⚠ Customisation Cuts the Buyer's Switching Cost the Other WayModerate threat

SK hynix (SKHY) — threat to the moat

The more of the stack the customer specifies, the more of the stack the customer owns -- and a design the buyer authored can be handed to somebody else.

The appeal of a custom base die is that it locks a customer in. The risk is that it teaches the customer exactly what it needs, in a form it can hand to somebody else.

Who owns what, as the base die gets customisedStandard HBM, to HBM3ESK hynix owned the base die outrightHBM4 onwardBase die on TSMC logic, configured per buyerConsequenceThe specification can be handed to a second supplierWhat still protects SK hynixStack yields, which cannot be specified56.4% of HBM rests on a stack whose bottom layer SK hynix no longer designs alone.
A design the buyer authored is a design the buyer can take elsewhere.

When a buyer specifies logic that goes into the memory package, the specification is the buyer's. The manufacturing is hard and the design intent is not, and a second supplier asked to build to the same specification starts from a much better place than one asked to invent it. The custom work that deepens the relationship also documents it.

This is the pattern visible elsewhere in this collection: co-development trains the partner, and the partner eventually has options. It does not happen quickly, and it happens.

What protects SK hynix is that the manufacturing remains the binding constraint — a specification does not yield a stack, and stacking is where the difficulty lives. The layer being specified is already outside the company — SK hynix used its own base die up to HBM3E and moved HBM4's onto TSMC's advanced logic process1.

The falsifier is a competitor shipping a custom base die to a customer SK hynix co-developed one with. That would show the design work transfers even when the manufacturing does not.

References
  1. ReportedSK hynix used a proprietary base die up to HBM3E and adopted TSMC's advanced logic process for the HBM4 base die.
    SK hynix newsroom — 'SK hynix Partners With TSMC to Strengthen HBM Technological Leadership'. The two signed a memorandum of understanding covering HBM4 and next-generation packaging: SK hynix used a proprietary base die up to HBM3E and adopts TSMC's advanced logic process for the HBM4 base die so more functionality fits in the same space, and the companies will optimise integration of SK hynix HBM with TSMC's CoWoS packaging. — HBM4 development · publ. 2026 · source ↗
Sources
Generated September 23, 2026