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

  1. Mar 17, 2016 #1
    Hi, I am a Reliability Engineer. Currently, I am facing difficulties in finding the root cause to a defective EMI filter bead. Resistance measurements across the four leads (vertically, horizontally, diagonally) show that they are all shorted. However, a good ferrite bead should only be shorted horizontally as tested. From the x-ray picture, I couldn't find any abnormality. It looks perfect with two wires running in parallel. I am hoping to find some explanation in the world of Physics? Thanks!
     
  2. jcsd
  3. Mar 17, 2016 #2

    sophiecentaur

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    HI
    How does it perform at the RF frequencies it's specified for? Do you know it is faulty?
    Was the device subjected to high volts or current before it exhibited this behaviour? Could you just have fried it?
     
  4. Mar 17, 2016 #3

    berkeman

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    Welcome to the PF. :smile:

    Can you provide the part number and a link to the datasheet? Can you upload a picture?
     
  5. Mar 17, 2016 #4

    CWatters

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    Do you have a new one to compare your faulty one with?
     
  6. Mar 17, 2016 #5

    wirenut

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    Was the unit installed on a board?
    Did it initially pass QC? or was found failed during QC?
    Did it fail after a time?
    Was it removed for testing/ x-raying?
    Can you upload the x-ray?
     
  7. Mar 17, 2016 #6
    Failed_FB1.jpg Failed_FB1.jpg Failed_FB2.jpg Good_FB sample.jpg The datasheet can be found here. Part number is CM3312R111R-10 from Laird Technology.
     
  8. Mar 17, 2016 #7

    berkeman

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    Well that's super helpful, Ken. Except for the part where you explain what those pictures represent... :wink:
     
  9. Mar 17, 2016 #8

    wirenut

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    Could you explain what each x-ray is?
     
  10. Mar 17, 2016 #9

    wirenut

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    Darn it, Berkeman.
    You beat me to it.
     
  11. Mar 17, 2016 #10

    berkeman

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  12. Mar 17, 2016 #11
    Sorry, not sure how to upload the files. The first 2 photos are of the same faulty ferrite bead which I had mentioned with no abnormality found. The 3rd photo is taken from another failed FB (2nd case) with cracked found inside. Both components are from different boards. The 4th is a sample from a good component.
    Some background:
    - Two cases of EMI filter failure. Both cases were found when the -12V input on the board was found shorted.
    - Suspect a high current/voltage surge occurred.
    - Components were removed from the boards for failure analysis.
    - Comparison on the resistance measurement of both failed and good components were performed. Resistance measurements of failed components showed all leads are shorted.
    - X-ray was taken on both failed components. Only one showed elements cracked within, and the other showed no symptom.
    - OEM mentioned that there was a shelf life issue on this component. When it is removed from the packaging, the shelf life will be reduced to 3 months from the original 12 months. This part was later removed by OEM in year 2012.
     
  13. Mar 17, 2016 #12
    Hi Berkeman, how can you tell there is a different in dimension?
     
  14. Mar 17, 2016 #13

    jim hardy

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    Like any transformer, inter winding short ?


    i never handled one of these tiny surface mount inductors. How in the world does one make them ?

    What i've seen in transformers hundreds of time bigger that that one :

    overcurrent heated it point of insulation failure ,,, it's a 5 amp device per datasheet you linked
    overly enthusiastic soldering at assembly heated it to point of insulation failure
    overvoltage between windings pierced insulation and spot welded the wires
    manufacturing error applied too much tension and varnish insulation crept away after time
    varnish insulation softened by solvent(flux remover?) or time and insulation crept away
    some insulating material not correctly placed inside
    solder bridge shorts terminals together - there's only 0.030 inch between the terminals and tolerance stackup could reduce that to 0.010 .

    You may have to break it open.
    or, the folks who made it can probably analyze it for you

    Our safety department had a binocular microscope for analyzing asbestos. I found it handy for examining small electronic devices.

    from datasheet you linked

    upload_2016-3-17_19-19-0.png


    Good luck, let us know what you find ?
    You may have to break it open.

    Surely your trade journal has advertisers for failure anaysis ?
    http://www.asminternational.org/materials-resources/journals/journal-of-failure-analysis-prevention

    old jim
     
  15. Mar 17, 2016 #14
    Hi Jim, thanks for your advice. How would you advice me to open up the component? Horizontally or vertically?
     
  16. Mar 17, 2016 #15
    Wirenut and Berkeman, is my explanation on the x-ray clear? :P
     
  17. Mar 17, 2016 #16
    Hi Sohpiecentaur and CWatters,
    Thanks for your questions. Do you need further information?
     
  18. Mar 17, 2016 #17

    jim hardy

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    Wow a lot of posts appeared while i way typing.. i hadn't seen the pictures.

    In that sequence? The one in first two photos is shorted as we speak , after removal from the board ?

    Ask OEM if he's having "Tin Whisker" issues.... i curse lead free solder. We refused to use it in my nuclear plant.

    Ideally i'd want to split it horizontally at center plane.That way you'd see the wires laying side by side undisturbed..and anything else in there that dpesn't belong.
    Where i worked we didn't have micro-machinery, any bolt under one inch was a small one. I'd have asked a tech with steady hand to see if he could remove the top with a Dremel tool.
    If it's hard ceramic ,,, wow.. diamond wheel ?

    Surely there's somebody here accustomed to surface mount ?

    i'll watch for now, better qualified folks than me are aboard.
     
  19. Mar 17, 2016 #18

    jim hardy

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  20. Mar 17, 2016 #19
    Haha...Jim, thanks!
    Yes, the first two photos are shorted component after removed from the board.
    Tin whisker? Hmm...I will look into it.
     
  21. Mar 18, 2016 #20

    Baluncore

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    I agree that tin whiskers are a probable cause of an external short between the conductors.

    Metal whiskers are like fuses, they can be temporarily blown, until they regrow. If you connect the two separate conductors to a high current, low voltage cell, (say 1.2V), does the short disappear? If not, increase the voltage to 12VDC, has the short been removed? If not then you have a problem bigger than whiskers.

    DC voltage in the presence of moisture can promote the growth of whiskers and is a real problem with tin solder. What is the potential difference between the two parallel conductors when operating normally on the PCB. Are the currents power supplies or signals?
     
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