[HN Gopher] Next-Gen Chips Will Be Powered from Below
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Next-Gen Chips Will Be Powered from Below
Author : jonbaer
Score : 122 points
Date : 2021-08-28 13:23 UTC (9 hours ago)
(HTM) web link (spectrum.ieee.org)
(TXT) w3m dump (spectrum.ieee.org)
| ChrisMarshallNY wrote:
| _> That 's up to three refrigerators, in your pocket!_
|
| Bit misleading, there. A != W
| marcosdumay wrote:
| Current is the one number you look when talking about power
| transmission.
| ChrisMarshallNY wrote:
| ...aother one, being Volts (which brings us back to...W).
| Also AC/DC, Waveshape, RMS/Peak, Frequency, etc...
|
| W is watt _( "Watt" -geddit?)_ cooks your junk, if the phone
| has a heat issue.
|
| Used to be an EE, back in the Dawn Times...
| marcosdumay wrote:
| Voltage is a completely different issue, that has only
| indirect impact on "how big will this thing be?", at least
| until you start measuring it by the hundreds.
|
| Of course, all of those are relevant, but the one number
| that sizes the thing is current.
| jagger27 wrote:
| > it's consuming 200 W to provide its transistors with about 1 to
| 2 volts, which means the chip is drawing 100 to 200 amperes of
| current from the voltage regulators that supply it. Your typical
| refrigerator draws only 6 A. High-end mobile phones can draw a
| tenth as much power as data-center SoCs, but even so that's still
| about 10-20 A of current. That's up to three refrigerators, in
| your pocket!
|
| This feels out of place coming from IEEE.
| nicoburns wrote:
| Amps aren't really relevant here, I have a wrench that will
| consume considerably more amps than that if you're able to
| supply them.
| marcosdumay wrote:
| I don't see anything wrong with it. That refrigerator will be a
| real constraint on the width of the power wires of any place
| it's installed on. And adding the current of your devices is
| exactly what you need to do to size your power lines.
|
| It being on IEEE, I can't imagine anybody on their target
| audience will be confused and imagine they are talking about
| power.
| amelius wrote:
| I don't know. I bet half of IEEE only worries about data/signal
| processing in their dayjobs and never thinks about power
| distribution. Such a comparison immediately makes clear what
| the problem is.
| [deleted]
| maccolgan wrote:
| >That's up to three refrigerators, in your pocket! This is the
| part where it feels out of place
| Keyframe wrote:
| What is Ohm's law? Come on, IEEE!
| rzwitserloot wrote:
| Especially considering the fact that this is harping on about
| Ampere. Which is _not_ the number to be looking at here; that'd
| be watts. That fridge is chugging down 6A at 110 or 220V
| (assuming it's a new fridge, unless its absolutely gigantic or
| incredulously inefficient, sounds like that'd be a 110V model)
| - not at 1 to 2 volts.
|
| If someone can build a fridge that is so efficient, it can make
| do with 6A @ 2V, dang. Where can I buy me one of those? That's
| 12W total, I can power one of these for a full hour with 4 AA
| batteries.
| wheels wrote:
| I thought the same initially, and do think the analogy is
| bad, but a few seconds later I wondered if the point that
| they were making was that the interconnects carry the same
| amperage: the required gauge for a connector (i.e. wire) is
| determined by amps, not watts. As a result you can send more
| power down smaller cables at higher voltages.
| mindslight wrote:
| That is the point they are making, but comparing it with
| "three refrigerators" is seemingly invoking the power of a
| fridge.
|
| To understand what they're saying, you need to understand
| current as distinct from power regardless of the scale. If
| you do understand this distinction, but you don't have a
| feel for what 100 amps is, perhaps a good comparison is
| starting a car.
| [deleted]
| willis936 wrote:
| Yeah the ohmic losses in the power delivery networks are
| the killer and the topic of this article.
| throwaway9870 wrote:
| As someone who has designed many chips, amperage absolutely
| matters because it is not DC, it has very rapid transients
| based on workloads and that, combined with inductance, can
| make power delivery very difficult. Additionally, the high
| current requires careful design of the package and routing
| because of resistance and electromigration even in the DC
| case.
| Dylan16807 wrote:
| > this is harping on about Ampere. Which is _not_ the number
| to be looking at here; that'd be watts.
|
| You say that like it's obvious. I don't see why.
| [deleted]
| dghlsakjg wrote:
| Amps is a measure of current. Then they switch to talking
| about power which is measured in watts, but use amps as the
| unit. Power measured in Watts is apples to apples. Amps are
| only one half of the equation for watts. So comparing a
| fridge to a phone is like.... comparing a fridge to a
| phone.
|
| Watts are a function of Volts * Amps. So them talking about
| a fridge using 6 amps at 120 volts is really quite a silly
| comparison to using 200 amps at 1 volt. The fridge is using
| 720 watts of power compared to 200 watts from the chip.
|
| In no way is it like having three fridges in your pocket.
| Dylan16807 wrote:
| They don't "switch to talking about power", they discuss
| both the power and current to phone chips as opposed to
| other chips. For fridges, they only mention amps. They're
| not using the best wording but they're not doing what you
| claim there.
| lazide wrote:
| It isn't, the example is confusing. It is about amps, as
| this is due to very low voltage, very high amperage with
| power movement. As the amperage is the interesting part
| here (due to conductor sizing, losses to resistance, etc.)
| dragontamer wrote:
| Agreed.
|
| In my electrical engineering classes, we use amps to
| determine the gauge of wires.
|
| The important calculation here is watts = I^2 * R
|
| Where R is the resistance of your wire, and watts is the
| power wasted in your wires.
|
| And I is amps. That little squared sign is a bit
| intimidating. Under normal circumstances, you want to
| increase voltage to reduce wire loss. But computer chips
| only operate at low voltage.
| unnouinceput wrote:
| And the reason they operate at low voltages has to do
| with the micro-scale these SoC transistors are separated
| from each other. Increase the voltage and you get a
| shortcut, which will render your chip useless.
| brennanpeterson wrote:
| This is true but also wrong. The transistors themselves
| have a specific operating voltage.
| ASalazarMX wrote:
| "[..] Austin Wilde held up the source of power that had
| enabled a Disinto to chew up a mountain in half a second -
| two flashlight batteries!"
|
| It's amazing how well Asimov's robot stories have aged in
| these A.I. times.
| bserge wrote:
| A modern fridge compressor uses less than 300W (for a big
| one). They're actually surprisingly efficient.
| deepnotderp wrote:
| Actually for power delivery networks current (amperes) is
| mostly what you care about
| whatshisface wrote:
| Voltage drop = current x resistance. Power lost to heat =
| current2 x resistance. I think they are making a reasonable
| point that resistance losses are likely to be a much bigger
| problem for a CPU than for a large appliance with similar
| wattage. 10-20A is an enormous current even on household wires
| (most household circuits are rated for 15-20A), and while wires
| on CPUs are shorter, they're also a lot thinner.
|
| The wires in the refrigerator would likely be unable to handle
| 20A at 2V.
| bserge wrote:
| No, they handle it fine.
| konschubert wrote:
| why?
|
| Amperage determines the wire diameter. High amperage means very
| wide wires.
|
| I think their point is that this is what ultimately drives the
| need to power from below.
| ReactiveJelly wrote:
| It's frustrating to see them not spare a couple sentences to
| clear up a misconception that _many_ laypeople suffer from.
|
| Sure, _we_ know the difference between amps, watts, and watt-
| hours, because we paid attention in science class, but most
| people still get them mixed up.
| wheels wrote:
| To be fair, this is not a publication for lay-people; it's
| obviously and explicitly a publication for electrical
| engineers, which would not need these things explained. But
| it's still a terrible analogy since the phrasing seems to
| imply that it's talking about power, when it's actually
| talking about current.
| asddubs wrote:
| because it would lead someone not already familiar with what
| those figures mean and how they relate to one another to come
| to the wrong conclusion. and someone who does know doesn't
| need the analogy. When I think "fridge", I don't think "what
| wire diameter do I need to deliver power", I think about a
| big old hunk of metal using a bunch of power
| JumpCrisscross wrote:
| > _would lead someone not already familiar with what those
| figures mean and how they relate to one another to come to
| the wrong conclusion_
|
| It's the IEEE. It's not designed for average consumption.
| That's almost OP's point, which make this counterpoint a
| bit comical.
| asddubs wrote:
| that's fair, although I would argue it's still a bad
| metaphor, I had to do a double take to get what they were
| getting at so it served to distract me rather than make
| the point.
| MayeulC wrote:
| Instantaneous power draw can be quite considerable too, when
| you have millions of transistors switching in a short lapse of
| time. Typically you cannot really include capacitors on the
| die, so those are close to it. It might have to do with it, but
| I haven't read TFA yet.
| bsder wrote:
| Actually, inductive ringing on the power grid is generally a
| bigger problem than lack of capacitance.
|
| Generally, not all the transistors in your chip switch. The
| transistors that don't switch provide a charge reservoir to
| draw from for the transistors that do.
|
| The problem is then backfilling all that current that got
| lost and you have to do that _within one clock cycle_ --which
| is the "lots of current" that this article is talking about.
|
| Because you have these pulses of current snapping from on to
| off at fairly high frequencies being fed over long distances
| with very little resistance to damp them, inductance kicks in
| and starts causing oscillations (LC tank).
|
| However, at this point Moore's Law about performance is dead
| (2x every 18 months), so this is not a very big deal.
|
| Moore's Law about cost is still alive (double the number of
| transistors/halve the cost every 18 months). So, the big deal
| currently is in the embedded space where leakage is more
| problematic because the die is mostly determined by RAM and
| flash sizes which goes directly to current leakage and die
| size.
| Zenst wrote:
| Question I have is how much heat is generated by that power layer
| and as it is copper, shifting that behind the silicon - would we
| not see more thermal mass shifted to the backend of the CPU and
| with a focus upon the top of the cpu for cooling solutions - how
| would that pan out? Would we also need some heat-sink upon the
| base of the CPU. Would we see extra heat shifted thru the silicon
| layer with this process?
|
| One aspect that I've pondered that would save power would be
| having the memory closer to the CPU and all that usable real-
| estate for slots upon the reverse of the motherboard. Sure you
| would be looking at new case designs in a way or existing ones
| with new design considerations upon the mounting plate to have
| gaps to accommodate sockets upon the reverse of the motherboard
| PCB. That without having to compete with the CPU airspace for
| cooling and in effect using the motherboard to zone things, could
| work out well.
| baybal2 wrote:
| Most high end chips with high TDPs are already packaged upside
| down.
|
| You CPU, or 9 out of 10 recent phone/tablet SoCs are all upside
| down chips.
|
| https://en.m.wikipedia.org/wiki/Flip_chip
|
| So, it's actually going to be an improvement from the thermals
| side, especially with TSVs carrying heat from the other side to
| the heatsink.
| ajaimk wrote:
| Article says this: 10% loss budget in Front side power delivery
| with a 7x improvement in the future => <2% of heat on bottom.
| formerly_proven wrote:
| Not really, no. Pretty much all LSI chips (and even a lot of
| power / analog stuff these days) is flip chip, i.e. mounted
| metal-layer down (if you see a BGA or waferscale package, all
| of these are flip-chip). So the stackup looks like this
| (roughly to scale): Heatspreader TIM
| Silicon Silicon Silicon Silicon
| Silicon Silicon Silicon Silicon
| Silicon Active layer Metal layers Metal
| layers Passivation Solder bumps and glue
| Solder bumps and glue Solder bumps and glue
| Solder bumps and glue Substrate/interposer/PCB
|
| What this article proposes is to put metal layers on BOTH sides
| of the active layer, so you get more metal closer to it. That's
| what they mean by "powered from below". If you look at a chip
| today, they're all "powered from below" in the sense that the
| metal layers are "below" the active layer and power (and all
| signals) are fed in from "below" through the interposer.
| Zenst wrote:
| Thank you and appreciate the layer listing - most elegantly
| done.
| baybal2 wrote:
| > they typically consume only about 200 to 400 watts per square
| centimeter.
|
| That's quite a gross inaccuracy coming from the ARM researchers.
|
| Most chips barely get above 100W/cm2, and it's a very hard limit.
| tus89 wrote:
| I sure am sick of those ICs with pins sticking out of the top.
| Progress at last!
| nimish wrote:
| This has already happened with cerebras and tesla dojo at least.
| blendergeek wrote:
| Source? I'd be interested in reading more about that.
| nimish wrote:
| https://fuse.wikichip.org/news/3010/a-look-at-cerebras-
| wafer...
|
| Power fed vertically. Not sure it's BPR specifically, but
| they do mention ~50% efficiency improvement in the PDN so
| it's comparable.
| rcMgD2BwE72F wrote:
| https://www.youtube.com/watch?v=QurtwJdb5Ew&t=229
| hliyan wrote:
| The metal-filled trenches in the silicon need to survive high
| temperatures, so according to the article copper is out of the
| question. They "experimented with ruthenium and tungsten" but
| it's not clear whether they actually built something or whether
| it was all simulation. Either way, this is likely to make the
| chips more expensive.
| unnouinceput wrote:
| It's a simulation, they don't even have a prototype yet. Quote:
| "Simulation studies are a great start, and they show the CPU-
| design-level potential of back-side PDNs with BPR. But there is
| a long road ahead to bring these technologies to high-volume
| manufacturing. There are still significant materials and
| manufacturing challenges that need to be solved. "
| brennanpeterson wrote:
| I doubt this is true. Mike Mayberry (Intel) softly announced
| buried power rail last year, as a 'within 5 years' tech. That
| suggests it has been prototyped, since from final tests to
| HVM is at least 2 years.
|
| Maybe not great.
|
| Within 10 means an idea with simulation. Within 5 means
| prototyped. Interesting means never. Or that, at least, is my
| decoder ring.
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