Soldering with Sn99.3Pb0.7 is like sailing with a headwind, and it also increases stress in the soldering iron's tips. Check here: Why do tips easily oxidize when they are used with lead-free solder? @ hakko.com
I recommend standard eutectic Sn63Pb37 or Sn60Pb40 classic solder alloys, namely if newbie.
Cheers ^:)
P.S.: editing Niko's above post to reduce image size would be nice, somehow messes up with the page view on mobile or small screen devices.
How is S-Bi57sn43 ?
I also have this.
…sorry for extra big photo
I remember it’s worse than SnPb.
Niko, you can fix the extra big photo by inserting a {width:100%} format tag in your photo link code. Example:
Before:
!https://www.nikohostwebsite.org/nikopicture.jpg!
After:
!{width:100%}https://www.nikohostwebsite.org/nikopicture.jpg!
Bi57Sn43/Bi58Sn42 should make your life a lot easier when soldering. Versus standard Sn63Pb37 or Sn60Pb40 you can do with around 50°C less in your iron.
Cheers :-)
Originally posted on Wed, 10/10/2018 - 19:06. Edited for a tiny explanatory addition.
! work, thx 
I found an article which largely rehashes what we’ve already found, but may be interesting for some. I only skimmed over it, so there may be some gems that I missed.
https://www.ledsmagazine.com/articles/print/volume-14/issue-8/features/manufacturing/understand-thermal-characterization-of-high-power-leds-for-reliable-ssl.html
Yes, Luminus makes great LEDs.
There is even better material than silver:
It’s even readily available for purchase. The price is a little high ($14K minimal order, good for 400 PCBs), but maybe one could get some samples? 
BTW, silver-diamond works even better but copper-diamond doesn’t: https://www.electronics-cooling.com/2008/11/advanced-metal-diamond-composites-love-and-heat-relationship/
Can you solder it?
The benefit of Silver is that you can solder the LED onto it just like with copper pcbs.
It’s gold plated, so you probably can solder to it.
Known as SC07. Good for soldering aluminum. High melting point, but very strong joint. I have 250gr 0,7mm spool lying unused. Bought for it’s “slightly” better thermal conductivity but the higher melting point made it my last choice.
- Clemence
If you don’t plan to remove the LED later, then we can easily braze the chip directly to almost any metal. The brazing material can be anything with melting temp lower than 1500°C. Copper or Silver is a good candidate.
- Clemence
But such temp will destroy the phosphor. Maybe a die as well.
Can someone explain how to wet aluminium with Sn99.3Cu0.7? Tried to do it over heatsink material in the past, to no avail. I can understand it has to do with the particular aluminium alloy of the heatsink, maybe some 6061 crap, and insufficient surface sanding down. Hello?
Cheers ^:)
Nanofoil
Asked for sample but heldback because the solder still limited to Sn based
https://www.indium.com/assets/videos/nanofoil/nanofoil-reaction.mp4
- Clemence
I’ll post a pic and video for you guys. But, I have to modify my hot plate (cloth iron) to 260ºC first. It’s only maxed at 210ºC. Rarely use lead free solder these days.
- Clemence
Tried to wet large area with SC07 and 60/40 at 260ºC max. and the result was not successful. I could only wet no larger than 3mm diameter blob. With 60/40 the blob was larger and easier but 5mm was the largest I could get. I had successful result but that was using my stove hot plate at 320ºC which I no longer have. Already converted the stove hot plate to a clamping fixture on my mini milling machine. 
FYI, soldering aluminum with Sn based solder is not reliable, unless there’s a continuous voltage in the junction to feed the galvanic difference. Adding Zn in the solder will last longer. I don’t recommend soldering aluminum directly in everything elsa but mechanical or structural projects. Most aluminum fluxes requires high temperature to properly activated (300ish ºC). The most successful way to solder aluminum in electronics is to first plate the aluminum with nickel - gold or copper.
- Clemence
Another thought….would vapor chamber work as a PCB base?
Plating aluminium with copper but what kind of aluminium we're speaking of here? The aluminium used to craft heatsinks or flashlights can be some sort of suckass aluminium alloy which won't plate easily, so I hear you better first zinc plate it then copper plate or something like that. 
Cheers :-)
A quick followup on the pure Indium:
I’ve really come to like the stuff, it makes reflows much less stressful especially with larger MCPCBs. You’re nowhere near the point where the emitters start to degrade and PCBs delaminate so you can take your time.
It also helps a lot with soldering wires where a large thermal mass sucks heat away quickly, which sometimes is unavoidable.