LED lamp pcb strange solder joints

[goaty]

Hi,

I disassembled a dead LED lightbulb and wanted to desolder some LED chips.
When applying hot air, the chips basically fell off the board.
What is that black stuff ? I tried the hot air with 450°C and nothing melted.

Thanks, Thomas

[sleemanj]

Thermal compound would be my guess, the oval areas in the middle being the solder.  Weird, never seen that before.

[goaty]

I think only the black areas are the "solder" areas, as the oval spots are plastic on the LEDs.
I was just very curious about the method of attaching the LEDs here. Seems to me more glued kind of than soldered.
It´s from a cheap LED light.

[sleemanj]

Could be an electrically conductive adhesive, but I don't know why they would use such in that application where normal reflow would have done, and the brown gunk around led outline looks like flux residue.

Take one of the LEDs you took off and apply a soldering iron directly to the remaining black stuff, see if it flows, could just be a weird looking, or just bad solderpaste.