What are the advantages and disadvantages of Manhattan-Style routing?

Question

Manhattan-Style routing being the use of expressly east-west planes and north-south planes, using a via and changing planes when a signal changes direction.

Comparing to freestyle routing, which lets define as routing signals in any direction on a given layer, would manhattan routing generally result in increased density, signal integrity, and more or less layers?

I know this is somewhat general and highly specific to a given application, but I'm generally interested in why one would decide to route in a manhattan-style -- surely the reasons relate to one or more of the above, and there should be some justification to that end.

One guess of mine is also that two adjacent layers, one E-W and one N-S would be fairly minimal in cross-talk due to the perpendicular nature of the traces, versus two adjacent layers where the layers are routed free-style. Would you agree?

Answer 1

The primary advantage of a Manhattan route is that it can always be completed. You just need to have enough board area to accommodate all of the traces — but otherwise, you'll never find yourself unable to complete a route. This can be important if you need to get a layout done on a fixed schedule — the amount of work is roughly proportional to the number of pins, and you won't spend days or weeks trying to complete the task because of blockages.

Other routing algorithms might be more efficient than Manhattan in terms of board area and the number of vias required (saving money on each board produced), but they cannot guarantee completion of the route in every case, which means that there's a nonrecurring cost risk in terms of the engineering effort required.