Hi John,
We too had the 1206 cap problem.
Ours was a .33uF, 50V, Z5U
We found many attributes causing failure.
List is not in order of occurrence:

1. position of cap on PCB (close to edge)
    PCB flex from depanelization caused micro-cracks (some showing at
test, mostly field failures)
2. orientation of components (we found stress when parallel to direction
of flow)
3. Vendor compatability (we found some vendors parts to be borderline)
4. Profile (temperature ramp-up and cool-down can significantly effect)

Also you may want to research John Maxwell's articles on cap failures.
Please feel free to call me and we can discuss further.
Hope this helped!
FNK


Frank N Kimmey C.I.D.
EM Designer
C&K Systems, Inc
916-353-5366
[log in to unmask]
> ----------
> From:         John Olivier[SMTP:[log in to unmask]]
> Reply To:     DesignerCouncil E-Mail Forum.;John Olivier
> Sent:         Tuesday, September 22, 1998 7:40 AM
> To:   [log in to unmask]
> Subject:      [DC] SMD Capacitors Cracking
>
> I was very interested to see that there are a number of people who
> have
> the same problem as we are experiencing regarding SMD capacitors.
>
> We have found a number of our 100 NF 50 Volt ceramic de-coupling
> capacitors are going short circuit after 9 months or more. The
> capacitors are not a dead short, but are pulling down the VCC line.
>
> We are using Kemet X7R 1206 caps. The caps are connected VCC to
> Ground.
> Once the cap or caps have been replaced the units work.
>
> The printed circuit board is FR4 and is 1.6 mm x 15 cm x 6 cm. The
> boards go through 2 soldering stages ie. one for the SMD components
> and
> then for the conventional components.
>
> The printed circuit board once completed is coated with a urethane
> MS472N conformal coating.
>
> I would appreciate any information or suggestions you could offer.
>
>
> Thanks Very Much
>
> John Olivier
>
> Quality Assurance Manager
>
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