my educated guess is your hard to control mini torch greatly exceeded the safe heating rate and peak temp the led can take and damaged it.
while none of the accepted methods we often use in the forums are 100% correct in reflow temp profile the torch is the worst.
and any solder voids in the thermal pad are deadly.overheating can also damage the die attachment to the ceramic base.
a failure here causes a near instant burnup.
Yeah it was dumb of me to do. I wonder why it worked fine for a few weeks though? Maybe it worked fine but had some unvisible damage that lead to failure? Could I reflow using a cloth iron next time? That seems to work too?
if you can use a ir thermometer or better yet a thermocouple meter yes.
set the peak temp via its control.start cold with the paste applied and the led placed.
when everything is showing correct like in the video just pull the plug and let it cool.
I dont see any problem in your heating device (torch).
If you dont want to buy prof equipment, you can use it this way:
-take a flat piece of metall (block). (see djozz link)
-buy cheap 300 degree C thermometer from ebay. Mount it on block near pcb place.
-found out proper position for led star and torch flame to folow requirements that are mensioned by Lexel in every second thread (nothing bad IMO).
Important:
You need to use good (good condition, not old, not laying several years, never heaten over 40 deg C etc) solder paste. It has lower soldering temperature than most usual wire solder. With right temperature raise curve, you will have lots of time to see how it is melting due process. After some practice, you wont need to check temperature. Surface structure of solder paste will say you what it going on right now and you will aprox know how much extra time you need.
If you cant see solder paste around led, you can always put same paste on wire pads and use them as indicator.
I tried the cloth iron and works good. i also have a thermal coupler.
Modern cloth irons are too delicate. It is hard to get them hotter than 200 degrees.
well I tried and it works to reflow the led just fine!