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Wave Soldering Defects - Solder Flags
Solder flags or spikes are due either to inconsistent
flux application or poor control of solder drainage from the wave.
If poor flux application is the cause, there will
be other evidence on the surface of the board, like thin whiskers
of solder similar
to snail trails on a garden path.
Poor control of separation
from the solder wave would tends to be a random fault, not on
the same
contacts every time. This is due to the back flow of the wave
not being set correctly on a lambda style wave. The solder should
flow
at the same speed and direction as the board during separation
from the wave. Running slightly faster will not normally cause
spiking, but running slow or not flowing at all will increase
spikes.
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| Figure 1: Solder flags or spikes. |
If the spikes are always on the tip of the leads
it may be a solderability
issue caused by bare cut ends. If leads are cut by the supplier
or cut and stored for a long period of time, the bare end of
the lead
will oxidise and be difficult to wet with solder. If the
lead is slow to wet, it will also be slow to drain, hence a spike
can
form.
On some occasions, the spiking can be a thermal
problem simply eliminated by a longer immersion time in the wave
or increase
pre heat. A
component that has a high mass may have a lead of the same
size as other parts. During pre-heat of the board there may
not be
an opportunity for heat to be absorbed by the lead to overcome
this
thermal load. As the leads of the component separate from
the wave they will cool much more quickly, leaving either spikes
or
shorts.
The solder flags shown in Figure 2 are probably
due to inconsistent fluxing, which should be visible on other areas
of the board. The length of the pins is also excessive. Lead length
should
not exceed
1.5-2.0mm below the level of the board. Shorting can also occur during
exit from the back of the wave if the component leads are dragged
through the
wave oxide layer rather the exiting from a clean surface.
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| Figure 2: Inconsistent fluxing likely caused these solder
flags. |
Wave Soldering Defects:
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