PCB layout and soldering nozzle design in selective soldering processes
dip solder
ing
1.0 mm
[0.04"]
> 2.5 mm
[0/1"]
> 1.27 mm [0.05"]
single miniwave in drag soldering mode
> 2.54 mm [0.1"]
Figure 8. Multi-nozzle dip soldering: pitch and lead length. Figure 9. Single nozzle miniwave soldering: pitch and lead length.
the strip conductor but will be held for has an impact on the solder’s peel strength the board is moved and usually a soldering
a longer time at the pad (Figure 7). to reduce the risk of bridging, or for sys- angle is used to improve the solder’s peel-
Flowing solder waves, also in dip tems featuring wettable soldering nozzles. off. The typical lead length here should be
soldering processes, should generally be Although pin rows with a lead distance around 1 mm (Figure 9). Shorter pins could
preferred. This ensures that oxide-free and smaller than 2.54 mm bear an increased cause poor meniscus formation and ball-
correctly heated solder alloy is continu- risk of solder bridging in a dip soldering shaped solder joints.
ously supplied to the solder joints. Even process, they still can be processed if some Particularly in the dip soldering pro-
during the contact phase, the solder alloy basic layout rules are considered. A smaller cess, an appropriate soldering nozzle design
does not cool down. This improves hole fill pad diameter, for example, can be helpful, can remarkably reduce the risk of solder
remarkably, even in the case of high-mass or, if possible, an oval pad form, which bridging. So-called debridging knives,
pins, at pins with connection to inner helps to spread the liquid solder into a for example, which mean wettable plates
layers or pins that are located at the outer different direction, off the component that are installed inside the solder nozzle,
edges of an assembly. leads. With specific modifications at the drain the liquid solder after dwell time
multi-nozzle soldering tool, a pitch to 2.0 (Figure 10). Debridging knives are suited for
reduced solder bridging mm can be realized as well. special applications where the design rules
Solder bridges are a major reason for The length of the component leads mentioned earlier could not be followed,
defects in selective soldering processes plays an important role in regard to solder where the pin length is smaller than 2.5
and are mainly caused by small distances bridging as well. mm and/or pitch is between 2.54 mm and
between the component leads. Multi-nozzle dip soldering processes 2.0 mm.
Whereas multi-nozzle dip soldering require a lead length greater than 2.5 mm
processes require a pitch greater than 2.54 (Figure 8). The peel strength of the solder Minimum solder balling
mm, single miniwave soldering processes is enhanced with longer component leads Solder balling is a phenomenon in all wave
allow remarkably smaller pitches of 1.27 and pulls the solder away from the solder soldering processes that always occurred in
mm. This applies for machine systems that joint to reduce the risk of bridging. the past and will occur in future as well. It,
facilitate setting of a soldering angle, which In single miniwave soldering processes, however, appears more frequently in lead-
free soldering processes, where process tem-
peratures are remarkably higher than in
traditional soldering processes. The higher
process temperatures can have a negative
effect on the solder resist. Depending on
the quality, the solder resist might soften
during preheating, which abets arising
solder balls to stick at the solder resist.
In traditional lead-bearing processes or
applications featuring high quality lead-free
solder resists, arising solder balls would just
bounce off.
Therefore, if possible, solder resist
close to the solder joint should be avoided
(Figure 11).
Particularly in multi-nozzle dip solder-
ing processes, special nozzle designs can
help to avoid solder balling as well. These
nozzle tools are featured with a defined
solder flow that is directed by means of
a flow plate. In addition, the complete
Figure 10. Debridging knives. nozzle tool is covered with a second top
18 – Global SMT & Packaging – January 2009 www.globalsmt.net
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