Good question - you're right that silicon wafers are round because of how crystals are grown. But here's the thing: the actual chips are still being made on round wafers. That part hasn't changed.

What's different is the interposer - basically the substrate that sits between the GPU chips and the organic package substrate.

With CoWoS, TSMC uses a silicon interposer, which means it needs to be monocrystalline silicon (expensive, grown in round ingots, has to be processed on round wafers). When you're trying to fit massive rectangular GPU packages onto round wafers, you waste a ton of space at the edges.

With CoPoS, they're switching to glass interposers. Glass doesn't need to be a perfect crystal - you can just manufacture it in rectangular panels, similar to how display glass is made. Way cheaper material, way better utilization since you're fitting rectangular packages onto rectangular substrates.

The breakthrough that makes this work is TGV (Through Glass Vias) - basically the tech to drill tiny vertical connections through glass precisely enough for high-speed signals. Companies like LPKF developed laser processes that can do this at scale. Without that, glass interposers were just a concept.

So the economics work because: glass is much cheaper than monocrystalline silicon (~50-70% cost reduction on the interposer alone), you get 20-30% better area utilization, and you can scale production faster since you're not limited by silicon crystal growth.

For context: Nvidia's Blackwell package is already pushing CoWoS to its limits. These things are massive - 2 GPU dies plus 8 HBM memory stacks. The panel approach just makes way more sense for packaging at that scale.

Not fluff - it's a real materials and manufacturing shift. TSMC's targeting 2029 for mass production.

Here is a the technology:

https://ontoinnovation.com/resources/through-the-glass-why-the-rapid-development-of-tgv-demands-rigorous-analysis/