RACKLINEEvening

 

RACKLINEEvening

 

 

Chips & SupplyUnited States

Same fab, different paths: Kioxia and Sandisk diverge on HBF

Credited to Blocks & Files · blocksandfiles.com

Useful
Photograph · Blocks & Files

NAND fab joint-venture (JV) partners Kioxia and Sandisk have differing views on the SK Hynix-led High-Bandwidth Flash (HBF) initiative, with Sandisk supporting it and Kioxia so far neutral and staking out an alternative architectural position.

Similar in concept to High-Bandwidth Memory (HBM), the HBF concept involves multiple layers of NAND with connecting wires (Through-Silicon Vias or TSVs) linking each layer to a base interposer logic and then unit, which connects them to a host GPU in parallel, not serially.

The HBF multi-die stacking step is done at the die level after the dies have been cut out from a wafer which is built using 3D-NAND, which is itself layered. Known-good dies are vertically stacked (typically 8-high or 16-high configurations in the first HBF spec) and interconnected with TSVs and microbumps. That means they have to have been fabricated with a design that facilitates TSV and microbump addition.

As we understand it, standard NAND dies (and the wafers that produce them) generally cannot be used as-is for HBF. The NAND dies in the HBF stack must be specifically designed and fabricated to enable TSVs and microbump connectivity. This is because:

TSV placement and area: TSVs must pass vertically through the dies. This requires reserved keep-out zones in the die layout, additional process steps (via etching, filling, etc.), and extra die area. A standard 3D NAND die designed for conventional SSD packaging has no such structures or space allocated.

Stacking requirements: Lower dies in the stack need TSVs that connect through them to the base logic die and interposer. Standard dies lack these pathways.

Internal architecture differences: HBF dies are typically optimized for much higher internal parallelism (many more independent planes/sub-arrays) to achieve the target bandwidth via concurrent reads.

What we are getting at here is that, because Sandisk owns no independent wafer fabs, currently Sandisk’s only access to a fab is through its JV with Kioxia. It would thus potentially need to access a fab that produces HBF NAND wafers. We don’t see this JV producing HBF NAND wafers just for Sandisk.

Sandisk and SK Hynix launched the HBF Spec Standardization Consortium and a dedicated workstream under the Open Compute Project (OCP) in February 2026. They are the main contributors to the first open technical specification, which covers NAND die stacks (8-high/16-high, up to 512 GB), bandwidth grades (~0.4–3.0 TB/s), UCIe interconnect, packaging, reliability, and software guidance. The spec was released via OCP in early August 2026.

Google and Tenstorrent then joined as consortium members.

HBF evangelist, Professor Kim Jung-ho of the School of Electrical Engineering at the Korea Advanced Institute of Science and Technology (KAIST) said in January “Samsung Electronics and Sandisk plan to integrate HBF into products from Nvidia, AMD, and Google by late next year or early 2028.”

In October Sandisk said: “HBF reached a milestone in FQ1 with tape-out of our first HBF memory die; samples of our first HBF-based products remain on track for next calendar year.”

Meanwhile, Kioxia has not joined the HBF consortium. Its own architectural vision flash and SSD developments include:

GP Series, Optane-class, SSD using gen 3 XL-FLASH with >20 million IOPS 512-byte random read performance.

Liquid-cooled NX1 Series E1.S NVMe SSDs for data centers and liquid-cooled version of its PCIe gen 5 CM9 datacenter SSD.

Plans to use fast NAND as a DRAM alternative, according to Japan’s Nikkei outlet. It intends to use a CXL (Compute Express Link) module wth memory-access speed across a PCIe link. Combining high-speed NAND with a CXL controller module reduces data-read latency to less than one-tenth that of conventional NAND products.

A prototype, large-capacity, high-bandwidth flash memory module with 5 TB capacity and 64 GB/s bandwidth in August 2025 for edge servers, not data center GPU servers. Its configuration has daisy-chained connections with so-called beads of flash memory, not stacked layers, plus high-speed transceiver technology enabling bandwidth of 128 gigabits per second (Gbps), along with techniques to enhance flash memory performance. It utilises differential serial signalling, not parallel signalling like the SK Hynix and Sandisk HBF.

There is then, a difference between Kioxia and Sandisk around HBF technology, with Kioxia considering serial connectivity without layer stacking, and Sandisk/SK Hynix favoring layer stacking with parallel TSV connectivity.

SK Group Chairman Chey Tae-won suggested in September that an SK Hynix partnership with Kioxia could be interesting.

As background, SK Hynix is a Kioxia investor through holding convertible bonds that could be converted into a 14.19 percent share holding with voting rights. This would be the largest single voting share holding. Kioxia also uses SK Hynix DRAM in some of its SSDs.

Kioxia CEO Hiroo Ota opposed the idea, telling The Japan Times: “We can’t just say, ‘Well then, let’s make it three companies,’” … We have no idea what prompted (Chey) to say what he did.” The spectacle of South Korea’s SK Group chairman and Japan’s Kioxia CEO disagreeing like this in public is surprising as such differences of opinion at this level are usually kept private.

If the Kioxia-Sandisk JV has no plans to manufacture SK Hynix-Sandisk style HBF NAND wafers then where would Sandisk get its source HBF wafers?

A TrendForce April 2026 document said: "According to ETNews, sources say the company has begun engaging materials, components, and equipment partners to build out an ecosystem for an HBF prototype production line. It aims to introduce prototypes in the second half of this year, with Japan emerging as a leading candidate for the production site. The industry expects the pilot production line to be completed in the second half and begin operation around year-end, with commercialization targeted for 2027."

This indicates that Sandisk could potentially be thinking about owning and operating its own NAND fab for HBF NAND, separate from its Kioxia JV. That would add tension to its partnership with Kioxia.

We have asked Sandisk about the existence of a separate HBF fab idea.

Original · Blocks & Files

Front