Page 7.5
7.0 Soldering profile management and production
Profile management 551.19 \ 12.2015 \
stantly. Otherwise the flux could be already exhausted before the
entry of the PCB in the peak zone (melting zone). However next
to the active parts the flux contains also a not small amount of
solvents, thixotropy agents and so on, which boiling points are in
the range between 125°C and 250°C. These agents are not nec-
essary for the soldering process itself and must be vaporised be-
fore the entry in the peak zone. Otherwise they can cause reflow
errors like solder balls or voids. Solder balls emerge by the explo-
sively evaporation of solvents in the solder paste and the resulting
skidding of paste grains out of the solder paste. Voids are bubbles
/ blow holes in the solder joint due to vapour locks cause by flux
elements which were not displaced out of the solder joint. So it is
necessary to observe the requirements of the solder paste for the
optimization of the preheating time, which means the preheating
should also be not too short. It is also important to observe the
max. heating gradients during the preheating phase for all compo-
nents of the PCB. ≤ 2,5 K/s are the generally accepted gradients
for the reflow process.
Time above liquidus
The time above liquidus t
L
above the melting point of the solder.
The time above liquidus should be adjusted in that way, that on
one hand all solder joints are melted, that there is enough time for
the wetting, which means the formation of a seam of intermetal-
lic phases and the liquid solder volume can fill the crack between
component and PCB as well as can form the meniscus. On the
other hand this time should not be chosen too long, to prevent the
accelerated growth of intermetallic phases under the influence of
temperature and to limit the leaching. According to experience a
time of up to 1 minute is optimal.
Cooling time
The cooling time is the time between the solidification point of the
solder (intersection of the soldering profile with the liquidus line T
L
)
and the achievement of an individual end temperature (e.g. 50°C,
end point of the data measurement). The cooling time should be
minimised but it is necessary to observe the max. allowed cooling
gradient to avoid damages at all components of the assembly. A
cooling gradient which is too high can cause microcracks in the
package. Aim of a short cooling time is the avoidance of dealloy-
ing effects and the undisturbed growth of the intermetallic phase.
At the same time a short cooling time
will reduce the stay time at
Time above liq-
uidus
Cooling time
Summary of Contents for 551.10
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Page 100: ...Page 6 26 6 0 Function and start up Function 551 19 12 2015 Menu structure Setup ...
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Page 222: ...11 0 Options and spare parts Optionen 551_19 12 2017 Page 11 14 Create user ...
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Page 226: ...11 0 Options and spare parts Optionen 551_19 12 2017 Page 11 18 Set up external operation ...
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