GDDR5: Getting the controller and PHY is difficult, not many companies have them and the ones that do (NVIDIA etc) don't have a strong interest in licensing the

GDDR5: Getting the controller and PHY is difficult, not many companies have them and the ones that do (NVIDIA etc) don't have a strong interest in licensing them. The PHY is a mixed signal design so requires specialized expertise and likely some debugging on the target process so not only would it take a long time but it'd be expensive and very possibly flaky in rev 1. HBM: The interposer means you get a complex, and risky manufacturing process.