3D Vertical Stacked DDR RAM, Why Not Coming to PC?

A few days ago, Samsung announced a DDR5 module with a capacity of 512 GB, the secret to getting it? The memory use of the 3DS in said DDR5, and is that the 3D Stack variants of the DDR memory standards are not something exclusive and unique, since they are part of the JEDEC standard.

Of course, we do not see it being used for home PCs and its use is limited to servers where large volumes of data are handled, resulting in a considerable increase in performance by having as many of them as possible in RAM at all times, and in that is key 3DS-DDR memory, but why don’t we see it implemented in PC?

How to solve the capacity problem: 3DS-DDR memories

The idea of stack several different chips vertically It responds not only to the need, but also to the argument of having to reduce the energy consumption of data transmission, something that is easy to visualize if we connect two hardware elements in series placed horizontally. The problem? Energy consumption is exponential with clock speed and voltage, where the latter grows linearly with the former. So the solution is to increase the number of interconnections, but in a normal bus it cannot be done by increasing the size of the chip. The solution? Connect the chips vertically, one on top of the other.

This approach was key to the development of HBM memory, but at the same time the JEDEC began to propose a similar low-cost solution for DDR memory within the standard, which led to the development of 3DS-DDR memory. The difference with HBM memory is that communication does not require an interposer, but rather the traditional 64-bit bus of DDR memory to communicate with the controller or interface on the memory side. This makes it possible to stack up to 4 memory chips.

Why hasn’t it been implemented on home PCs?

3DS DDR conceptual render

At a glance the implementation of 3DS-DDR memory should not be a problem, especially in laptops where space is important, since it would be as simple as soldering 2 or 4 batteries depending on the number of memory channels we want to have in the system. Of course, the ability to expand the RAM would be lost, but this is also limited in those that make use of LPDDR4 or LPDDR4X memory.

So how come 3DS-DDR4 memory hasn’t become standardized in notebooks? It is not due to the fact that its implementation could limit the expansion capabilities seeing the trend that certain manufacturers are going, but because completely break the consumer advantage the use of vertical interconnections by not using an interposer for interconnection. So that 3DS-DDR4 memory power consumption is the same as DDR4, since the processor-memory interconnection is the same. To all this we have to add the high costs of manufacturing stacked memories that make use of pathways through silicon.

It is precisely the high cost of manufacturing that makes DIMMs with 3DS-DDR memory exclusively in the server market where they are used in RDIMM and LRDIMM modules instead of the conventional modules that we use in our PCs. In any case, the space advantage of stacking memory chips is attractive in notebooks because every cubic millimeter inside counts.

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