2.5D vs 3D Packaging: Chip Integration Compared

Both approaches put several chips into one package to get more bandwidth than a circuit board can offer. In 2.5D packaging, chips sit side by side on a silicon interposer that wires them together at fine pitch. In 3D packaging, chips are stacked on top of each other and connected through the silicon with through-silicon vias (TSVs) or by hybrid bonding. 3D gives the shortest connections but is harder to cool and to build.

Aspect2.5D3D
StructureChips placed side by side on a silicon interposer (or a silicon bridge)Chips stacked vertically on top of one another
InterconnectMicro-bumps to the interposer, with very fine wiring layers inside itTSVs through the chips, joined by micro-bumps or by hybrid bonding
Bandwidth densityHighHighest, because connections are shortest
HeatEasier: each chip can be cooled from the topHarder: heat from lower chips must pass through the chips above
FootprintLarger: the interposer must hold all chipsSmaller
Manufacturing difficultyNeeds a large interposer, which adds costNeeds wafer thinning to tens of micrometers, TSV formation and careful testing of each die
Typical examplesAI GPUs with HBM on the same package (such as TSMC CoWoS)HBM itself (stacked DRAM dies), and stacked cache on a processor
Typical usesConnecting processors, memory and chipletsStacking memory, or stacking logic and cache for the shortest paths

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

2.5D
When you want to combine a large processor with several memory stacks or chiplets and still cool them easily.
3D
When connection length, power and footprint matter most, as in stacked memory.
Together
High-end AI accelerators use both: HBM is a 3D stack of DRAM dies, placed beside the processor on a 2.5D interposer.

FAQ

What is a silicon interposer?

A silicon layer with very fine wiring, and often TSVs, that connects several chips placed side by side in 2.5D packaging.

Why is 3D packaging harder to cool?

Heat from the lower chips must pass through the chips stacked above them, while in 2.5D each chip can be cooled from the top.

Is HBM 2.5D or 3D?

Both. HBM is a 3D stack of DRAM dies, and it is placed beside the processor on a 2.5D interposer.

What is a TSV?

A through-silicon via is a vertical connection that passes through a thinned silicon chip, used to link stacked chips.

Check your understanding

Which correctly describes the difference between 2.5D and 3D packaging?

  1. A2.5D places chips side by side on an intermediate substrate (such as an interposer), while 3D stacks chips vertically
  2. B2.5D stacks chips vertically, while 3D places chips side by side
  3. CThere is no difference between 2.5D and 3D
Show answer and explanation

Answer: A. 2.5D places chips side by side on an intermediate substrate (such as an interposer), while 3D stacks chips vertically

2.5D packaging places multiple dies side by side on an intermediary substrate such as a silicon interposer and connects them through its fine wiring. 3D packaging stacks dies vertically, connecting them with TSVs or hybrid bonding. 2.5D dissipates heat more easily, while 3D gives the shortest wiring. Combined structures are also used, such as 3D-stacked HBM placed beside a GPU in a 2.5D arrangement.

Which correctly describes the role of the silicon interposer used in 2.5D packaging?

  1. AA heat sink for cooling chips
  2. BAn intermediate substrate on which several chips are placed side by side and connected at high density with fine wiring
  3. CA photomask used for exposure
Show answer and explanation

Answer: B. An intermediate substrate on which several chips are placed side by side and connected at high density with fine wiring

A silicon interposer is a silicon intermediary substrate on which wiring of 1 µm or finer is formed using semiconductor back-end processes. Multiple chips, such as a processor and HBM, sit side by side on it and are connected with wiring far denser than ordinary organic package substrates, whose lines are tens of micrometers wide, can provide. The interposer connects to the package substrate through TSVs. It is unrelated to cooling parts or photomasks.

Which correctly describes a TSV (through-silicon via)?

  1. ALateral wiring formed only on the chip surface
  2. BAn electrode that passes through the silicon substrate in the thickness direction, connecting stacked chips vertically
  3. CA solder ball used as an external package terminal
Show answer and explanation

Answer: B. An electrode that passes through the silicon substrate in the thickness direction, connecting stacked chips vertically

A TSV (through-silicon via) is a conductor filling a hole a few to over ten micrometers in diameter drilled through the silicon substrate, electrically linking its front and back. It enables thousands of connections between stacked dies over far shorter distances than wire bonds, so it is used in 3D stacking, HBM and silicon interposers. Lateral wiring on the surface is ordinary multilevel interconnect, and solder balls are external terminals.

  1. Which is especially likely to be a challenge in 3D packaging, which stacks chips vertically?
  2. To stack chips with TSVs, approximately how thin are wafers ground from the back?
  3. In AI GPUs, what is the main reason the GPU chip and HBM are placed side by side on one silicon interposer?

Practice Advanced packaging (3D and chiplets) questions

Key terms explained

2.5D packaging
Placing several chips side by side on an intermediate substrate such as a silicon interposer and connecting them with dense, fine wiring. It is used, for example, to connect GPUs and HBM.
3D packaging
Stacking chips vertically and connecting them with TSVs or hybrid bonding. Shorter connections improve speed and power, but heat removal becomes a challenge.
TSV (through-silicon via)
An electrode that passes through the silicon substrate. It connects stacked chips vertically over a short distance.
Silicon interposer
A silicon intermediate substrate that holds chips side by side and connects them densely with fine wiring. It is used to connect GPUs and HBM, among other things.
Chiplet
A small chip made for a specific function. Several chiplets are connected inside one package to build a larger system.

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