What could have caused an EMI Filter Bead shorted by itself?

[Ken Leong]

Hi Guys, thanks for sharing and giving me all the valuable feedbacks and suggestions. Just to answer to some of your queries.
- The first two pics are from the same component (sorry for the confusion) and yes, it is shorted diagonally across the terminations when measured standalone after removal from the board.
- The board was conformal coated, not sure if tin whisker can be the possible cause of failure. Will further check on that.
- The OEM did mentioned about the storage issue and that's probably why they had discontinued this part.

Jim, the shadows you saw are the solder remaining after removal from the board.
Baluncore, thanks for your explanation on condensation impact on the component during storage.

P.S. How do you guys paste my comments when replying?

 

[Baluncore]

P.S. How do you guys paste my comments when replying?

We highlight your text, then a tab drops and we click on quote or reply. It pastes your quote in our new post box.

 

[davenn]

- The first two pics are from the same component (sorry for the confusion) and yes, it is shorted diagonally across the terminations when measured standalone after removal from the board.

so it should also be shorted between both terminations on each end as well then ?

Dave

 

[Ken Leong]

so it should also be shorted between both terminations on each end as well then ?

View attachment 97668


Dave

Yes, Dave. All four terminations are shorted.

 

[davenn]

Yes, Dave. All four terminations are shorted.

OK

pretty crazy when there is no obvious sign of a short
Is it really a short ... 0.0 Ohms or is it just low resistance a few 10's of Ohms or similar ?
I would warm up a couple of faulty ones ( that look OK in x-ray - like the one I posted) and see if it drives off any internal moisture
and test resistance again

I just find it so hard to see how you can have a short cct without anything obvious

 

[jim hardy]

Yes, Dave. All four terminations are shorted.

To Dave's question

Ahhhhh, terminology... One man's short is another man's leakage...

I always use a an analog meter for such tests. That's because on the RX1 ohms range it applies tens of milliamps to the device under test.

Wetness gives a part-scale reading determined by the electrochemistry at the fault.
A hard short circuit that we'd expect to see on those Xrays will read very near zero

A digital meter on a high ohms scale will report a low number for a few hundred ohms of something conductive like carbonized insulation.

I'll bet Dave was thinking the same thing.

Individual tin whiskers will usually get burnt away by the current from an analog multimeter on low ohms scale.

I have seen however a "tin beard" , thousands of whiskers in a clump, growing between two adjacent terminals on a barrier terminal block. It passed enough current to actuate a solenoid valve and trip the power plant .
Tin is not the only metal susceptible

http://nepp.nasa.gov/whisker/reference/tech_papers/2007-brusse-metal-whiskers.pdf

That's why i suggest somehow getting the top off one of the failed ones so you can see inside. A clean interior says "well, it wasn't that."

old jim

 

[Baluncore]

We need some idea of the circuit application / impedance. Does the bead isolate high impedance, low power signals or does it isolate low impedance, higher power supplies? The presence of intact single whiskers in a power supply situation is unlikely, but there may be a surface coating of metal due to vaporisation of many whiskers over time.

An accurate resistance measurement would be most informative.

At some point you need to use destructive testing to further the research. It is almost certain that any whiskers present will be lost if you attempt to physically expose the component's internals for visual examination. That is why in my post #20, I suggested fusing whiskers with a DC supply. By using sequentially larger charged capacitors as an energy source you might get a good estimate the mass of the whisker(s) or short involved.

 

[Clark Magnuson]

That is a 5 mOhm 5 amp single turn balun made of Copper and Ferrite. It it shorted out, that must be an isolation test with an Ohm meter. No debris could represent a short that would shut down that kind of power line. My guess would be contaminated part or board or the part was damaged when soldered, caused a crack. Pieces of Ferrite do not conduct much.