Why HBM stacks are diluted bandwidth: 20 layers leaves you at 20% of single-layer speed, slower than DDR5
knowrohit07 · x · 2026-09-25
An engineer argues HBM stacking is the wrong path: a single DRAM layer has 20 TB/cm² of cell-level bandwidth (derivable from refresh calculations). Stack 20 layers (roughly double HBM4) and each layer gets only 4 TB/cm² — 20% of single-layer throughput, hugely diluted. At 20 high, average HBM4E speed is slower than DDR5. He questions why SK Hynix and Samsung prioritize height over speed instead of intelligently placing cheaper memory around the stack.
Related event: Engineer Questions HBM Stacking: Bandwidth Diluted to 20% at 20 Layers(4 posts)→
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