How Big Can an SSD Get?

SSD circuit board - courtesey of Intel Corp.Someone recently asked The SSD Guy to guess what would be the largest amount of flash that could be fit into an SSD’s case.  This sounded like a fun problem, so I did a “Back-of-the-Envelope” estimate to try and figure it out.

First of all, I would judge by this post’s picture that you could get no more than 20 chip packages (4 x 5) on one side of a PC board for a 2.5″ SSD.  That’s probably an optimistic estimate.

If you ignore the controller that would allow you to squeeze 40 packages onto a single circuit board.

Certain high-capacity SSDs use a “Butterfly” design to fit three circuit boards into a single 2.5″ HDD housing.  With three 40-package circuit boards you could fit 120 chip packages into the 2.5″ HDD housing.

Today’s densest flash chip stores 128 gigabits or 16 gigabytes.  Samsung and SanDisk can stack 16 of these chips within a single package, making a 16 x 16 gigabyte or 256 gigabyte package.  SanDisk just announced a 512 gigabyte SD Card that doubles that figure, probably by using two 16-chip stacks.  If we were to put 120 of these 256 gigabyte packages into an SSD case it would give us a 30 terabyte SSD.

Compare this to the 500TB raw capacity of Skyera’s highest-capacity 1U box, which was designed with an obsessive focus on getting the largest amount of flash into the box.

The 30TB SSD I guessed about above would be a “Hero” SSD.  More reasonable SSDs use the same 16 gigabyte chips but only stack 4 or fewer chips in a single package.  That would cut the 30 terabyte number to 7.5 terabytes, provided that the SSD used 3 internal PC boards.  Since you have to put the controller somewhere then a reasonable SSD would have a smaller capacity than 7.5TB.

The 3-board design is uncommon and expensive.  Most SSDs (like the one in the picture) don’t use more than a single PC board.  2TB would be a reasonable capacity for a single-board SSD.  At today’s MLC prices the flash alone for a 2TB SSD would cost $800, limiting the market for such a product.

No matter whether you opt for a 2TB SSD or a 30TB SSD it’s going to cost an awful lot of money!  Of course, eventually NAND flash (or its replacement technology) will change that situation, and through the same mechanism (Moore’s Law) the largest SSD that can fit into a 2.5″ HDD case will grow as well.  But for the time being those two numbers are about the limit of what can reasonably fit into this industry-standard 2.5″ HDD form factor.

9 thoughts on “How Big Can an SSD Get?”

  1. Hi Jim,

    Your estimation may be somewhat right or wrong. Physically and logically, your calculation makes sense to get 7TB ~ 30TB in 2.5″ form factor. Typical 2.5″ FF has 7mm, 9.5mm or 15mm thickness. Therefore, 7mm thick 2.5″ may have only one PCB because of the thickness of NAND packages, passive devices and/or PCB thickness. (FYI, most 16-die-stacked NAND packages are 1.5mm max. Some capacitors are thicker than NAND packages, but SSD controller and DRAM packages are thinner. Considering the case of SSD, 7mm and/or 9.5mm case can have only one PC board, and 15mm case may have dual PC boards for the most practical implementation even though it is rare and expensive. Or as you guessed, 3 PCBs can be squeezed inside 15mm 2.5″ case if very special and thin or flexible PCB materials and ultra thin passive devices are used.)

    However, the truth about SSD’s capacity is not the physical limitation mentioned above. It is because of “NAND flash IO bus” architecture’s speed limitation.

    Currently ONFi 3 or Toggle Mode 2 NAND’s IO speed is maximum 400MB/s according to the vendors’ spec. Those 400MB/s is mostly achievable if less than 4 NAND dies (=not packages to be clear!) are attached to the single and common IO bus in parallel. We call it as ‘channel’, and the channel speed dramatically drops below 400MB/s if we add more and more NAND dies to the channel because of RC loading effects on IO bus. Therefore typically one channel can have maximum 8-NAND-dies to operate at 200MB/s ~ 333MB/s in best scenario. And as you mentioned, some NAND vendors are selling 16-die-stacked packages, but they have 4 channels per package, not single channel, because of the IO speed issue. So each channel has 4 NAND dies inside the package. That being said, typical SSD controllers (or NAND flash controllers) have 8-channels (some has 10-channels for redundant), and your maximum number of NAND packages per SSD is limited to maximum FOUR, because each channel can be reasonably operated with only 4 ~ 8 NAND dies!

    That’s why Samsung’s 840Evo 1TB 2.5″ SSD has 8 NAND packages inside with only using 2/3 of available space of 2.5″ estate. See below link for the proof;

    To overcome that kind of capacity limitation, some flash controller vendors (= like L company or H company) tried to make 16-channel or even 32-channel controllers but all miserably failed because bigger channels mean bigger chip size, bigger package size, more ball counts, more power consumption, more ECC engines inside etc, etc. And it may require huge cooler on top of that to handle heat issue. (We all have seen that on some PCI card based SSDs at FMS). So, 16-channel or 32-channel controllers are definitely not suitable for 2.5″ SSDs.

    Novachips’ HLNAND technology, however, utilizes unique serial ring topology (instead of parallel ‘butterfly shaped” IO bus system used in ONFi/Toggle) to make 4TB in 7mm 2.5″ SSD or 8TB in 15mm 2.5″ SSD. With Novachips’ technology each NAND channel can have unlimited # of NAND dies at the IO speed of 800MB/s ~ 1600MB/s.

    1. HakJune Oh,

      Thanks for a very thorough reply!

      This very simple analysis wasn’t meant to focus on anything other than the mechanical issues of getting the most NAND into a 2.5″ package, so I overlooked the (very important) issues that you mention.

      Thanks for sharing your expertise. This is a very solid analysis.


    1. Jerrod,

      Thanks for the comment. You are correct, but I didn’t make this a thorough exercise so I ignore a number of things that would reduce capacity and only focused on the mechanical aspects.

      Kevin Rowett, formerly an engineering manager at Violin, told the audience at HotChips 2013 that, if he had it to do all over again, Violin’s engineers would have considered thermal issues FIRST.

      It’s an important part of high speed design.



  2. Jim,

    From the dimensions of the 512GB SD card and the 2.5″ HDD, we can estimate that approximately 40 SD cards can fit into the same volume of an 2.5″ HDD, which will result in a 20 TB 2.5″ SSD even in today’s technology. I assume that the total volume overhead of 40 SD cards, i.e. volume occupied by the controllers, PCBs, passives, housing, etc is about the same as that of a single SSD.

    Which means that pretty soon, ~ 1 yr when flash density doubles again, it may be possible to fit ~40 TB of flash in a 2.5″ SSD.


    1. Yanjun,

      That’s a pretty interesting way of looking at it. I limited myself to seeing how the more traditional approach would work, but your thinking is perfectly legitimate.

      I wonder just how much flash you could squeeze into a 2.5″ form factor if you completely disregarded cost and allowed even more exotic packaging approaches to be used?


  3. Jim,

    Indeed I am amazed that Sandisk can pack 512GB in a SD card. Thanks for posting that news.

    To your question – A low hanging fruit might be to design the flash die in the right shape so they can be packed more squarely in a 2.5″ SSD. In reality, as you and others have commented, the heat dissipation issue will likely limit the packing density and exotic packing options. Perhaps one can coat layers of graphene between dice so heat can be removed from the middle of the die pack more efficiently.


  4. 18 months later and 13TB drives are available ($13k apiece!)

    What will the market look like in another 18months?

    1. Stoat,

      It all depends on whether NAND can continue to follow Moore’s Law density growth, which 3D NAND is designed to support. Assuming that it does, then you can count on SSD capacities doubling every year or two.

      Other technologies can accelerate the SSD capacity increase: Samsung’s 48-layer V-NAND chips are shipping in 16-die packages. What if someone started shipping 32-die stacks?!?!?

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