How to reflow solder an LED emitter on a PCB or MCPCB.

Flashlight Modding and DIY Parts
66

So you would solder mcpcb with domed emitter to flashlight and then you would de solder it, de dome it and then solder again?

Nah… This is not right technique.

I usually reflow around 10-20 emitters and check them on my test rig and then de dome all of them at the same time so there are ready to go.

We are talking about curious cases when everything works on testing rigs and yet it does not work when assembled in flashlight.

Edit:

I just saw you were not replying to me. But it is ok lol :smiley:

67

Um, no… Solder the LED to the mcpcb you want and then dedome it.

Where did you get the other part from?..

Edit: Saw your edit :slight_smile:

68

Mitko and myself just want to point on this:

69

Wow that is great, thank you!
I particularly appreciate the intentionally casual technique, and lack of precision shown, to highlight the effect of surface tension with the wetted solder.

Very illuminating video, I had no idea that the leds could tolerate that duration of time at the solder’s melting point, or that thermal shock was the greater issue. I would have hurried it and had poor joints and raised, heat shocked emitters if I’d tried it on my own experimentally.

This is something that I’ve been putting off learning to do, I’m now quite inspired.
Thanks again.

70

If you reflow an LED try to stay within the heat up and cool down speed in the reflow diagram, too fast is bad for the LEDs lifetime

71

So the vertical side of the T is the - (right) and the horizontal side is the + (left).
How can I know the + and - of the PCB?

72

Hi Zoomie,

Most mcpbs are marked with + and - near the pads.

73

You can also verify with a multimeter by checking continuity.

74

Is an iron hot enough?

75

I have a 60W adjustable iron and it is perfectly fine for reflowing. My smaller 45W also works fine.

76

That’s great. Never done this before, but it looks easier than with a skillet.
Any advise about the temp setting? Cotton?

77

I think you all are talking about two different type “irons”. :wink:

Clothes vs. Solder

78

I think you are right. I was wondering 60w was one of those small traveling irons.
Now I think about, LEDs get very hot. Hotter than a clothes iron I guess.

I have a Weller solder station.
I’ll scrape the corn bread out of my skillet and use that :slight_smile:

Tnx teach :wink:

79

We were indeed talking about different irons, but, coincidentally, I have also used an ironing iron before! That also worked, it was a different project though. My solder paste has a melting point around 175° (measured with IR temp).

you can use skillets, mug warmers, hot plates, anything you have that gets hot enough.

80

:+1: … I love it when a plan comes together…. :wink:

81

its easy to overheat the LED qwith little thermal mass and high power heat source
slow and steady heat up and cool down is important especially for flip chip LEDs

and same as for food if its too hot the LED gets crispy, may have a bad taste when it is dead or quite working after very short of time

82

With the standard reflow procedure used here (melt, let auto-center, tap the dome, cool), what is the resulting solder bond thickness?

83

Hi Argo,

I don’t have a measurement I can give you, but I will say it’s quite thin. In most cases the LED will appear to sit perfectly flat with the mcpcb. If you can see a gap, the emitter is probly sitting to high.

84

Thanks. It tells my I botched my first reflow which I did yesterday. :wink:
But the purpose of my question was to get better understanding of thermals of solder joint.
With XHP35 running at full power we’re talking about temperature delta exceeding 20 °C per 0.1 mm of joint thickness. And that’s assuming no voids, which is probably unrealistic.
I see that Osram / Cree recommend reflowing with stencils resulting in 70-100µm joint. But I think that standard reflow doesn’t involve applying pressure to the LED, does it? So maybe what we do here is better? Or maybe no, I’ve seen some post here on BLF telling that manual reflow can’t result in a joint as thin as that with a stencil. I also found a joining process from Indium Corp which leads to 40 µm joints (though ones of poor thermal conductivity, overall not better than good paste).

85

OK, I spent far too much time seeking about how to best do a reflow.
I’d like to dump here some info that I found. I did not organize the search well enough, so I won’t quote all the sources, sorry.

  1. The thermal shock causes damage to the LED. It is normal. The only hard data that I found is here:
    https://www.nichia.co.jp/specification/products/led/ApplicationNote_SE-AP00032-E.pdf
    It shows that 260°C reflow for 10 seconds reduces output of Nichia NS3W183 by 10%. I’ve seen somewhere that flip-chip LEDs are less susceptible to being damaged like that. Also, normally you heat the LED twice. To reflow it and to solder the leads.
    Conclusion: low-temperature solder should improve output (use it for all LED pads and leads). A conductive glue should be even better. There’s also a soldering process from Indium Corp. called NanoBond that reduces thermal stress too.
    But I’ve seen no LED manufacturer suggesting really low-temp solders. Why? I don’t know. Maybe normally the effect is less pronounced than with NS3W183?
  2. Some LEDs don’t have high power density in relation to the thermal pad size. Some do. The worst affected is XHP35 HI. Osram Oslon Black is quite bad too. Using a highly-conductive solder may bring some real improvements for them. So can the use of liquid metal.

Overal glueing anode and cathode + pasting thermal pad with Conductonaut seems like the best attachment method to me.
But:

  • pads suck solder, but not glue. You need to cover them carefully. And then place the LED precisely because it won’t self-center. And then tap it without moving it. Overall much harder to do. I’m not about to try any time soon.
  • I’m somewhat afraid of thermal paste pump-out. Laptop forums recommend using CLU / Galinstan instead of Conductonaut due to it being thicker. This would provide somewhat different tradeoff, but probably not full safety. Maybe some adding non-conductive glue around the thermal pad would be a good idea?

The second best looks to be using a low temperature high-conductivity solder. Not too low temperature to avoid the LED unsoldering itself during work though. There’s one option that really stands out. Indium

However:

  • it is very expensive
  • it is not available as a paste

You can buy indium scraps for a moderate price . Is it really pure though?

Thoughts?