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I have a red LED that doesn't light up

Discussion in 'LEDs and Optoelectronics' started by Morgann, Dec 4, 2018.

  1. Morgann

    Morgann

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    Dec 4, 2018
    I have a red LED that doesn't light up, but I don't see bad soldering and open circuit after opening. What other reasons will cause it to be off?
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    LED defect?
    Current control defect (series resistor or current source circuit)?
    No power?

    Show us pictures. Do you have a multimeter to test for power and continuity of traces?
     
  3. Morgann

    Morgann

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    Dec 4, 2018
    LED problem, but I did not see any problems
     

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  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Do you have another LED to test the rest of the circuit by swapping LEDs?
     
  5. Morgann

    Morgann

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    Dec 4, 2018
    Yes, I have,and the LED test is invalid using the instrument.
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Please explain, I do not understand this.
     
  7. hevans1944

    hevans1944 Hop - AC8NS

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    Did you de-cap the plastic lens to expose the LED chip, as shown in the image you uploaded in post #3? If so, why would you do that instead of just replacing the "defective" LED? WTF is going on here?

    The chip wire-bonds (welds) might have broken because of the stress applied to de-cap the lens. Is there a reason for wanting to use the exposed die instead of the packaged LED? Have you tried to perform both forward- and reverse-biased continuity measurements on the LED? Is there any conduction at all with, say, two volts bias applied through a one hundred ohm current-limiting resistor?

    More information, please.
     
    davenn likes this.
  8. davenn

    davenn Moderator

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    a wider view showing the rest of the circuit + a schematic and a link to the LED datasheet would be really helpful
     
  9. kellys_eye

    kellys_eye

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    The clue is in your own statement 'after opening'....... you DON'T 'open' LEDs to check/test them. You 'open' LEDs to break them - and that is what you've achieved.
     
    hevans1944, davenn and Harald Kapp like this.
  10. Morgann

    Morgann

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    Dec 4, 2018
    I have found a possible reason with my partners. We are analyzing LEDs, not circuit problems. The LED is determined to be bad.
    The welded silver paste cracked
     
  11. hevans1944

    hevans1944 Hop - AC8NS

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    A "possible reason"? If you want to "analyze" LEDs, you should purchase bare (un-packaged) dies and probe these with micro-adjustable wire test leads. No need to worry about cracking "welded silver paste" (or whatever) if you analyze the LEDs in that manner.

    All the "big boy" semiconductor manufacturers do it this way, sometimes with hundreds of individual test leads probing a die simultaneously, depending on die complexity. The companies that make the micro-adjustable lead fixtures and wire test lead probes go to a lot of trouble to electrically characterize their probes, which allows the effect of probe lead resistance, inductance, and parasitic capacitance to be taken into account when reviewing the test results.

    All this is usually done in a Class 1 clean-room environment, by workers encased in bunny suits with filtered breathing-air supplies. Of course you can do it on the cheap by just washing your hands, putting on a clean set of blue jeans, and squeezing a packaged LED with a pair of Vice Grip pliers to "de-cap" the lens, but your test results may be variable.

    LEDs appear to be mass produced by injection molding plastic around pre-assembled lead frames attached to LED dies. Perhaps you can contact one of these manufacturers to obtain the lead frames with dies attached before the plastic molding is applied.
     
  12. Cannonball

    Cannonball

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    May 6, 2017
    Take a 1k resistor, a 9v battery, clip leads. Connect resistor to negative contact on the 9v battery. Use clip lead to connect a clip lead to the other side of the resistor. Connect the other end of the clip lead to the cathode of the led and touch the anode to + side of the battery.

    If they are red leds you can check them with the diode checker of your multimeter.

    I hope this helps.
     
  13. hevans1944

    hevans1944 Hop - AC8NS

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    They (for some unspecified reason) want to test the LED without it's plastic envelop/lens. So they de-capped it, but the strain induced by that operation broke one of the wire bonds to the LED die. Or, the packaged LED could have been defective "as received," and they just wanted to get a better look at the wire attachment to the LED die. Whatever the reason, they don't seem to be having any trouble testing an LED.
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Testing to destruction is a valid test. What it's a test of in this case is unknown :)
     
  15. Morgann

    Morgann

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    Dec 4, 2018
    I think it seems that I did not express it clearly? I am an LED engineer who specializes in analyzing LEDs, so what confirms that LED problems are the most basic thing for us. Just confirm that it is an LED problem, the LED will be handed to me. I have to analyze the LED to confirm the problem...so I don't understand what the above answer means.
    Anyway, thank you for your reply.
    Let's take a look at the picture of the cause of LED failure.
     

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  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Yeah, that looks damaged. How are you sure that the fault didn't happen when you opened it? Your original question implies the LED worked until you opened it.
     
  17. Morgann

    Morgann

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    Dec 4, 2018
    Thanks for reminding.It seems that I have expressed the wrong problem. This lower part is slowly ground using a matte paper. This LED is determined to be ineffective by using an oscilloscope test. At present, only the adhesive is cracked, so this reason is most likely.
     
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