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June 2005

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Subject:
Ceramic MLCs and Leadfree, John Maxwell case?
From:
"Ingemar Hernefjord (KC/EMW)" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Ingemar Hernefjord (KC/EMW)
Date:
Fri, 10 Jun 2005 09:12:08 +0200
Content-Type:
text/plain
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Ave America et omnigenum.

Read a bulletin from Kemet. About MLC flex cracks and leadfree soldering.
 
Cut:
"Once the crack is created, it may not be readily detected electrically. A small DC voltage may not create sufficient stress to create a runaway state of increasing current within a short time. In the case of the flex crack, the crack can exist in a nonconductive state for a long time after creation. With time, temperature cycles, moisture exposure and increased stress levels, the crack can grow an ionic path that at first becomes noticeable leaky, then quickly deteriorates into a  highly conductive path within the ceramic dielectric. The time to failure is UNPREDICTABLE, AND CAN OCCUR IN MINUTES, HOURS, DAY, MONTHS, AND LONGER." John D. Prymak, Application Manager, Technical Marketing, July 7, 2004. Kemet USA.

The author tells that the above is now very actual because leadfree solder joints will increase the probability of flex cracks due to a harder solder. Small MCL caps are not exposed as much  as the big ones.

Personally, I thought the ductility properties of SnPb vs Leadfree were neglectable, but I may be wrong. The Kemet article rang a bell, the specialists here ran to the battle stations...
I'm too old, so I remain on my seat and just await a comfortable answer from John. Or any other proffessori ceramico...

Ingemar Hernefjord
Ericsson Microwave Systems

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