Well its been a long time since i have seen such a board, especially if it has modern fine pitch SMD parts on it. So care to show examples?
I would love to, but NDAs, y'know?
Includes 4 and 6 layer designs, too. Unsure if they were autorouted but they mostly adhere to a layer bias anyway.
Personally, I prefer layer bias, but I allow for changing orientation around the board. For example, around a QFP, same-side pins are radial, so opposite side routes should be tangential. Some routing can be resolved underneath the chip (typically making the shortest path), but don't go crazy with it, as that blocks other routes, or ground fill if applicable (i.e., 2 layer).
Of course any designs that are even close to dense will have large ares of the board that are flowing in a certain direction since its the only way to make good efficient use of board area.
I would dare say it's the other way around: bias is done for expediency. Or six layers used when four or even two layers will suffice, just to get it finished.
Mind, I mostly work with smaller quantity designs, where NRE is a larger fraction of total production cost, and PCB cost, isn't so much. So I've seen a lot of those cases. (I've also "fixed" a number of those cases, taking just a few hours to remove a practically redundant layer pair.)
Whereas, truly dense layouts -- consider cellphones with HDI boards, 8+ layers, and surfaces absolutely stuffed with parts -- can't afford much layer bias, because everything is so dense. HDI also facilitates flexible layout, which may or may benefit from layer bias anymore. (The businesses making them, can probably afford a better autorouter than Altium, too...)
But a single layer might have multiple of these directional areas on it depending on what that particular section of the board needs or the areas might flow and curve organicaly to get around places. As opposed to the classical style of having pretty much all traces going strictly vertical and then on the next layer all traces going strictly horizontal. Tho to be fair modern electronics tend to be a lot more point to point on a PCB compared to the oldschool digital designs where nets tend to fan out hugely over many components all over the board.
Yeah, which is my style as just said. I think not everyone can afford to make those allowances without confusing themselves, so a lot still prefer stronger layer bias.
I could point to any number of classic computer boards, with strong bias -- many of which were done with early autorouters, with square corner traces and all. But as you note, designs change over time, and what worked back then, may not be the best approach today.
Tim
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