LEDs are the most heat-sensitive device of all the components. When soldering surface-mount LEDs, you should hold the LED with tweezers and "tack" one end.
Then wait for the LED to cool down and solder the other end very quickly. Then wait a few seconds and completely solder the first end.
Check the glow of each LED with 3 cells in series and a 220R resistor. If you have overheated the LED, its output will be dim, or a slightly different color, or it may not work at all.
They are extremely sensitive to heat - mainly because the crystal is so close to the soldering iron.