I bought 20 xml2 u3 from ledna.
I used one in a dd setup. After a minute it failed. When I tried to replace it, I found that all my leds had a dark spot in the bottom. The led which failed clearly didn’t had solder in the dark spot, and I think this was the reason of failure.
In the next led I cleaned the dark spot with alcohol (last pic after 2 reflows) .
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I believe that is oxidation where there is a hole in the packaging. I sand them off with wet/dry 2000 grit. Use water and just sand enough to removed the oxidation.
Looks like oxidation but why in the middle of every led and not over the whole base?
The plastic packaging (reel) each LED comes in is composed of a tiny black plastic tray with a layer of clear plastic sealed over the top. There’s a small round hole in the center of the tray directly below the center of the LED. This hole is right underneath those oxidation spots.
It might have something to do with binning procedure - each LED is tested in factory so those can be thermal accelerated oxidation marks in this region under die.
Those bottom pieces of plastic with holes in it are not airtight around the rest of LED base so this do not explain why whole base is not oxidized.
If an LED is laying flat rightside up inside one of those trays then gravity would hold the LED on top of the hole. While it’s not completely air tight, it is likely that that the bottom of the LED over the hole might get more exposure to outside moisture then the rest of the LED.
At least that is my guess. Only other possibility I can think of is maybe CREE spraying the reels with something after the LEDs are sealed inside. And a bit of whatever they are spraying is passing through the hole and hitting the bottom of the LED.
Sorry, but this is almost never the case. LED reels are packaged in flat cardboard boxes inside plastic airtight envelopes (called MBP by Cree - resealable Moisture-Barrier Packaging) sideways and even after cutting smaller lengths they tend to coil on themselves so LEDs lie on their side. Even if that was not the case, LEDs are just too light to seal themselves gravitationally to the underside of reel plastic and they are not fit tight inside reel holes as they readily rattle if you shake the reel. LEDs are very sensitive to moisture and should not be stored removed from airtight packaging(first picture below) as they might be damaged during soldering then. Cree specify in LED datasheets storage conditions and baking procedure for overexposed LED to avoid said damage. Last picture is taken from Cree datasheet, reels dimensions etc are also specified for feeding to automated soldering machinery.
Unpacked reel in box.
Vacuum sealed resealable moisture-barrier packaging (MBP).
I still believe the most probable explanation is those marks are thermally accelerated oxidation resulting from binning procedure(testing) and will do so unless somebody will provide plausible explanation to the contrary.