important to note that damage may during placement and reflow.
occur when removing a component with
 
conduction tips. Method 3: Solder paste deposition and
Figure 4. Dispensed paste pattern under heat sink.
The size of the package and the size hot air method
of the PCB will determine if a preheater The third method is usually done in
needs to be used with hand-held tools, or military applications where higher the solder under the part and would most
if a rework system with top and bottom reliability and better joint formation are likely burn or discolor the substrate.
convection heating with vacuum pick up is required. This method starts by cleaning
required. the pads of old solder. New solder is Replacement
PLCC 28-44 pins are relatively easy, as deposited onto the pads using a mini Solder paste is used to achieve good solder
size—6 mm to 18 mm—and mass are small. stencil or a time-pressure solder paste coverage on the heat sink. Tinning the pad
Hand-held tools and a preheater are used in dispenser. Dispense two small dots of with solder wire is not a consistent method
this case. PLCC 68 and 84 pins, which are solder paste to every pad, one dot on each of applying the correct solder volume
much heavier in mass and size, 25 mm and extremity. Place the component in between to a heat-dissipative pad. Solder paste is
30 mm, need a small hot air rework system the dots of paste and the J-lead contacts the easily dispensed with a time pressure paste
to remove and place, especially on larger, pads in the middle. Use a vision system to dispenser. Dispense paste in a pattern
higher mass PCBs. place the PLCC, then reflow with hot air. under the heat sink that will provide at
least 80% coverage after reflow and a line
Replacement reworking a T0-220 power or dot on the power device leads (Figure 4).
Method 1: Drag solder transistor X-ray analysis is the only way to evaluate
T0-220 power transistors have a heavy adequate solder coverage under the
ground pad that transfers heating from the transistor heat sink. Components can also
transistor to the PCB ground pad (Figure be printed with a stencil if required. Various
3). This feature is to keep the component solder print patterns exist from component
cool during operation. These components suppliers.
need to be soldered to the PCB with good The solder paste reflow can be done in
solder coverage on heat sink in order a process with multiple zones of heating, i.e.
to keep component safe and not allow a small rework machine or a basic reflow
component overheating. with hand-held hot air from the top and a
preheater warming PCB surface up to 80-
Removal 100°C from below.
Because of the solder connection to the If electrolytic capacitors are close to
ground pads, reworking this device requires power transistor, as in Figure 3, the use of a
 
top and bottom convection heating. If only rework system that shields capacitors from
top heat is used, the temperature would heating is an advantage as these capacitors
Figure 2. Drag-soldering a PLCC.
be have to be very high in order to melt can explode when exposed to high air
16 – Global SMT & Packaging – October 2009 www.globalsmt.net
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