Results 1 to 6 of 6
  1. #1
    mtbr member
    Reputation:
    Join Date
    Jul 2007
    Posts
    1,124

    emitter reflow problem

    Dilemma:

    I blew up one XPG2 on a 3-up 20mm star and need to replace it.

    I've removed the blown emitter already.

    The star is AA'ed to it's backplate, removal would be difficult. For extra problems, it is actually a pair of 3-up stars that I didn't disconnect from each other, because it helped with alignment in my housing.

    I removed the blown emitter from the board using my big wattage solder-gun by applying heat to the edge of the emitter itself, since I didn't care about damaging a blown LED.

    In that process, the dome detached, so I know that is not a method I can use to heat the replacement LED up. I can't apply heat directly to the star, since it is attached to a 1/8" thick piece of angle in the housing I'm using.

    I also know that too much heat makes AA epoxy give up too. - That's the method I use for debonding stars when I don't want them there anymore - I use a butane torch to heat up til the star (and emitter usually) fall off the backing.

    I've heard of hot-air reflow, but have no idea who to do it with a heat gun? from top or bottom? what concerns should I have about the other LED's? Will it even work since I can't expose the back of the star?

    My current thought is applying heat using my soldering gun to the edge of the star near the area I'm trying to reflow?

    Other Ideas?

  2. #2
    mtbr member
    Reputation: mattthemuppet's Avatar
    Join Date
    Jul 2004
    Posts
    2,296
    you're going to have to bite the bullet and remove the star. I can't see any other way that will a) work and b) won't stuff up the rest of the LEDs.

  3. #3
    mtbr member
    Reputation:
    Join Date
    Apr 2010
    Posts
    488
    If your soldering iron is powerful enough, you should be able to heat the star enough locally, to reflow the LED. Keep your soldering iron right next to the pads and hold solder right on the the just to see if it melts.

  4. #4
    mtbr member
    Reputation:
    Join Date
    Jul 2007
    Posts
    1,124
    It's a 140-watt weller soldering gun. Up to 900 degrees F I believe the box said.

  5. #5
    mtbr member
    Reputation:
    Join Date
    Jan 2007
    Posts
    754
    I use a hot air rework station for general surface mount soldering rework. The components are heated from the top of the PCB and the rework station has different nozzles to direct the super heated air to the appropriate places based on the package's footprint. It's not hard to imagine what the nozzle looks like for an IC with pins down each side of the chip.

    I've never tried it on an LED because I've always been able to use safer methods, such as a reflow heat plate, but I'd guesstimate a 50/50 chance of success without cooking the dome.

    Having the star bonded to the aluminum housing is going to make it quite a bit harder no matter what technique you're using to apply heat. That's because the case is a heat sink and will pull the heat away from the LED. That means you have to apply a lot of heat locally near the LED which makes it much more likely damage will occur.

  6. #6
    mtbr member
    Reputation:
    Join Date
    Apr 2010
    Posts
    488
    That should be more than enough power to heat the star locally and not desolder other two LEDs, but you need to be very quick. Have your LED and tweezers prepaired and please MIND THE POLARITY of the LED, so you don't have to go through all over again.

    First tin the solder pads and let them cool down so you'll be able to see when the solder starts melting (use some flux if you have some on hand), keep the iron on the star and hold the LED with tweezers. As soon as you notice solder melting, pop that led on the star and align it. Since your star is still attached to the heat sink, you shouldn't worry about hurting other LEDs because the heat will spread very quickly.

    Wish you best luck!

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •