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

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.

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.

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.

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 :slight_smile:

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?

So Agro you tested this loss when you soldered the driver leads to the contacts on a DTP mcpcb, that you heated the LED enough to cause damage to the LED?

” Also, normally you heat the LED twice. To reflow it and to solder the leads.”

I didn’t. Nichia tested their LED, heating it twice and measuring output before, after the first heating and after the second heating.
I posted the link a couple of posts ago.

In the part that you quote simply I notice the fact that a similar double-heating occurs in what we do.
It’s not exactly the same because you heat up pads several mm from the LED. Heat from it goes either through board traces or through a layer of insulation and then the board core (which has minimal thermal resistance).

In the meantime I found glue that works up to 190°C:

A followup…Previously I didn’t realize pure indium was so weak mechanically. Over 26 times weaker than SnPb. Is it bad enough to cause problems?
http://alasir.com/reference/solder_alloys/

20 kgf/cm² would be 1.4 kgf for XP footpring LED. Assuming a perfect reflow with solder covering entire pads (unrealistic). Sounds good enough, but some drop tests would be interesting….

Give up… Nothing will beat solder. :smiling_imp:

Nah. Indium might be good enough. If it’s not…
By using indium on the thermal pad and commonly available (and reasonably cheap) Bi58Sn42 on electrical ones you get 53% strength of the recommended SAC305. Should be enough. But the melting temperature drops to 138 °C, quite low?
Other (harder to get and likely more expensive) alternatives could be:

  • Sn51.2Pb30.6Cd18.2
  • In70Pb30
  • In97Ag03
  • In80Pb15Ag05
  • Sn70Pb18In12

[quote=Agro]

If you do it correctly the LED will not become as hot as it does during reflow soldering (you can tell by just looking at the solder on the sides of the LED). Use a powerful soldering iron (80W for example), a wide, clean tip and it should be possible to do it very quickly preventing heat buildup in the pcb.

Also you could probably find a way to also solder the wires during the reflow of the LED by fixating them carefully.

Yet another followup.
After reflowing take a toothpick and glue the sides of the LED to MCPCB with epoxy. Assuming 0.2 mm bond with XHP35, it should be 8 times stronger than Indium alone (with strong glue).
I wonder how would Arctic Alumina work….I guess thermally not much differently…

I can keep emitters under 220°C with ease in my electric induction stove, aided with an infrarred thermometer. Reflowed a 100-pack of Osram 5730s more or less recently without issues. Emitters hovered at ≈200+°C for minutes while seating and centering them.

If this did some damage to the leds I do not really know. They're now doing a bunch of hours per day.

Cheers ^:)

There is damage and there is damage. It’s a different thing to damage them so they have their lifetime shortened, you probably didn’t do that. And it’s different do damage them to the point of having sub-optimal performance.
We won’t know unless somebody tries to actually test it.

I have to say I have mixed feelings about writing my musings about possible future improvements in this thread….on one hand I don’t know a better one. On the other it stopped being based firmly on the ground. There are data points which suggest future improvements, I’m discussing mostly with myself how to get them, but we don’t even know if they are real, let alone worth the effort.
Personally, I won’t be able to make such test in a long while and I highly regret it.

On to the future:
I contacted Amepox about electrically conductive epoxy needs. I was told that to get the desired resistivity I need a compound which requires 150°C curing for an hour. That’s actually not above the maximum temperature stated for most LEDs, but I have mixed feelings nevertheless….I need to contact others. It may be required to separate keeping LED and PCB together from actually conducting (either electricity or heat)…

what do you think about this?
is it good for reflow emitters?
https://www.ebay.co.uk/itm/T962-Infrared-Solder-Led-Free-Reflow-Oven-Windowed-Drawer-IC-Heater-180x235mm/253416094785?epid=2271205256&hash=item3b00c6bc41:g:~WgAAOSwi49bHzgK

Or for a small fraction of the cost… :smiley:

Cost only as better flashlight.

That would ok if you are using a solder stencil to ensure that you get the exact amount of solder needed on the LED pads. Personally I think the electric skillet is perfect because if something goes wrong like one of the LEDs sliding out of place I can make adjustments while it is still hot.

What’s the trick with smaller emitters like nichias? Those who have reflowed nichias likely knows what I’m talking about.