[_Jim]

Exactly when did flight 990 explode and catch on fire?

At the point, I presume, where breakup of the airframe took place (if at all - approx 16,000 feet) ...

Check the charts in this thread:

http://206.112.97.70/focus/fr/571151/posts

[Fred25]

1.14 Fire. Very few pieces of wreckage have been recovered; no signs of fire have been found yet.

LINK