LPDDR is low power because it is a more sophisticated device with more complex protocol compared to regular DDR, because it has a higher pinout allowing it to have a more efficient electrical connection and because the computer manufacturer can optimize the connection with specific chips in mind instead of having to support a wide range of devices.

I think the number of pins is a practical limitation. Current DDR DIMMs use 260-280 pins if I remember correctly, while an average LPDDR4X BGA package is 400 balls or higher. Apple uses chips with 877 balls. It is definitely possible to design modular RAM with high-pinout, but would it be economically feasible?

You can't compare balls on the BGA package to signal pins. The vast majority of these are for power and ground. LPDDR4 actually has fewer data lines than DDR4.

It is if it's a standard that everyone decides to support...

You can fit FAR more signals on the bottom of a chip and in a multi-layer PCB than you can on any socket you might think of. There is a practical limit to how small the pitch of the pins on a socket can be if it's going to be field-replaceable; at high pin counts, the length of the socket itself becomes a driving factor.

As an example, even moving away from edge-connectors (which are just a non-starter for 800+ pins) Samtec's highest-density high-speed connector, the NovaRay line [0] advertises 224 signals per square inch, so an 800 pin module would require 4 square inches of connector space alone, not to mention making the system PCB assembly thicker and compromising on speed, power and thermals.

[0] https://www.samtec.com/connectors/high-speed-board-to-board/...

So design it like a CPU socket? Laptops usually don't have socketed CPUs because they also need to mount a heatsink, but RAM doesn't have that issue.

So an FM2 socket (to pick one of the more reliable ones), which has ~900 pins has a 4.5in^2 area and adds ~.25" in thickness, while reducing the maximum speed you can get through the chip at the very low voltages necessary to run LPDDR, which is what I meant by compromising speed and power.

Not to mention that socketting parts reduces your production yield over SMT, increases your assembly cost, and cuts reliability (it's fine on a desktop because you almost never move your desktop, and there's usually a big thermal assembly bolted on top).

Pentium m used to be socketed, pretty reliable, and available for thin laptops. I'm not trying to dismiss the difficulty, just saying they could perhaps be overcome

Hardware engineering is ever thus: Difficulties can always be overcome, but every cost adds 5x that cost to the MSRP.

That's the thing, who is everyone? Such technology would only be relevant to premium-level ultraportable notebooks. Desktops, gaming laptops as well as budget laptops would stick with the cheaper and more ubiquitous DDR. Smartphones and tables will continue to solder RAM on due to space constraints. Apple (who is the driving force in this market segment) won't be interested since they are going fully custom and I doubt that an LGA socket will work with their on-package custom-build wide RAM interfaces. So in the end we are left with things like Dell XPS 13, MS Surface and few others. These are all popular and impactful brands, but they are just a drop in the bucket compared to the total PC sales. And I doubt that technology reserved to only selected premium laptops is going to be cheap.

If LPDDR is faster won’t desktops and gaming laptops also want it?

One possible other compromise could be soldered LPDDR RAM, but with a DIMM expansion slot for DDR as well. I wonder whether the DDR could be switched off and the baseline power draw eliminated if the RAM usage was below needing it.

So I will preface this with that I'm not an expert. But as I understand it, multicore systems are already frequently NUMA, but to a degree that most stuff (I am eliding categories of code here, but you get the idea) don't have to really care. From my understanding of how all this works, the difference of speed and bandwidth for LPDDR versus DDR seems like risking significant complexity for an expandability that few people would use and in turn cause an everything-or-nothing problem, where either high-performance code is written assuming anybody might have this, or nobody assumes it and you get a pile of stalls.

> If LPDDR is faster won’t desktops and gaming laptops also want it?

The desktop version of LPDDR4 is there, it's just called DDR5.

We have cpus with lots of pins, nobody is stopping ram from switching to a cpu socket layout..

Laptop CPU is onboard for the same reasons RAM is. This is not about the amount of pins, it's about power requirements for signal clarity.

I've never done anything beyond amateur electronics, so sorry if I'm way off base here, but it at least seems intuitively plausible to me that soldering the components to the board allows them to work reliably at lower power. With a pin and socket connection, you've got to try and be at least reasonably resistant to corrosion or other sources of less-than-perfect connection.

That said, I don't know that RAM chips are the hill I want to die on, anyway. Could we get to a better place if we dispensed with the whole, "just the one big motherboard," design, and looked to the old backplane-style physical layouts? Then you could have a module that contains the CPU and RAM and suchlike all in a single package, and move other things that may not want to be quite as tightly integrated off to another package.

Up until relatively recently, a lot of laptops and even phones would put the connectors on separate PCBs connected by a ribbon, so that damaging those wouldn't mean a $500 motherboard replacement.

I definitely remember ordering a replacement headphone socket board for an old MacBook and just putting it in myself, and I've done similar with, eg, the connector port board on a PS4 controller.

But I guess this is one of the hazards of Apple having their AppleCare programme— they can do the math and calculate that saving a dollar on the cost of making and mounting and connecting that extra component still makes them money even if they have to eat the occasional full motherboard in the first three years of ownership.

Not just for the easier replacement, but also to avoid translating the mechanical stresses directly to the motherboard. The separate PCB can mount in a way that lets it move a little.

If I am not mistaken the ports on recent Apple machines are on a separate, easily replaceable board.

Seems like this stuff maybe comes and goes— like here's a repair guide for a 2010-vintage MBP which definitely shows all the connectors being right on the logic board:

https://www.ifixit.com/Guide/MacBook+Pro+15-Inch+Logic+Board...

Doesn’t appear to be the case on the 16” MBP: https://www.ifixit.com/Teardown/MacBook+Pro+16-Inch+2019+Tea...

Looking at this photo from that page it appears that they are: https://guide-images.cdn.ifixit.com/igi/uavG2A3vFrVmwv6J.hug...

You can see the ports still in place after the motherboard has been removed.

Interesting, didn’t notice that. It looked like the logic board went all the way to the edges.

Part of it comes down to signal drive strength.

If you design to a socketed DIMM package, you actually have two trace domains, the CPU to the socket, and the socket to the RAM chips. For that to be reliable, you have to adhere to standards for DIMM (or SODIMM) so that multiple manufacturers can build compliant interchangeable parts.

If you solder the chips down to the main board directly, you only have once domain for the traces, CPU directly to RAM. At that point you can tweak the design for several goals, speed, power, etc.. And you can tune the drive strength on the signalling the same.

That is how you end up with more efficient soldered designs over socketed designs.

This sounds conspicuously like an Intel NUC, I think?

Yes, as I wrote somewhere below, LPDDR4 signal is actually more robust than DDR4 signal, and LPDDR4 controllers have more features to fix degraded signals.

DDR5 has more roots in the LPDDR4 than in the regular DDR4.

ribit and others are correct. While this may have been possible in the past, at modern memory frequencies and timings, it's not really plausible to deviate from what the platforms are designed for: LPDDR being soldered and placed a specific layout area relative to the CPU, and for socketed DDR, the sockets being placed in one of a few placements relative to the CPU, with a little layout wiggle room.

Edit: I guess to clarify, the plausibility challenge is that whether or not it is theoretically possible, the platform owner (Intel, AMD, etc) or the memory vendors (Samsung, SK Hynix, Micron) are unlikely to support the lark.

There’s no technical reason not to (I’m no EE but I’m sure it could be done somehow), but there’s no demand for removable memory. The entire consumer PC market is already a rounding error for the semiconductor industry, and the enthusiast market that would be interested in this is a very small portion of that market.

Like it or not, repairable/upgradable laptops are a thing of the past. There’s just not component support or sufficient market demand to make the parts removable anymore.

Way overstating your case. PC's might be on the decline but they are still far far from being a "rounding error for the semiconductor industry".

Laptops also make up a major component of that still very alive market so no, I disagree that there is not enough market demand for having laptops that can be repaired or upgraded. It's the industry themselves that have worked to hamper the right of repair while giving nebulous pseudo-technical rationalizations for it.

Modern laptop motherboards can easily be about the size of a cell phone. Most of the case is battery and the main board is a shrinking percentage of the BOM cost. The entire motherboard is the replaceable component. It’s less “right to repair” than the additional components and design required to make them repairable are more expensive than just replacing the whole board.

Seems like a deliberate design choice, not some inevitable consequence of modernity. Why exactly does the board need to be cell phone sized when the actual form factor of even a netbook is substantially larger?

To fit more battery. Other than my Mac mini, every computing device (including watch and phone) I own is mostly battery inside.

Which you could also do by simply making the device a little thicker and have a battery run the entire width of the device. Point is past a certain point, thinness becomes more of a drawback than a benefit, especially given the trade-offs involved.

You might want that trade off, but consumers in general have been leaning towards thinner and lighter for years. Just look around at complaints about the thickness of early android watches being too big and chunky. Or the huge popularity when the first MacBook Air came out.

Given the option between thicker w/ a bit more battery life or thinner and lighter, consumers are choosing thinner and lighter.

Of course engineering is always a trade off, and it is possible to go too far in one direction. But it's not all or nothing, Apple is the one feeding us that false dichotomy.

> ...consumers in general have been leaning towards thinner and lighter for years.

> ...consumers are choosing thinner and lighter.

A) I'm guessing actually lighter more than thinner.

B) Are consumers really clamouring all that intensely for "Thinner! Thinner!", or is that a) because that's all that's offered, and b) the relentless hype from manufacturers -- above all, bendy phone maker Apple -- amplified by rarely-critical media?

B. Definitely B. I’d happily choose a phone that’s chonkier if it was an option.

Battery is a big part of mobile devices of all kind, but it's not “mostly battery” for most of those.

For instance, in a iPhone 11, the battery occupies less than half of the surface of the phone[1]. On a Macbook pro, it's about the same.[2] [1] see this ifixit video if you want to see it by yourself: https://www.youtube.com/watch?v=Feyqwf3cYtw&t=161s

[2] https://guide-images.cdn.ifixit.com/igi/noPvQ2CLs1jptX2p.hug... You can see that the motherboard is much, much bigger than a smartphone.

Looking at the picture from [2]. it looks like the motherboard is really not much much bigger than a smartphone. There's a lot of space dedicated to ventilation (note the two large fans and the black cooling ducts under the hand in the picture. Some fraction of that motherboard is also the solid state storage. The picture here of the internals of an iPhone 11 https://www.zdnet.com/article/heres-whats-inside-your-new-ip... (I don't do videos) sure looks like it's mostly battery. If I discount the empty space, the insides are more battery than not battery.

It is mostly only a deliberate design choice in the sense that consumers have leaned towards designs that are smaller, lighter, and with longer battery lives.

It doesn't matter that the rest of the form factor is larger: there's not a lot of empty space in these, so a larger main board will still mean a larger and heavier device.

I don't know what kind of cell phone you have, but it's not an usual one: here is a picture of an open modern thinkpad https://gearopen.com/wp-content/uploads/2019/12/internals-15...

That’s a laptop from 2 years ago; the new M1 MacBooks are more indicative of where things are headed in the future. Things like memory and any custom ICs are just going to be integrated on the same silicon as the CPU in most cases.

Here you go: https://eshop.macsales.com/blog/wp-content/uploads/2020/11/M...

Still bigger than a smartphone (it occupies the entire width of the laptop (when counting the heatsink, 2/3 of the width otherwise), and around one quarter of the whole area).

Also, your original sentence was

> Modern laptop motherboards can easily be about the size of a cell phone.

Here we see that the one and only best laptop in this regard comes close to the cell-phone-sized motherboard. That's far from what “modern laptop can easily be” means. Most laptops sold today have a much bigger motherboard than this.

(BTW, as much as I'd like to see non x86 laptops going mainstream, software compatibility is huge issue for an ecosystem as diverse as the Windows one (and Roseta 2 being too much of a coinflip isn't encouraging…), we might get there at some point, put it's gonna be slow and painful.)