This may no be a good reference but here is the story so far. There are a number
of possible reasons why chip capacitors have been found to fail during
production and in the final products and here are just a few.

The design and method of component manufacture can affect the components ability
to stand up to modern manufacturing methods. In cases where components are
selected at the top of capacitance range for a given package size due to the
number of plates, the components can be more sensitive to fracture failures. 

Often the component manufacture will suggest de-rating parts for a given
process. The internal construction and lay up of the internal electrodes may
also affect the robustness of the parts. This does vary with different part
suppliers and should be evaluated during component assessment.

Components, when they are supplied, may be damaged prior to removal from the
packaging; this has certainly been seen in the past. Internal cracking may be
present and is then exaggerated by the assembly process. Examination of selected
components in their packaging can provide examples of defective parts. 

Machine centring and placement of components on to the board has been a problem
area over the years. Again this is exaggerated where the parts are less robust.
Recently it has been the use of board support pins supporting the board without
any flexture. Poor positioning of pins under components does not allow the board
to move when the component is placed. Minor cracking may occur and is then shown
up during soldering and test. 

(For those older engineers who remember a specific type of machine that was
called the chip cracker, remember the old America magazine Nut Shell News!!)
 
Wave and reflow soldering temperature settings again have been shown to affect
component reliability.  Controlling peak temperature and temperature rise during
reflow must be monitored. Minimising the temperature of solder baths to between
235/240oC and controlled board pre-heating has mostly eliminated the wave
soldering problems in the industry. 

It is uncommon for the reflow soldering process to cause failures unless parts
have already been damaged. Unless the quality of the capacitor is poor with
internal voiding or stress cracks the reflow process is less likely to cause
problems.

In the case of wave or reflow soldering if the board is poorly supported and the
board is allowed to sag the board may be warped after soldering. If any
subsequent process straightens the board it may cause component cracking. A
possible example is in-circuit test where the board is straightened flat during
pin contact.

Hand soldering and rework has also been shown to damage parts if correct
procedures are not followed or poorly trained staff are used. It is common where
components come into direct contact with solder bit rather than using the solder
to provide effective heat transfer. During any hand soldering operation using
soldering irons the solder is used to transfer the heat to the board and
component.  Today it is more common to use hot air pencils to rework surface
mount component minimising physical contact with parts.

Board flexture during board handling, break out, in-circuit test or final
mounting of the board into the product is more likely to cause failure. This is
again true if the component is not correctly specified and tested during initial
assessment of the supplier. 

Most of the cases currently being examined for customers .

Existing methods of board break-out needs to be reviewed the board if not
supported during separation will flex and cause component damage. Again this is
the most common method of failure.

The flow solder pallets need to be examined as the boards are not held correctly
or supported to minimise warpage. This is causing distortion during the final
circuit board during test or breakout. Boards held too firmly in pallets can
result in failure, operators tend to peel the board out of a jig if it is a
tight fit, crack goes the chip.



Bob Willis
Electronic Presentation Services
2 Fourth Avenue, Chelmsford, Essex CM1 4HA. England.
Tel: (44) 01245 351502
Fax: (44) 01245 496123
Home Page: http://ourworld.compuserve.com/homepages/bwillis
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