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Thread: Reflow issue leds not lighting up

  1. #1
    Unenlightened
    Join Date
    Mar 2018
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    2

    Default Reflow issue leds not lighting up

    im using sink pads but im having an issue with the leds not lighting up after reflow.
    leds all are fine i think its the stencil ive been using isnt getting enough paste under the leds . im thinking of taking all the leds off and putting more paste on an re doing them .

    do i need to do this or can i just put blobs of solder paste net to each led would this get sucked under each led ??? and fix my issue ??

    whats the best solution

  2. #2

    Default Re: Reflow issue leds not lighting up

    Quote Originally Posted by oly999 View Post
    im using sink pads but im having an issue with the leds not lighting up after reflow.
    leds all are fine i think its the stencil ive been using isnt getting enough paste under the leds . im thinking of taking all the leds off and putting more paste on an re doing them .

    do i need to do this or can i just put blobs of solder paste net to each led would this get sucked under each led ??? and fix my issue ??

    whats the best solution
    There could be a few things going wrong but here's my perspective.
    I never use a stencil and I use the cook top in my kitchen which is the flat glass type, not so scientific but it works.
    When reflowing I go about it two different ways:
    If removing an LED from a board to swap in another LED I seldom use any extra solder as there will already be plenty left in place (e.g. doing a basic LED upgrade using the existing hardware).
    When starting with a fresh MCPCB and LED I use a syringe of liquid solder with a very fine nozzle so need to stencil. Sometimes I even pre tin the PCB by heating it until the solder melts and conforms with the pad and goes from wet and grey to shiny and solid.
    Either way, pre tinned or wet, place the LED on the PCB ensuring the right orientation and gently heat until the solder melts. The LED will be sucked into position by the liquid hot solder and you're good to go.
    A few finer points you'll pick up as you get the hang of it... a light press with a finger tip can press out excess solder, too much solder can be a bad thing and so on.
    Hope this helps get you on the right track.

  3. #3

    Default Re: Reflow issue leds not lighting up

    when that happens to me, it's usually too much paste on the thermal pad which raises one of the electrical pads too high.
    i've gently pushed down on the LED while the solder is melted and the excess will squirt out.

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