Hacker News Reader

Comment by bob1029 | Hacker News Reader

It really depends on how you interpret Moore's law. I personally think that counting all your transistors across multiple discrete dies is kinda cheating.

But, I absolutely love the concept of chiplets and can see a trivial 10x in logical transistor density, even if we don't shrink the process nodes any further or even make major changes to the current generation of chiplet-based CPUs.

We are currently only arranging these things in 2D. If AMD/TSMC develop a way to stack chiplets in 3 dimensions, we could wind up with another huge bump in logical transistor density per socket/node.

Chiplets are ultimately about solving the manufacturing and business problem of defect density. There isn't much additional magic going on here in terms of beating Moore's law.

View on HN

> We are currently only arranging these things in 2D. If AMD/TSMC develop a way to stack chiplets in 3 dimensions, we could wind up with another huge bump in logical transistor density per socket/node.

We already hit up against Dennard scaling more than strict Moore's law issues (although there's hard problems on that side too). Cooling those transistors is already such an issue that we have concepts like dark silicon, even when you assume that the transistors themselves are in a 2D plane immediately backed by cooling.

> If AMD/TSMC develop a way to stack chiplets in 3 dimensions, we could wind up with another huge bump in logical transistor density per socket/node.

Stacking is pretty close to being a solved problem. Cooling a stack of high-power chips is the challenge.

Right. Memory can be stacked. Flash memory, especially. Only a small fraction of memory cells do anything on each cycle. But parts where most of the gates do something on every clock, like CPUs, heat up too much.

> If AMD/TSMC develop a way to stack chiplets in 3 dimensions, we could wind up with another huge bump in logical transistor density per socket/node.

Isn't that already done for HBM (high bandwidth memory)

Yeah, but it'd be way more complex if you changed those chips to being regular logic, or your memory access pattern to HBM changed.

GPU memory is overwhelmingly accessed as long streaming transfers, which means that the hotspots (like, literally tiny points of high temperatures) migrate around the chip which is ideal for cooling. That's why you haven't even seen HBM for general purpose CPUs yet (I guarantee you that a threadripper could consume that bandwidth under the right circumstances).

There is nothing preventing usage of the same microvias and wafer thinning for non-memory ICs.

Sure, but the hotspots are going to be intrinsically more localized in logic chips, or even memory chips with non streaming access patterns.

And? Semiconductor thermal engineers are paid salaries to solve exactly that.

I mean, we've thrown tens of billions at that problem and don't have a good solution beyond "chips with consistent hot spots array their transistors on a 2D plane, and even then you have to play games with turning parts of the chip on and off to keep it from melting itself".

If you've got a solution that's manufacturable, I'm all ears, and so is every fab out there.

I am a layman here, but couldn't they just use more aggressive cooling on the outside? If the exterior were cooled to -20°C, wouldn't that keep the interior from getting too hot as well?

That's even worse, frequent thermal shocks are not good for IC longevity.

ICs frequently passing zero degrees, which aren't packaged in completely water impermeable packaging like ceramics or metal are also having issues

Sure, they'll last shorter, but at least they will work.

Besides, couldn't they keep it at a stable -20 °C?

> ICs frequently passing zero degrees, which aren't packaged in completely water impermeable packaging like ceramics or metal are also having issues

Issues with condensation?

Well, that makes sense. So why can't they be packaged in metal or ceramics?

> Well, that makes sense. So why can't they be packaged in metal or ceramics?

Money

> Sure, they'll last shorter, but at least they will work. Besides, couldn't they keep it at a stable -20 °C?

I can't say that. I'm not a thermal engineer

A CPU costs at least $50 already. Is it that much more expensive to package it in metal?

Also, aren't they already packaged in metal? On graphics cards, the dies are directly exposed.

Liquid nitrogen cooling can already get good results in overclocking, so it seems like it should work here too.

I think an appropriate way to measure Moore's law is computational power per dollar. Whether that's mips or floating point math, there's some discussion there.

Reply on news.ycombinator.com