That advice is interesting. I have in my hand a 2 ounce can of Alpha metals brand soldering paste that must be more than a decade old and it still functions flawlessly. I live in the Mojave Desert so the garage where this can is stored certainly reaches 120 degrees F (maybe more) in the summertime. The active ingredient, according to the label, is zinc chloride. It is an inorganic salt and therefore is not degradable. I apply small amounts of it using the end of an artistās paint brush. The one I generally use is about 1/8ā wide but I could use a pointed brush for more precise work. Nothing on the can mentions any special storage requirements.
@Vegas LED Fan, the trick is the flux inside of the solder paste as well as the little solder balls.
If you can keep the solder bass together and have some nice flux, it can work very well 
@Gunga, I only have experience on stove top, not inside of the oven.
I will never dare to use an oven, as that is far too expensive, energy intensive, and risky for food if I use the same oven.
Thatās why I just use a time temperature controlled hotplate for reflowing now.
It was 100% worth the time investment, and even the money investment if I were to buy it new
Can you send a link? Iām in Canada too.
I actually bought this one used a few years ago, it was a super lucky buy, and I even outfitted something near it with a relay timer to be able to control when to shut it off and run a fan at low RPM to cool it down a bit more quickly to get close to the ideal soldering profile:
https://www.aliexpress.com/item/1005002584173802.html
Absolute overkill, but it nicely beat the scientific hotplate I was using in the past 
Ah ok. Iām not that much into itā¦ 
Iāll just practice on the stove
I used a cheap IR thermometer to measure the temp increase of the pan i use on the hob, i monitored each power setting to see which one increased the temp closest to the rate the LED manufacturers recommend (basically took a reading every 5 seconds) and worked out when to turn off the power so that the pan temp peaks at the right temp.
In my case the hob needs to be on for 80 seconds at max heat, 30 seconds after this is reflow time. All rough measurements but the temp vs graph time is close to the manufacturers recommendations.
A well seasoned cast iron pan works best! Donāt forget the oil. 
/s
Thatās funny. I just ordered an ir thermometer yesterday. How much time and effort is it to do multiple LEDs this way?
Solder paste intended for electronics usually has quite specific performance characteristics, particularly with respect to stenciling. Older or poorly stored paste can have issues with viscosity (thinning) or coalescence (the individual balls donāt come together as well). This stuff is very important on a production line where you need the paste to behave predictably at all times, and following the manufacturerās refrigerated storage guidelines ensures it will behave exactly as expected for the intended shelf life. For a lot of hobbyist use it doesnāt matter as much, and I doubt many flashlight modders store their paste in the fridge unless they regularly build drivers from scratch and need reliable stenciling.
This is also good to read. When I look up solder paste handling info, itās usually telling me to refrigerate and discard after warming up for use. Which is a non starter for me.
Btw. Does anyone have the data for recommended pan temperature again? And 80 seconds is good to ramp to that? I like this plan. Would a tiny cast iron pan work? I got a tiny 4ā one for chocolate Iāve never used (and will never use) so no worries about food contamination.
80 seconds is specific to my hob so not a generic thing, could be quite a difference on yours. And iām going from memory from what i was doing a couple of years ago, i have it written down somewhere but that 80 seconds well may actually be 120 seconds instead of the 1min 20 seconds iāve remembered it as
The cast iron pan would be fine, if you measure the temp in the pan it doesnāt matter what itās made of really.
My hob is an old electrical one that has a lot of residual heat, hence having to allow for the temp still increasing once itās been turned off. I use a 4ā aluminium camping pan, it makes little difference to the temp because of the hob. I would guess a 4ā cast iron one wouldnāt make too much difference but a large cast iron skillet would likely slow down the rate of temp increase and as my hob is on max i couldnāt increase it, which wouldnāt be ideal i canāt see it making much difference to me as a hobbyist.
If you have a more instant hob, such as infrared or induction, the cast iron may help control and maintain the heat slightly better that alu, and take a bit longer to cool down afterwards which would be better for the LED.
The soldering temperature recommendations are in LED spec sheets, for example a standard XP-G2 spec sheet: https://cree-led.com/media/documents/XLampXPG2.pdf
If you look at p32 in this one it shows a graph of the temp/time recommended for reflowing, takes a bit of time to match the numbers to the markers on the graph but once done it makes sense.
As for multi reflows, it depends on your speed and preperation but iāve done triple boards comfortably without rushing. I miiight be able to do a board with 6 LEDs but i would need to work out how to prolong the pan staying at reflow temp, mine cools off too quickly.
My main focal points were on keeping the LEDās temp shock to a minimum by not heating it too quickly, and staying below itās max recommended temp, which is why i used a thermometer rather than guesswork. I just wanted to know i hadnāt cooked the LED so i didnāt have to factor it in as possibility when troubleshooting a build.
Remember, folks on here do reflows with butane torches and soldering irons heating the MCPCB directly which would indicate a lot tolerance in the process, so anything even close the manufacturers specs should be fine.
I also remember one reflow (i purposely ignore the other couple :person_facepalming: ) where i forgot to heat the LED with the MCPCB so flowed a cold LED straight onto a hot MCPCB, itās still working fine.
Itās funny. I used a soldering iron on mcpcb and have had good results. I used to use an aluminum pan. That was generally fine. I was never successful with triples and occasionally cook a mcpcb. I had a disaster that ended up working (we shall see for how long) the other day trying to adjust my techniques. Iām planning on using the small pan, ir thermometer, gas stove, and solder paste from now on. Iāve rarely had issues but my last mod has prompted me to clean up my technique (got a shipment of 519s to play with).
I can see that, if youāre doing a single LED itās easy to take temperature cues from when the solder starts and stops flowing, and once it starts flowing itās generally that way for long enough to do the reflow without having to heat it again, and if not you can give it a quick hit with the iron to get it flowing again.
With a triple you want to keep the solder flowing for a longer period of time and there are no clues as to how hot things are actually getting so itās very easy to overheat things.
If you werenāt getting a thermometer you could mitigate this by just letting the solder cool down enough to stop flowing between each LED on a triple so you know youāre not getting too hot with each hit of the soldering iron. Not ideal but better than disaster.
Oh, and as gas is instant the cast iron should act as a bit of a heat buffer to smooth the heat transference to the MCPCB. Nice.
here is a nice little hotplate option for $16
https://www.amazon.com/Remover-Heating-Soldering-Welding-Station/dp/B07W1ZZH8T
I store my solder paste at room temp, the flux tends to separate out after about a year (I think thats what the expiration date is based on)ā¦ makes it harder to squeeze out, and spread thin, but it still works
Thsnks Jon. Iām in Canada so different selection and prices.
this āPTC Heaterā is 110V and 220C,
a good temperature for a reflow plate:
https://www.amazon.ca/Thermostat-Constant-Temperature-Heating-Element/dp/B08LK7VN54/
not a full kit ā¦ needs legs and a plugā¦
hope you find something you like
the Bob_McBob approved model (lol) is this one for $50 https://www.aliexpress.com/item/33022471796.html
the main difference is the temperature is adjustable. I use 200C for reflows, but I also use 100C for softening stubborn loctiteā¦ works nicely!
Thanks for the links Jon. The second one looks interesting but itās $25 shipping on a $50 item.