Hi Ingemar,
I see that you are observing human reaction to potential danger, the "when
in danger, when in doubt, run in circles and scream and shout" syndrome.
Interesting speculation about lead free solders increasing flexure
cracking. Flex cracks occur under rapid high energy conditions unless being
tested and that is performed at 1mm/sec, still pretty fast. I suspect that
these times are well below what we could expect fro SnPb solder alloys to
respond to by creeping to relieve appreciable stress. Werner and/or Guenter
can better answer that specific issue.
Test results require careful examination as Pb free solders wet differently
and works published by a number of observers have shown fillet shape and
size have a significant impact of flex crack sensitivity. Also pad size
(width) has a big impact on flex crack sensitivity. One could almost see
"Better" performance from Pb free solder if pads wider than the test
capacitors were used and did not fully wet the pad and termination. Clearly
reflow profile also becomes an issue.
I commend your calm demeanor as those around you provide some entertainment.
But once we do have cracks then it can take a long period of time to fail
depending on a number of conditions including voltage, humidity and circuit
sensitivity to reduced impedance.
John
At 12:12 AM 6/10/2005, you wrote:
>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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