onCollisionEnter2d spawn - unity 4.6

my understanding of onCollisionEnter2D was that a collision would not be detected if the object was spawned inside Another collider.

I have airplanes that cross the screen and drop bombs, which I want to explode when they hit the ground. the planes can pass through walls. If I instantiate the bomb inside the walls collider, the onCollisionEnter2d fires and it explodes immediately. what I want is for the bomb to drop and only explode if it's hitting a wall after it's exited the wall it was spawned in. Bombs aren't always spawned inside a wall, sometimes there is nothing there in which case the current method works fine.

How come the onCollisionEnter2d is firing during instantiation? any simple solutions for this. I could set an armed flag when the bomb exits the wall but they aren;t always spawned inside a collider.

any suggestions?