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

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luminarium iaculator
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vestureofblood wrote:

I’ve had a similar result when there is power bleeding off to ground to led negative on the driver board. High mode always. If there is a short of some kind in the MCPCB the best solution would be to just dump the mcpcb and use another one. Trying to prevent a short with thermal epoxy would likely result in having to use a layer so thick it would negatively effect the thermal path.

No first version is not for sure. Triple checked that.

It is pita to reflow dedomed emitter on new mcbcp since I can’t use tap from above cause I’ll damage perfect emitter.

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Great video. I guess on a chemically dedomed LED you don’t tap the emitter. How do you remove the excess solder then?

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You reflow them before dedome Wink

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OK….
Thanks. Smile

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Sledgestone wrote:
You reflow them before dedome Wink

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 Big Smile

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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 Smile

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Mitko and myself just want to point on this:

Mitko wrote:

Something else that is important and worth mentioning: sometimes the emitter + pad and the core pad could solder toghether yet the emitter will work( xml, xpl, xpg est).

It will work even if you connect the proper driver for testing outside the flashlight, but then inserted in the host the MCPCB will contact the host body and ofc this will cause a short connection.

Thats valid for DTP MCPCBs, non dtp in this case wont suffer that issue yet the emitter core isnt electricaly neutral: sometimes it could affect driver memory mode and/or correct driver modes

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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.

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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

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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?

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ZoomieFan wrote:
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?

Hi Zoomie,

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

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You can also verify with a multimeter by checking continuity.

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Is an iron hot enough?

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ZoomieFan wrote:
Is an iron hot enough?

I have a 60W adjustable iron and it is perfectly fine for reflowing. My smaller 45W also works fine.
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FPV wrote:
ZoomieFan wrote:
Is an iron hot enough?
I have a 60W adjustable iron and it is perfectly fine for reflowing. My smaller 45W also works fine.

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

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ZoomieFan wrote:
FPV wrote:
ZoomieFan wrote:
Is an iron hot enough?

I have a 60W adjustable iron and it is perfectly fine for reflowing. My smaller 45W also works fine.
That’s great. Never done this before, but it looks easier than with a skillet.
Any advise about the temp setting? Cotton?
I think you all are talking about two different type “irons”. Wink

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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 Smile

Tnx teach Wink

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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.

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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

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

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

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.

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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).

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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-A...
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?

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On new thought already.
Regardless what you do with the LED, it is easy to avoid heating the LED while soldering leads with the use of conductive glue. All the difficulties mentioned with glueing LED don’t apply here. It may even be fairly inexpensive.

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Excellent video vestureofblood! I learned a lot. If I ever need to reflow an LED, I’ll remember slow heat up (and not too hot), and slow cool down. Makes sense to not damage the LED.

I’d rather use my flashlight around the house than turn on the lights.

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Agro wrote:
On new thought already. Regardless what you do with the LED, it is easy to avoid heating the LED while soldering leads with the use of conductive glue. All the difficulties mentioned with glueing LED don’t apply here. It may even be fairly inexpensive.

Conductive glue? That is C.R.A.P

I have it home and it is ridiculous. I have no absolutely single use for that and it is fragile like glass when it hardness.

Why would you want to avoid heat? Isn’t solder paste good enough for you? It has very low melting point.

Don’t complicate things. Do Vestureofblood method or Old Lumens soldering Iron method and call job done.
I use soldering Iron method since it is virtually impossible to reflow certain emitters with VOB method. Sometimes you need to manually place and tap emitter because or it is to small or it does not getting sucked in pads.

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luminarium iaculator wrote:
Agro wrote:
On new thought already. Regardless what you do with the LED, it is easy to avoid heating the LED while soldering leads with the use of conductive glue. All the difficulties mentioned with glueing LED don’t apply here. It may even be fairly inexpensive.

Conductive glue? That is C.R.A.P

I have it home and it is ridiculous. I have no absolutely single use for that and it is fragile like glass when it hardness.

Why would you want to avoid heat? Isn’t solder paste good enough for you? It has very low melting point.

Don’t complicate things. Do Vestureofblood method or Old Lumens soldering Iron method and call job done.
I use soldering Iron method since it is virtually impossible to reflow certain emitters with VOB method. Sometimes you need to manually place and tap emitter because or it is to small or it does not getting sucked in pads.


I find 12% output loss due to heating the LED twice to be extreme and would like to avoid it, as simple as that.
And that’s with not-so-bad temperature profile that they use, not semi-random as we do.

In fact even if the output loss on chips other than the Nichia is far lower than that I would like to avoid it as I simply like to optimize stuff. I’m not fully aware of the tradeoffs involved (like glue selection which as you indicate may not be trivial), but half bin upgrade is worth a lot for me.

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You will not upgrade anything. Gluing the led is ridiculous. Glue maybe is electrical conductive and it could be used for some tiny wires repair or something like that but what about thermal properties of glue? How it will transfer heat from led to mcpcb? It will crack like glass once when you turn on the light(I give it 30 seconds – 1 minute before cracking).

Your idea would be possible if glue would have thermal and mechanical properties like solder but that kind of glue sounds like sci-fi to me. Yes I would also like if possible. “Cold re flow” sounds great but with today conductive glue impossible Smile

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Yes, at this moment I find the all-glue way challenging, there’s quite some R&D involved in making it right. And actually in validating whether it’s worthwhile too, the singular data point that we have is very promising, but my guts tell me that if it was so important, manufacturers would try harder to remove thermal stress themselves. I don’t find it impossible, actually I’m certain it’s possible.

There are many conductive glues. Some are based on polyurethane, some on acrylic. Some are filled with silver, some with graphite, some with diamond. They have various additives. There are likely other bases and fillers too. Are all of they brittle? I doubt it.
saabluster used conductive glue with XR-E. But he did so for structural reasons because he thinned down the ceramic substrate in order to shorten the thermal path.

How would I glue the the thermal pad? I wouldn’t, I would use liquid metal thermal paste instead.

And in the meantime I found a potential problem with indium as well. Will the bond be strong enough at near-melting temperatures?

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