Package substrate

The carrier inside the package that fans the die's tiny connections out to the board.

What you’ll learn

What a substrate is and what it does
How it bridges fine die pitch to coarse board pitch
Why substrates matter for advanced packaging

Why it matters

The substrate is the hidden workhorse of a package — it carries every signal and power connection between the die and the board.

Explanation

A package substrate is a small, multilayer board that the die sits on inside the package. Its main job is “fan-out”: the die's pads are only micrometres apart, but the board's pins/balls are far larger and further apart, so the substrate routes the connections from fine die pitch to coarse board pitch.

It also delivers power to the die and provides mechanical and partial thermal support. Modern substrates have many routing layers, like a miniature printed circuit board.

In advanced packaging the substrate — or a silicon interposer placed on top of it — must carry far more and denser connections, which is why substrate technology is a bottleneck and an area of heavy investment.

Visual explanation

A die on a multilayer substrate that routes its fine pads outward to the larger solder balls underneath the package.

Key terminology

Pitch: The spacing between connections.
Fan-out (routing): Spreading fine die connections to a coarser board pitch.
Interposer: An extra dense-wiring layer placed above the substrate, used in 2.5D.

Example

A high-pin-count processor uses a large multilayer substrate to route thousands of connections to the motherboard.

Common mistakes

Watch out for:

Confusing the package substrate with the circuit board it's soldered to — the substrate is inside the package.

Real-world application

Substrate supply has been a real constraint on shipping high-end CPUs and GPUs.

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