1. Purpose and scope
In a single-chip product a die that slips past wafer sort is caught at final test and only that die is lost. In a multi-die product one faulty die scraps the whole assembled unit, including every good die, the interposer and the substrate. The quality of each die going into assembly therefore decides the assembly yield and the cost of the product.
This stage brings each die to known-good-die quality at wafer sort. That usually means more thorough testing than a die destined for its own package would get: speed testing where practical, tests of the die-to-die lanes through loopback or self-test, stress or screening steps that would otherwise happen after packaging, and careful handling so probing does not damage the fine-pitch bumps.
It also covers the die bank: storing tested dies with their test results and traceability, and supplying the right mix of dies to assembly. Where dies come from outside suppliers, it covers the quality they deliver against the expectations agreed at the supply gate.
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