Detailed application notes intended to guide and assist our
customers in using multilayer ceramic capacitors in surface mount
technology are available on the Syfer website www.syfer.com
The information concentrates on the handling, mounting,
connection, cleaning, test and re-work requirements particular to
MLC’s for SMD technology, to ensure a suitable match between
component capability and user expectation. Some extracts are
given below.
Handling
Ceramics are dense, hard, brittle and abrasive materials. They are
liable to suffer mechanical damage, in the form of chips or cracks, if
improperly handled.
Terminations may be abraded onto chip surfaces if loose chips are
tumbled in bulk. Metallic tracks may be left on the chip surfaces
which might pose a reliability hazard.
Components should never be handled with ngers; perspiration and
skin oils can inhibit solderability and will aggravate cleaning.
Chip capacitors should never be handled with metallic instruments.
Metal tweezers should never be used as these can chip the product
and may leave abraded metal tracks on the product surface.
Plastic or plastic coated metal types are readily available and
recommended - these should be used with an absolute minimum of
applied pressure.
Counting or visual inspection of chip capacitors is best performed
on a clean glass or hard plastic surface.
If chips are dropped or subjected to rough handling, they should
be visually inspected before use. Electrical inspection may also
reveal gross damage via a change in capacitance, an increase in
dissipation factor or a decrease either in insulation resistance or
electrical strength.
Transportation
Where possible, any transportation should be carried out with the
product in its unopened original packaging. If already opened,
any environmental control agents supplied should be returned to
packaging and the packaging re-sealed.
Avoid paper and card as a primary means of handling, packing,
transportation and storage of loose components. Many grades
have a sulphur content which will adversely affect termination
solderability.
Loose chips should always be packed with sulphur-free wadding to
prevent impact or abrasion damage during transportation.
Storage
Incorrect storage of components can lead to problems for the
user. Rapid tarnishing of the terminations, with an associated
degradation of solderability, will occur if the product comes into
contact with industrial gases such as sulphur dioxide and chlorine.
Storage in free air, particularly moist or polluted air, can result in
termination oxidation.
Packaging should not be opened until the MLC’s are required
for use. If opened, the pack should be re-sealed as soon as is
practicable. Alternatively, the contents could be kept in a sealed
container with an environmental control agent.
Long term storage conditions, ideally, should be temperature
controlled between -5 and +40°C and humidity controlled between
40 and 60% R.H.
Taped product should be stored out of direct sunlight, which might
promote deterioration in tape or adhesive performance.
Product, stored under the conditions recommended above, in its “as
received” packaging, has a minimum shelf life of 2 years.
Mechanical considerations for mounted ceramic chip
capacitors
Due to their brittle nature, ceramic chip capacitors are more prone
to excesses of mechanical stress than other components used in
surface mounting.
One of the most common causes of failure is directly attributable
to bending the printed circuit board after solder attachment.
The excessive or sudden movement of the exible circuit board
stresses the inexible ceramic block causing a crack to appear at
the weakest point, usually the ceramic/termination interface. The
crack may initially be quite small and not penetrate into the inner
electrodes; however, subsequent handling and rapid changes in
temperature may cause the crack to enlarge.
This mode of failure is often invisible to normal inspection
techniques as the resultant cracks usually lie under the capacitor
terminations but if left, can lead to catastrophic failure. More
importantly, mechanical cracks, unless they are severe may not be
detected by normal electrical testing of the completed circuit, failure
only occurring at some later stage after moisture ingression.
The degree of mechanical stress generated on the printed circuit
board is dependent upon several factors including the board
material and thickness; the amount of solder and land pattern.
The amount of solder applied is important, as an excessive amount
reduces the chip’s resistance to cracking.
It is Syfer’s experience that more than 90% are due to board
depanelisation, a process where two or more circuit boards are
separated after soldering is complete. Other manufacturing stages
that should be reviewed include:
1) Attaching rigid components such as connectors, relays,
display panels, heat sinks etc.
2) Fitting conventional leaded components. Special care
must be exercised when rigid terminals, as found on large
can electrolytic capacitors, are inserted.
3) Storage of boards in such a manner which allows warping.
4) Automatic test equipment, particularly the type employing
“bed of nails” and support pillars.
5) Positioning the circuit board in its enclosure especially
where this is a “snap-t”.
Syfer were the rst MLCC manufacturer to launch a exible
termination to signicantly reduce the instances of mechanical
cracking. FlexiCap™ termination introduces a certain amount of give
into the termination layer absorbing damaging stress. Unlike similar
systems, FlexiCap™ does not tear under tension, but absorbs the
stress, so maintaining the characteristics of the MLCC.
SM Pad Design
Syfer conventional 2-terminal chip capacitors can generally
be mounted using pad designs in accordance with IPC-7351,
Generic Requirements for Surface Mount Design and Land Pattern
Standards, but there are some other factors that have been shown
to reduce mechanical stress, such as reducing the pad width to
less than the chip width. In addition, the position of the chip on the
board should also be considered.
3-Terminal components are not specically covered by IPC-7351,
but recommended pad dimensions are included in the Syfer
catalogue / website for these components.
Handling notes
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