Ed,
Capacitors are better today than they were in 1986 when I wrote my first
paper on capacitor cracking. Today 1206 and smaller chips are wave soldered
with solder pot temperatures between 250 and 260 degree C. This is because
ceramic materials are much better and there are fewer internal flaws that
tend to propagate across opposing electrodes during the solder process. I
suggest that you get copies of papers written by Kemet on capacitor
fractures and soldering.
When investigating cracks in capacitors look for first order effects first
because that is where most sources or root cause will be found. In the 15 or
so years looking at defects in ceramic capacitors I have had to look beyond
first order sources in only a hand full of times and then I needed back
scatter SEM and EDAX to find capacitor manufacturer processing related
problems.
When we cross section the reference document is RS-469 which limits our
methods of investigation because parts are mounted with electrode
perpendicular to the section plane. With thinner dielectrics it becomes more
and more difficult to find the actual flaw site because ceramic can pull out
during sectioning leading an investigator to assume that there is a porosity
or void problem when it may not exist. Accurate ceramic porosity is viewed
when a part is fractured and examined using SEM.
Current external crack sources include board flexure, hand or wave soldering
large parts (>1210), large solder filets coupled with excessive cooling
rates after soldering and impact damage. Board flexure after soldering is
still the biggest source of capacitor cracking I see today and generally
occurs during handling, depaneling or assembly installation into final
product. Large solder joints, excessive cooling and of course soldering
irons are also in the top five for external defects. Modern pick and place
machines are unusual if they are a source of cracking. Using the wrong
solder process with large parts like wave soldering chips >1210 is a good
way to observe capacitor cracking first hand.
Internal crack sources include delamination or lack to adhesion between
layers, inadequate ceramic grain growth across electrodes, inconsistent
dielectric layer thickness, poor cut registration resulting in inadequate
side or end margins, electrode ink clumping resulting in localized
dielectric thinning, organic fibrous inclusions during layer build up
resulting in thin dielectric, termination over firing and ceramic over
firing. Most capacitor internal defects are easily dismissed as "consistent
with mechanical or thermal over stress" so an investigator needs to look
beyond RS469 sectioning techniques. Sectioning techniques parallel to
instead of perpendicular to electrodes reveal a host of capacitor
manufactures problems that are not detected using normal methods.
While not traveling I am rewriting my old crack paper to include the good,
bad and ugly when it comes to cracks and defects. I am constantly getting
new source material to add to the paper.
If there are any further questions please contact me directly at
[log in to unmask]
John Maxwell
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Edward Hare, <SEM
Lab, Inc.>
Sent: Tuesday, August 24, 1999 8:07 AM
To: [log in to unmask]
Subject: [TN] Chip Capacitors
Hi all,
Thanks for all replies to my original question about chip capacitor
failure rates. I am currently involved in an issue where high leakage
current failures (~300 microamps at 50V) occur at a fairly significant
rate on chip caps that are on the top side of a mixed PTH/SMT assembly.
SMT components are reflowed in an IR oven and then the PTH components
are stuffed and wave soldered. I read through John Maxwell's (AVX)
papers and it appears that he recommends a solder wave temperature of
232 +/- 2 degrees C for assemblies with chip capacitors. My clients use
a 260 degree C wave (which I have always thought was typical). Do any
of you follow Maxwell's recommendations?
Best regards,
Ed
--
SEM Lab, Inc.
Scanning Electron Microscopy and Failure Analysis
Snohomish, WA
(425)335-4400
http://www.sem-lab.com
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