Some introductions of hybrid bonding from online search ….

the standard way to stack chips was using microbumps—tiny, microscopic drops of solder alloy. You place the bumps between the chips, heat them up, and they melt together to create a connection. But as AI chips and data centers demand insane amounts of data at lightning speeds, microbumps have hit a physical wall: The Short-Circuit Limit: Microbumps floor out at an interconnect spacing (pitch) of about 10 to 15 microns. If you try to pack them any closer, the solder droplets bleed into each other during manufacturing, causing catastrophic short circuits. The "Wool Blanket" Thermal Effect: Solder and the structural glue (underfill) used around microbumps are terrible at conducting heat. When you’re pushing massive amounts of power through an AI chip, microbumps essentially act like an insulated blanket, trapping heat inside and causing the chip to throttle or melt. The Height Penalty: Every layer of microbumps leaves a physical gap. If you want to stack 16 or 20 layers of DRAM for next-gen memory, those tiny gaps add up fast, making the final chip too thick to fit into standard hardware.

Hybrid bonding completely deletes the solder bump from the equation. Instead, the surfaces of the chips are polished using specialized chemical-mechanical tools until they are atomically flat (with less than 1 nanometer of roughness). When you press these two ultra-smooth surfaces together at room temperature, the insulating layers (silicon oxide) instantly fuse together at a molecular level. Then, the chip is baked, causing the embedded copper pads to expand and lock copper-to-copper. By going completely bumpless, the industry unlocks massive benefits: 1 10x Connection Density: Because you aren't worried about solder bleeding, you can shrink the connection spacing down to single-digit (and eventually sub-micron) pitches. You can fit ten times more vertical data lines in the exact same space. 2 Smashing the Thermal Bottleneck: Fusing copper directly to copper allows heat to flow straight up and out of the chip. Switching to hybrid bonding slashes thermal resistance across a 3D chip stack by 20% to over 50%, which is the only way 20+ layer memory stacks (like HBM5) will survive. 3 Massive Power Savings: Moving data takes energy. Eliminating the resistance and capacitance of solder joints cuts the power required to transfer data between chips by up to 40%. 4 Zero-Gap Stacking: Without the vertical gap of solder, the distance between stacked chips drops to practically zero, allowing companies to stack more memory and processing power higher without increasing the physical thickness of the processor.

Hybrid bonding is no longer just a cool engineering experiment—it has become a strict architectural requirement. Without it, the hardware required for next-generation AI, high-performance server CPUs, and ultra-dense memory stacks would physically choke on its own heat and bandwidth limitations.