PCB layout and soldering nozzle design in selective soldering processes
Figure 3. Minimum required clearance for multi- Figure 4. Minimum required clearance for single Figure 5. Minimum required clearance for single
nozzle dip processes. miniwave processes miniwave process, wettable nozzle.
ing nozzle in multi-nozzle dip soldering component should be equal or less than ratio between the pin diameter and the
processes are 5 x 8 mm. the distance to the solder joint. via. If this ratio is too large, no capillary
To avoid difficulties caused by edge action will emerge. Should this ratio be
clearance, multi-nozzle dip soldering improved hole fill too small, flux cannot soar through the
processes require a distance of at least 3 The phenomenon of poor hole fill is most- via, and therefore solder joints cannot be
mm between the edges of the joints to be ly based on an insufficient heat transfer formed properly. As a rule of a thumb, the
soldered to surrounding components or rate, which also can be improved with an diameter of the via should be equal to the
joints that should not be soldered. With appropriate PCB layout. The length of the diameter of the pin plus 0.2 to 0.4 mm.
a minimum nozzle size of 5 x 8 mm this component leads plays an important role Lead-free processes even can require a plus
results in a ‘clear area’ of at least 11 x 14 in this regard, particularly in multi-nozzle of 0.5 mm.
mm (Figure 3). dip soldering processes. Thermal energy also will be transferred
Depending on the specific process Multi-nozzle dip soldering processes better when the pad size is enlarged to a
conditions, smaller clearances can be require a lead length greater than 2.5 mm. certain extend, or if oval pads are used.
realized as well. This, however, needs to This is related to the energy transfer rate, If possible, avoid solder resist close to
be checked thoroughly. It mainly depends which directly affects hole penetration. the solder joint. This helps to keep the
on the type of neighboring components Longer component leads are dipped deeper heat at the solder pad and helps to avoid
and may require special measures, such as into the liquid solder, which improves the solder balling.
grippers with centering pins or wettable heat transfer, which finally results in an Attention should be given also to
solder nozzles. improved hole fill. thermal decoupling. With an appropriate
For miniwave soldering processes, Another issue that should be con- thermal decoupling of the PCB, the heat
board designers should consider 2 mm on sidered in respect to hole fill is an ideal will not be completely withdrawn to
three sides around a pin or a pin row and
5 mm on the side where the component
leaves the wave, to allow a proper peel-off
(Figure 4). If a clearance of 5 mm is not
possible, leaving the wave at an angle or
using wetted solder nozzles can be helpful
(Figure 5).
If board designers are not able to keep
the required 2 mm distance on at least
three sides, neighboring SMD compo- Figure 6. Alignment of neighboring SMD components.
nents should be aligned inline (Figure 6).
The advantage of an inline alignment is
that if the neighboring reflow-soldered
component is wetted during the selective
soldering process, it will not immediately
be washed away
Single miniwave soldering in a drag
process moreover requires consideration of
the distance between the solder joint and
a neighboring component higher than 10
mm on the soldering side. When soldering
at an angle, components higher than 10
mm could touch the soldering nozzle or
gassing hood. As rule of a thumb for these
Figure 7. Thermal decoupling.
specific components, the height of the
www.globalsmt.net Global SMT & Packaging – January 2009 – 17
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