That certainly matches my experience, which I had
put down to being supplie dfrom a reject batch.

So, what can we do as Radio Hams to prevent such
a problem?

How hot does the whole board have to be raised before
applying a solfering iron? Would a hairdryer do?

"Terry Given" wrote in message
...
Larry Brasfield wrote:
Hand soldering can be very hard on SMD ceramic capacitors.
The high temperature gradiant created by applying heat suddenly
at one end can fracture the ceramic. This can lead to excess noise
or a tendency to break down at a lower than rated voltage as
moisure gets into the crack(s). The insidious aspect of this kind
of damage is that it can show up in the field, quite some time
after the parts perform alright in initial testing.

This is a VERY good point. Reflow ovens have very well controlled
thermal profiles, slowly ramping temperature to a plateau, holding,
slowly ramping up to final tmep, holding etc. Mostly to avoid this
thermal shock related mechanical failure mechanism. High voltage
ceramics are especially prone to this - hand soldering them is a risky
process.

I once used 2 x 15nF 1000V smt X7R caps in series across an 80-800Vdc
supply for a smps application. During testing one smps failed
catastrophically (two others ran fine). Detailed examination of the
corpse showed a blast pattern radiating outward from one of the caps,
which had ruptured. The resulting mess sprayed directly across the legs
of one of the FETs, thereby toasting the unit. At the time it was
operating at a DC bus voltage of around 400V, so the cap was nowhere
near its rated voltage, more like 20%. One of the guys I worked with had
extensive experience in this area (hi-rel smps hybrids for
il/aerospace), and showed us what went wrong. We immediately replaced
the capacitors, carefully using a manual hot air station, to both
preheat and solder. The units operated continuously into a dead short at
800Vdc, no problems - there were other issues of course, it was a
pre-production protoype, but none of the explosive kind.