Fluke Datapaq Reflow Tracker User Manual page 27

Checking Profiles
Some soldering faults which can be caused by incorrect temperature profiles are
described below.
• Incomplete soldering – There are numerous possible causes, one of
which is insufficient heat energy input to the solder joint. This would usually
be caused by too low a temperature before the reflow zone or too short a
time above the liquidus temperature of the solder.
• Tombstoning (drawbridging or Manhattan effect) – A leadless
component is lifted at one end during soldering, due to uneven wetting of
the two terminations. This can be caused by many factors including PCB pad
design and component layout. To help ensure that the two ends of the com-
ponent are soldered at the same time, the profile shape should be adjusted
to minimize the rate of heating as the liquidus temperature is reached.
• Misalignment of soldered components – This is a less severe
manifestation of the problems that cause tombstoning, and thus the same
remedies can be applied. Careful profiling, placing many thermocouples in
the area affected, may reveal large temperature gradients in that area of the
assembly. These may be alleviated by passing the PCB assembly through the
process in a different orientation.
• Component cracking/mechanical failure – The cracking or fracture
failure of small ceramic components can be caused if the change in
temperature (heating or cooling) is too rapid. Use Insight software's Slopes
analysis mode to find the maximum slope (temperature gradient, °C/s) and
compare this to the limits set by the component manufacturer; these limits
may well be different from any limits specified by the solder-paste supplier.
• Solder balling/beading – Often caused by excessive deposits of solder
paste and then outgassing as the product is heated. It can be improved by
reducing the rate of temperature rise in the ramp zone.
• Poor wetting of solder joints – If the cause is thought to be within the
thermal processing of the assemblies, there may be oxidation of the leads
and/or pads before the solder reaches liquidus. Possible remedies include
reducing the dwell time in the middle part of the temperature profile or
reducing the temperatures in this area.
• Voiding – Voids are generally caused by air trapped in 'via holes' in the
component pad or by outgassing of the solvents within the solder paste.
Changes to the temperature profile may improve this: either shortening the
soldering time or slowing the ramp rate.
Statistical Process Control
SPC is a powerful feature of the Insight software which allows easy analysis of
the results of your profile runs over time. Thus, by highlighting trends in the
REFLOW TR ACK ER
Reflow Temperature Profiles
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