Ken,

We recently had a similar problem with chip caps (.08 uF).  Modules
built with these parts tested OK thru standard module manufacturing
tests. The problem was only noticed when the resistance got low enough
to start putting a load on the circuit.  These parts also got 'better'
when exposed to heat (as we found when we did an elevated voltage stress
test).  The heat actually helps by driving out the moisture, one of the
3 factors needed for electrochemical migration (bias and ionic species
are the other two).  We performed cross sections and found cracks
extending from the ends of some internal plates. These cracks either
extended to the termination at the other end of the component or made
there way an adjacent plate.

Turns out the manufacturer finally admitted to having a bad batch of
parts.  The cross section photos put the issue to rest though.  Contact
me offline if you have additional questions. Might be the same supplier!

Sincerely,


Paul Terranova
DIGITAL's Analytical and Environmental Test Services Lab
200 Forest Street
Marlboro, MA  01752

Phone: 508-467-3109
FAX: 508-467-6796


                -----Original Message-----
                From:   Ken Patel [SMTP:[log in to unmask]]
                Sent:   Wednesday, November 19, 1997 8:41 PM
                To:     [log in to unmask]
                Subject:        [TN] Cap shorting at low voltage high
impedance circuit

                Cap shorting Problem????

                Failure Analysis Expert,

                In a low voltage high impedance circuit we are finding
~700ohm short
                across a .1uF capacitor in a low voltage, high impedance
DC circuit.  We
                find that over time the short develops, but a thermal
cycle (touch-up) of
                the location causes the problem to disappear.

                We tried using X-ray of the cap's but proven
inconclusive due to the density
                of the ceramic cap body and the suspected low mass of
the short.  Our initial
                impression is that dendritic growth is occurring, and
over time the two pads
                short through this growth but we were not able to find
any dendritic growth
                (?) using microscope (30X). Is dendritic growth possible
through epoxy which
                porous in nature? This cap is glued on the bottom side
using epoxy and goes
                through wave soldering operation.

                We are looking for any answer cum suggestion from anyone
who has seen this
                kind of problem were able to resolve.

                Thanking you in advance.

                Re,
                Ken Patel

                ______________________________________________________
                Ken Patel                       Phone:  (408) 490-6804
                575 Cottonwood Dr.              Fax:    (408) 490-6859
                Milpitas, CA 95035              Beeper: (888) 769-1808


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