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What is the silicon spacer used for? And where is it located - there's not a picture of the three pieces of silicon in the package.

Also fun to see this next to the 8086 die shots. How things have progressed in 40 years.

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You can see an illustration of the RP3A0 system-in-package in the official blog post[1].

From what I can gather[2] the spacer is actually an interposer, which for all intents and purposes functions as a PCB between the two chips, allowing for signals to be routed between the chips. Think of it as two IC's on either side of a PCB with vias connecting them. No expert though so maybe I'm mistaken.

[1]: https://www.raspberrypi.com/news/new-raspberry-pi-zero-2-w-2...

[2]: https://sst.semiconductor-digest.com/2011/06/silicon-interpo... (section "Alternative 3D technology")

I was assuming they called it an interposer for a reason. However as the sibling comment by mkoc mentions it might just be a spacer, and the X-ray image[1] of the package seems to confirm that.

[1]: https://www.jeffgeerling.com/blog/2021/look-inside-raspberry...

have a look here [0], Fig 1; the spacer serves really as a mechanical spacer (if dies are same or similar size); also nice for heat spreading.

[0] https://www.semanticscholar.org/paper/Productivity-improveme...

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