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Comment by ajross | Hacker News Reader

> on smaller scales you should avoid making right-angle turns in metal lines because of electromigration

Someone needs to tell the semiconductor industry that, then, because all traces inside a modern IC are at 90 degrees. The whole chip is just a big array of parallel lines in two orientation that get cut by photolithography to form circuits. The vias between levels are 90 degrees too, of course.

It's certainly true that electromigration is going to concentrate more at tight bends. But in practice if it isn't killing your 7nm interconnect you're... probably going to be fine with that 1 mil trace that is a million times wider.

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The reason it isn't killing that 7nm interconnect is that those semiconductor physical implementation designers run lots of EM simulations to determine the reliability and lifetime of all traces, taking into account the trace dimensions, electrical properties and the expected current flows and the design is only signed off when those metrics meet or exceed the design requirements. Note also that specialized metals (tungsten, ruthenium) are used on really fine lower level traces of advanced nodes, due both to their resistance to EM as well their better compatibility with silicon (smaller passivation thickness).

I think you missed the sarcasm. Electromigration is simply not an issue for PCB design, period. The relative scales of the devices are separated by many orders of magnitude. The effect wasn't even discovered until photolithography pushed scales to values much smaller than we see on even the tiniest printed circuit.

Maybe, just maybe, there's a high current, high voltage board out there with tiny traces that was designed with some kind of trivial electromigration mitigation. But if there is, I've never heard of it. And electromigration is absolutely not a reason to avoid 90 degree turns on your digital logic board, don't be ridiculous.

Electrochemical migration is an issue in high-rel PCBs, though I don’t think that is true electro migration from charge carriers. Silver and tin dendrites have failed satellites.

Yeah, different things. Electrochemistry involves motion of charge carriers through some kind of medium, and can happen over very large distances even if the medium is just the polymer surface of the board. And needless to say it doesn't care about junction angle at all.

"Electromigration" is the motion of individual conductor nuclei out of the wire due to scattering with electron momentum. Effectively, when electrons are asked (by the electric field in the conductor) to make ultra-sharp turns at nanometer scales, they sometimes collide with the aluminum/copper/tungsten/whatever nucleus instead, knocking that nucleus away from where it should be in the crystal. Over time, this erodes the junction, increasing electrical resistance, and eventually the chip starts to fail at voltages and clock rates it used to handle well.

Yeah, poor wording on my part. You're not going to be able to avoid 90-degree corners in any case when 45-degree (or any other non-axis-aligned) lines are not allowed in designs for many processes. What I should have said is that the metal line widths should take electromigration into account.

There are also fun things like optical proximity correction [1], but of course these things are mainly handled automatically by simulations/EDA tools now.

[1] https://en.wikipedia.org/wiki/Optical_proximity_correction

> Someone needs to tell the semiconductor industry that, then, because all traces inside a modern IC are at 90 degrees.

This occurs almost never in a modern VLSI process.

Practically every 90 degree turn in a modern process involves a via or a contact to a different layer--VLSI tends to run different layers in orthogonal directions from one another.

I actually suspect that the really advanced processes nowadays actually ban 90 degree turns in their design rules as it may make the phase shift masks impossible to compute.

Uh... a via IS a 90 degree turn. We live in a universe with three spatial dimensions.

The wording was loose, but I stand by the point: there are no interconnect technologies in the modern world that use anything but axis aligned conductors. All turns are 90 degrees. (It's just that to turn 90 degrees in the plane of the wafer you need to make two of them)

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