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Thread: Question about led surface mount soldering

  1. #1

    Default Question about led surface mount soldering

    Hello everyone,

    I have skillet reflow soldered leds to star boards a couple times but they have always been 3w chines style leds. I am now looking to use a little higher quality leds such as the luxeon rebel.

    My method for attaching the china leds has worked but is a little different from any other methods i have read about as i dont use solder paste. In my method I pre tin the pads on the star boards with rosin core wire solder and a soldering iron. The amount has to be just right forming a smooth flat pillow look and then i add just a little extra flux/rosin afterwards. Then balance/set the led over the pre tinned pads and set it on the skillet over a hot plate. Then slowly turn up the heat watching it with a laser temp gun to be sure i dont crank it up to fast (generally pre heat for 1.5 minutes till it reaches about 140*C). I then crank the heat up so that the solder melts and sucks the led down (this generally takes about 15-20 seconds and happens around 180*C).

    So my question is, does anyone see any reason why this method wouldnt work with smaller surface mount style leds such as a luxeon rebel? I feel the most difficult part would be pre soldering the star boards with just the right touch of rosin core wire solder. If done properly i would think it should still work. I dont have any solder paste, it seems expensive, has a shelf life, messy, and to many kinds to know what to buy. What are your guys thoughts? Thanks

  2. #2
    Flashaholic
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    Default Re: Question about led surface mount soldering

    I've done similar and had success with rebels and cree xp LEDs. I do find using paste is easier though. Just watch the temps like you said.

  3. #3
    *Flashaholic* wquiles's Avatar
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    Default Re: Question about led surface mount soldering

    Quote Originally Posted by Optical Inferno View Post
    I do find using paste is easier though.
    +1

    The other/main reason to try solder paste (besides being "the" proper method), is that solder paste has built-in the right amount of flux to allow the solder to flow and create the surface tension which allows the LED to re-orient itself properly. Solder paste is CHEAP, so there is absolutely no reason not to try it.

    Do a Google search for this string, for a 10cc syringe: "http://www.ameritronics.com/zephpaste_63-37_solderpaste_noclean_10cc.htm"

    Will
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    Flashaholic* bshanahan14rulz's Avatar
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    Default Re: Question about led surface mount soldering

    as long as you use new, clean solder, and it is nice and shiny when you tin the pads of the mcpcb and clean the pads of the LED, solder should be just fine.

    I used to heat only the tinned mcpcb so that I could see the solder melt, and then carefully drop the LED on top and let it center before removing it from heat. This may lend itself to cold solder joints if you do it too fast, but I've never encountered one that I could detect. Giving it time to realign also gives it time for the layer that solidified under the LED to remelt and wet the LED pads. That said, I only have a 40W iron, so being able to reflow with that at all is impressive, in a ghetto-hack sort of way, and I've had to give consideration to how long it takes to fill up the mcpcb with enough heat. I would think with hotplate reflowing, I probably wouldn't be heating the MCPCB as long as I have to with the iron.

  5. #5

    Default Re: Question about led surface mount soldering

    Quote Originally Posted by wquiles View Post
    +1

    The other/main reason to try solder paste (besides being "the" proper method), is that solder paste has built-in the right amount of flux to allow the solder to flow and create the surface tension which allows the LED to re-orient itself properly. Solder paste is CHEAP, so there is absolutely no reason not to try it.

    Do a Google search for this string, for a 10cc syringe: "http://www.ameritronics.com/zephpaste_63-37_solderpaste_noclean_10cc.htm"

    Will
    Thanks for the link. Sounds like pretty legit stuff! I guess if its actually easier to work with why not give it a try. Roughly how many luxeon rebel leds do you suppose the 10cc syringe can solder? Thanks for the help!

    Lou

  6. #6
    *Flashaholic* wquiles's Avatar
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    Default Re: Question about led surface mount soldering

    Quote Originally Posted by ledlou View Post
    Thanks for the link. Sounds like pretty legit stuff! I guess if its actually easier to work with why not give it a try.
    Yes, this stuff works great. Same stuff I use to reflow my Nichia 219's:




    Quote Originally Posted by ledlou View Post
    Roughly how many luxeon rebel leds do you suppose the 10cc syringe can solder? Thanks for the help!
    One to two trillion LED's

    Seriously, you will run out of LED's before you run out of solver paste, or it might dry out before you have a chance to use it all (keep it in the fridge in a sealed plastic bag when not being used).

    Look at this Nichia LED about to be reflowed (top right) - note how little it takes per tiny pad:



    Will
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  7. #7

    Default Re: Question about led surface mount soldering

    Nice, I'm starting to like this solder paste idea. One last quick question which is a little off topic. For all of my cheap China leds I have been using the natural gas stove top in my kitchen with a stainless bowl and a solder fume sucker. I want to relocate this project to the garage for obvious reasons and have been thinking about buying a $20 dollar 800w hot plate from walmart. Do you suppose this could still do the trick? They also sell a 1500w hot plate at home depot for $40 and looks nicer. Since you have a lot more experience then me what do you recomend? What do you use? Thanks for the help!

    Lou

  8. #8

    Default Re: Question about led surface mount soldering

    Paste is definitely easier if you have a stencil, if you dont have a stencil then youre just dropping a blob on, and its easy to have excess squeeze out, and have little micro-beads of solder form that can potentially short out stuff. If youre getting good results with pre-tinning, its fine. If you have fine solder, I find it easier to get a precise amount on the pad with solder wire and a iron vs paste in a syringe imo. But you can put on too much either way if youre not careful. If you have a "hoof" tip, its even easier to get consistent results pre-tinning with wire solder.

  9. #9
    *Flashaholic* wquiles's Avatar
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    Default Re: Question about led surface mount soldering

    Quote Originally Posted by ledlou View Post
    Nice, I'm starting to like this solder paste idea. One last quick question which is a little off topic. For all of my cheap China leds I have been using the natural gas stove top in my kitchen with a stainless bowl and a solder fume sucker. I want to relocate this project to the garage for obvious reasons and have been thinking about buying a $20 dollar 800w hot plate from walmart. Do you suppose this could still do the trick? They also sell a 1500w hot plate at home depot for $40 and looks nicer. Since you have a lot more experience then me what do you recomend? What do you use? Thanks for the help!
    Since I do a lot of projects for myself and for sale, I have a Hobby/semi-professional pre-heater and SMT heat air gun (Aoyue 853A and 968), but you don't need anything this "fancy" to reflow parts. I just do enough to justify the "nicer" equipment. Plenty of youtube videos showing how to reflow with dirt cheap stuff, with excellent results


    Quote Originally Posted by CKOD View Post
    Paste is definitely easier if you have a stencil, if you dont have a stencil then youre just dropping a blob on, and its easy to have excess squeeze out, and have little micro-beads of solder form that can potentially short out stuff.
    When you have LOTS of pads to do, then yes, a stencil is the way to go since you can do all of the pads at the same time, with the right amount of solder paste. But for a single LED (3 pads at most), using the syringe for the tiny amounts I showed in the photo above only takes a little bit of practice. It is very easy to do. You can of course practice, swipe off if you mess-up, clean with alcohol, do it again, etc., until you get it right. After a few parts it becomes second nature to put a very small bead.

    Will
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  10. #10

    Default Re: Question about led surface mount soldering

    I want to relocate this project to the garage for obvious reasons and have been thinking about buying a $20 dollar 800w hot plate from walmart.
    I have once used a hot plate for soldering XR-Es, but I found a heat gun to be much better.
    I put the star in a third hand, and plance enough books under it, so that it rests about 10cm over the heat gun, wich stands upright on the workbench. Then I switch it on, wait until the solder melts, and remove it. This works excellent, and keeps the LED vibration-free.

    Just leave the LEDs in a normal oven for 2 hours or so at 80°C prior to soldering, to remove internal moisture.
    This message was morsed into the open end of a fiber-optic cable with a flashlight.

  11. #11
    *Flashaholic* wquiles's Avatar
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    Default Re: Question about led surface mount soldering

    I had to reflow some XP-G's for a project I am doing for a guy in California, so I took more pictures last night.

    This one unfortunately is a little bit blurry, but it shows the relative size of the solder paste serynge, the 10mm boards, the XP-G's, and an US penny for comparison:



    After wiping the boards with alcohol to clean them, here I applied a small bead to each pad (a little bit more in the middle pad):



    After you position the XP-G's with tweezers, you lightly press it down - don't have to go all of the way down:



    Then pre-head for 5 minutes:



    And use heat-gun to reflow the already warm solder - just a few seconds:



    Let them cool off for 5-10 minutes (the metal core PCB stays warm for a little while):



    All done. Nichia 219 on top, and XP-G (the ones I just reflowed here in these photos) on the bottom:



    Will
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  12. #12
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    Default Re: Question about led surface mount soldering

    For me i always found the easiest method was to pick up a spare toaster oven and do some re-flow work in there. I bought a small tube a solder paste from Digikey and i've done hundreds in that toaster oven so far. Just be sure Never to use that toaster oven for food after that though.

  13. #13

    Default Re: Question about led surface mount soldering

    IF anyone wants a visual look at led assembly we have put up some assembly videos at youtube on how to assemble leds manually, using basic tools like the George Foreman grill, mmmm tasty

  14. #14
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    Default Re: Question about led surface mount soldering

    Quote Originally Posted by WeLight View Post
    IF anyone wants a visual look at led assembly we have put up some assembly videos at youtube on how to assemble leds manually, using basic tools like the George Foreman grill, mmmm tasty
    Huh, that one shows laying down one strip of paste across all three pads! Don't know if I'd trust surface tension / solder mask that much personally, but I guess if that's the way Cutter does it and it works...?

  15. #15

    Default Re: Question about led surface mount soldering

    Quote Originally Posted by RoGuE_StreaK View Post
    Huh, that one shows laying down one strip of paste across all three pads! Don't know if I'd trust surface tension / solder mask that much personally, but I guess if that's the way Cutter does it and it works...?
    I have seen a few videos of real pros doing reflow, and it really seems like magic, the paste. Soldering BGAs by just making strips over the dots, and then _sloooop_, everthing fits like magic.

  16. #16
    Flashaholic* bshanahan14rulz's Avatar
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    Default Re: Question about led surface mount soldering

    solder mask keeps solder from sticking to the circuit board. A bare circuit board doesn't hold solder too well to begin with either. paste is basically tiny solder pieces and flux.

  17. #17
    Flashaholic bobjane's Avatar
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    Default Re: Question about led surface mount soldering

    I use a soldering pot to reflow LEDs. $15-20 on DX depending on which size you get. Works well and keeps me out of the kitchen.

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