Can't offer an answer, Danny, but may add a bit more fuel to your fire.
I've just had a board returned to me by the Test Engineer, who found two
X7R 0805 caps "missing". By "missing", I mean that the solder joints and
the component end caps were still there, but the ceramic centre portion had
disappeared.
One of several possible suspects that immediately sprang to mind, was this:
There appears to be more solder in the solder joint than is strictly
acceptable, though this board is a development model. I wondered if the
large amount of solder, while contracting when cooling, was pulling the
ends of the caps, allowing the middles to fall out. Anyone got any thoughts
or experiences about this? I have a couple of not-terribly-good pictures I
can show you, if they might help to focus thoughts.
Peter
[log in to unmask] 24/04/2002 12:10 AM
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cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: [TN] TAIYOYUDEN CERAMIC CAPS
Fellow TNers,
Wanted to see if any one else has experienced this.
We are using a 1uF X7R 1206 of this particular cap and using the
recommended reflow profile for the solder paste. The test is to ground one
end of the cap and measure the noise level from the other end through a
330kohm impedance circuit. What we have seen is that after the PCB is
reflowed and tested, the noise level is at some specific amplitude. We can
then re-solder the cap by hand and the noise level will drop an order of
magnitude. The reflow soldering appears to be good - nice fillet, wetting,
structurally sound. We were speculating if perhaps the termination ends of
the cap were disconnecting internally during the reflow (CTE problems) and
that the hand soldering is somehow connecting it back. I have noticed that
the recommended reflow profile of the cap specifies a much shorter
liquidous time than what the paste profile requires. This could also be the
problem. I have not yet been able to speak with the component manufacture
to get ! their opinion.
Sorry for the lengthy message and thanks for your feedback,
Danny
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