TCB (Thermocompression Bonding)

Notes

The throughput bottleneck in HBM and 2.5D assembly, since it is a serial per-die operation rather than a batch process.

Overview

TCB (thermocompression bonding) is a flip chip assembly method in which a bonder picks up one die, aligns its bumps with the pads beneath, and joins them by pressing down with a heated bond head. Because heat and force are applied locally and under close control, it suits fine-pitch microbumps, thin dies and stacks of memory dies that would warp or misalign in a reflow oven. The trade-off is speed: dies are bonded one at a time.

How it works

The die is held face down on a heated bond head and the substrate, interposer or lower die sits on a bond stage. Optics look up at the die and down at the target at the same time so the two sets of bumps can be aligned to sub-micron accuracy. The head then lowers the die and follows a programmed profile of temperature, force and vertical position, so the solder caps on the copper pillars melt and form joints under controlled conditions.

Keeping the stage well below the solder's melting point limits warpage of the whole assembly, and control of tilt and flatness keeps every bump in contact across a large die. This is why TCB is used for chip-on-wafer assembly onto interposer wafers, fine-pitch chip-to-substrate placement, and stacking thin memory dies for high-bandwidth devices.

Traditionally, flux is applied to clean oxide from the joint and removed afterwards, which becomes very difficult when the gap under a large die is only tens of micrometres high. Fluxless variants use plasma cleaning and formic acid vapour to reduce oxides instead. Equipment makers report solder-based TCB at pitches down to about 10 micrometres; at and below that pitch, hybrid bonding, which joins copper pads directly without solder, is generally seen as the way forward.

Examples

• Stacking thin DRAM dies on top of each other with microbumps to build a high-bandwidth memory stack.
• Placing logic and memory dies onto a silicon interposer wafer in a chip-on-wafer 2.5D flow.
• Fine-pitch chip-to-substrate placement of a large die on an organic substrate where warpage must be tightly controlled.
• A fluxless TCB process using formic acid to reduce oxides, avoiding a post-bond flux clean under a tight gap.

Why it matters

TCB is used for memory die stacking and 2.5D assembly, so its throughput and yield bear directly on the cost of the high-end packages used in AI and high-performance computing. Die attach and assembly process engineers develop the bond profiles, equipment engineers keep the bonders running, and package mechanical engineers model the warpage it is meant to control.

Related terms

• Microbump
• Hybrid bonding
• HBM
• Warpage

Where it appears on the Hub

Used on 9 pages of the Hub. In each group, the pages that mention it most come first.