Differential pairs in top layer

[Bermell]

For a custom board, i am routing, the most of the high speed signals are in inner layers.

In the top layer, in the bga components, the high speed signals tracks, are very short.

I have a question about routing the differential pairs in the BGA component. I mean, because of the controlled impedance, i should route the tracks, with the width and gap needed, but It is not possible to set the gap between tracks because the tracks are very short in top layer .

Other question is about very short single tracks in top layer in bga components. Should these tracks be routed with controlled impedance?

[robertferanec]

If they can't be routed by required impedance, there is not really much what you can do. In some designs, thinner tracks and smaller gaps are used under BGA but as soon as signals are away from the BGA footprint, the track geometry is adjusted to meet the impedance requirements.