Kools Develops Ultra-Large Low-Warpage Bonding Technology Aimed at Rubin Ultra's Four-GPU Single Interposer

Kools (쿨스), a South Korean semiconductor bonding technology company, said on September 10 that it has completed development of an ultra-large low-warpage bonding technology intended to enable a four-GPU single interposer for Nvidia's Rubin Ultra.

The method attaches a high-stiffness lid before bonding to hold the oversized package flat, and uses a device the company calls a thermal clutch to heat and cool only the bonding interface. Kools said the approach cuts heating and cooling time for a 180x70mm-class package to under 10 seconds and holds post-bond warpage below 0.1mm.

The initial concept for Rubin Ultra was to integrate four GPUs and 16 HBM4E stacks in a single oversized CoWoS-L package. A series of reports has since said the four-die package was scaled back to a two-die structure because of manufacturing execution and package yield problems. In the industry, warpage that occurs when a large interposer is bonded to an ABF substrate is cited as one of the main causes. As packages grow, the difference in thermal expansion between the interposer and the substrate accumulates, making it difficult to keep the bonding surface flat with conventional oven reflow alone.

Kools' first answer is the high-stiffness lid. Unlike the conventional sequence, in which the lid is attached after bonding is finished, the lid is joined beforehand so that the whole package is constrained flat while the interposer and substrate are pressed together, with pressure maintained until the solder has fully solidified. Rather than correcting a warped package afterwards, the package is held so that it cannot warp in the first place.

The drawback is that a high-stiffness lid is thick and has a large heat capacity, so with ordinary thermal compression bonding, heating and cooling alone takes more than a minute — controlling warpage at the cost of mass-production throughput. The second core technology, the thermal clutch, addresses that bottleneck. During heating it blocks the thermal path so that heat from the bond head does not escape to the cooling section, raising the temperature of the bonding interface quickly; once bonding is complete it immediately switches the thermal path to the cooling side to draw heat out of the lid and the joint. Pressure is maintained during cooling as well, holding the package flat through the point at which the solder solidifies.

Kools believes there is a strong possibility that the ultra-large CoWoS-L bonding structure recently disclosed by TSMC infringes patent applications the company filed earlier. It argues that the core structure and process order — joining a high-stiffness lid before bonding, constraining a large-area package flat, and maintaining pressure until solder solidification — overlap directly.

"The combination of a high-stiffness lid and thermal compression bonding that TSMC appears to have applied overlaps in its core structure with the direction Kools developed and patented earlier," said Cho Jin-hyun, chief executive of Kools. "Kools added the thermal clutch to that, eliminating oven reflow and solving the warpage and productivity problems of ultra-large packages at the same time."

Comparing a four-GPU single interposer with two separate two-GPU packages, Cho used a poker analogy. "A four-GPU single interposer is four of a kind, while two two-GPU packages are two pair," he said. "Just as the same four cards can be worth different hands, the same number of GPUs can deliver a different level of performance depending on the degree of system integration."

Kools said it plans to propose to TSMC that the two sides verify the technical configuration, discuss licensing and jointly evaluate ultra-large low-warpage bonding equipment. It also plans to commercialise 180x70mm-class ultra-large low-warpage bonding equipment on the basis of a portfolio of 277 intellectual property rights, and to extend its target applications to next-generation AI packages at Samsung Electronics, SK hynix and global outsourced assembly and test providers.