HBM (High Bandwidth Memory) is vertically stacked DRAM that sits right next to the logic die in the package. Why it exists: AI workloads are memory-bandwidth limited. Regular DDR memory on the motherboard is too far away and too slow. HBM solves this by stacking multiple memory dies vertically and connecting them directly to the processor through the package. The packaging challenge: integrating HBM requires advanced techniques like [[CoWoS and 2.5D Packaging]] or hybrid bonding. The dies must be precisely aligned, thermally managed, and densely interconnected. This is hard to manufacture at scale. Why this matters for investing: HBM integration is a primary driver of [[Packaging Capacity Bottleneck]]. Every AI accelerator needs HBM, and only a few companies can package it reliably. [[OSATs - Outsourced Semiconductor Assembly and Test]] with HBM capability have pricing power. Future generations (HBM4 and beyond) will push toward even tighter integration, likely requiring hybrid bonding from players like [[BESI - Hybrid Bonding Leader]]. Links: [[Advanced Packaging MOC]], [[Chiplets and Heterogeneous Integration]], [[Why Advanced Packaging Matters Now]] --- #semiconductors #deeptech