Surface Mounting Guide
MLC Chip Capacitors
POST SOLDER HANDLING
COMMON CAUSES OF
Once SMP components are soldered to the board, any
bending or flexure of the PCB applies stresses to the soldered
joints of the components. For leaded devices, the stresses are
absorbed by the compliancy of the metal leads and generally
don’t result in problems unless the stress is large enough to
fracture the soldered connection.
MECHANICAL CRACKING
The most common source for mechanical stress is board
depanelization equipment, such as manual breakapart,
v-cutters and shear presses. Improperly aligned or dull cutters
may cause torqueing of the PCB resulting in flex stresses
being transmitted to components near the board edge.
Another common source of flexural stress is contact during
parametric testing when test points are probed. If the PCB
is allowed to flex during the test cycle, nearby ceramic
capacitors may be broken.
Ceramic capacitors are more susceptible to such stress
because they don’t have compliant leads and are brittle in
nature. The most frequent failure mode is low DC resistance or
short circuit. The second failure mode is significant loss of
capacitance due to severing of contact between sets of the
internal electrodes.
A third common source is board to board connections at
vertical connectors where cables or other PCBs are con-
nected to the PCB. If the board is not supported during the
plug/unplug cycle, it may flex and cause damage to nearby
components.
Cracks caused by mechanical flexure are very easily identified
and generally take one of the following two general forms:
Special care should also be taken when handling large (>6"
on a side) PCBs since they more easily flex or warp than
smaller boards.
REWORKING OF MLCs
Thermal shock is common in MLCs that are manually
attached or reworked with a soldering iron. AVX strongly
recommends that any reworking of MLCs be done with hot
air reflow rather than soldering irons. It is practically impossible
to cause any thermal shock in ceramic capacitors when
using hot air reflow.
Type A:
Angled crack between bottom of device to top of solder joint.
However direct contact by the soldering iron tip often causes
thermal cracks that may fail at a later date. If rework by
soldering iron is absolutely necessary, it is recommended
that the wattage of the iron be less than 30 watts and the
tip temperature be <300ºC. Rework should be performed
by applying the solder iron tip to the pad and not directly
contacting any part of the ceramic capacitor.
Type B:
Fracture from top of device to bottom of device.
Mechanical cracks are often hidden underneath the termina-
tion and are difficult to see externally. However, if one end
termination falls off during the removal process from PCB,
this is one indication that the cause of failure was excessive
mechanical stress due to board warping.
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KYOCERA AVX [ KYOCERA AVX ]
