Still not reflowing my own led’s, I always buy them on a pcb. Especially the alu ones tend to be a bit warped. Every new pcb+led that goes into one of my lights, gets a flattening trip on a lapping stone. But a sheet of fine sandpaper on a glass plate also will do the trick. It makes better contact (IMHO) and saves on thermal grease. You only need a hint instead of a blob.