Competition in sixth-generation high-bandwidth memory (HBM4) has changed. The Joint Electron Device Engineering Council (JEDEC) has eased the HBM package thickness standard, raising it to about 775 micrometers from 720 micrometers. Samsung Electronics and SK hynix are recalculating when to introduce the next-generation bonding technology known as hybrid bonding.

Before the change, meeting the 720 micrometer thickness rule for HBM4 required DRAM to be ground down to an extreme thinness during stacking. That is effectively difficult to achieve with current process technology. For this reason, the industry had seen a shift to hybrid bonding from HBM4 as inevitable. But that premise has been shaken as JEDEC participants Samsung Electronics, SK hynix, Micron, Nvidia and AMD agreed to raise the thickness limit.

The additional 55 micrometers of room has brought back choices in stacking methods. Space has opened up to stack 16 layers without eliminating bumps, reopening a path to upgrade existing bump-based processes. Because what has been eased is a physical constraint rather than a performance specification, the bandwidth and capacity required by HBM4 remain unchanged. What has changed is only how to achieve the same target through packaging.

The HBM4 specification itself is already fixed. Under the JESD270-4 standard disclosed by JEDEC, HBM4 operates at 8 Gb/s per pin on a 2,048-bit interface, delivering up to 2 TB/s of bandwidth per stack. Independent channels increase to 32 from 16 in HBM3, and each channel has 2 pseudo-channels. Stacking supports 4 to 16 layers, and stacking 32 Gb dies into 16 layers can secure 64 GB per stack.

◆The HBM4 landscape changed by 55 micrometers... Samsung and SK’s hybrid bonding clock stops

HBM stacks DRAM vertically, and the tiny protrusions connecting chip to chip, called bumps, along with adhesive materials filling the gaps, take up space.

SK hynix uses advanced MR-MUF, which injects and hardens liquid resin, while Samsung Electronics has applied TC-NCF, which inserts a film-type adhesive between chips and bonds them with heat and pressure. In both methods, the thickness burden grows as layer count rises. That is why there was little choice but to remove bumps altogether to fit 16 layers within 720 micrometers.

Even adopting hybrid bonding, which had been almost the only option, was not easy. Hybrid bonding removes bumps that affect thickness, directly bonding copper and insulating layers on chip surfaces to reduce thickness and improve heat dissipation and power efficiency. But it entails a heavy equipment investment burden to build dedicated lines, and there are many variables that determine yield, including surface planarisation and ultra-clean environment management.

But with 55 micrometers of margin, the possibility has opened to implement 16 layers even with existing bump-based processes. A mood has formed to push the application of hybrid bonding back to the seventh generation (HBM4E) or later.

The two companies’ choices have diverged. SK hynix is putting more weight on raising its proven MR-MUF to 16 layers to secure stable yields. The structure focuses on profit-centred supply by delaying the timing of investment in dedicated hybrid bonding equipment. Samsung Electronics has been considering an early introduction of hybrid bonding as a card to reverse technological leadership. The easing of specifications leaves a variable in that customers have less incentive to demand a new technology with high costs and risks.

The ripple effect is already appearing in the supply chain. Orders for new hybrid bonding equipment are being pushed back, while orders in existing processes and testing are being brought forward. Semiconductor test equipment maker Unitest on the 23rd filed that it signed a contract to supply SK hynix with HBM4 wafer testers for 22.68 billion won. The amount is equivalent to 20.58 percent of its latest consolidated revenue of 110.19503 billion won.

The contract period runs from July 23, 2026 to Oct. 8, less than three months. It is a firm contract, not conditional, and payment terms are 90 percent after equipment delivery and 10 percent after setup completion. Unitest posted revenue of 110.2 billion won last year and an operating loss of 21.1 billion won. It has performance executing similar contracts with SK hynix over the past three years.

The time gained by easing the thickness standard is not expected to be long. In the custom HBM range, where stacking exceeds 16 layers and the number of input-output terminals surges, requirements for signal transmission and heat dissipation again reach a threshold. The industry expects that while HBM4 mass production develops into a competition to upgrade existing bonding, the two companies will simultaneously put resources into securing hybrid bonding yields aimed at the HBM4E timeline. An industry official said, "This is a phase in which we have to handle the development fronts of HBM4 and HBM4E at the same time."