Not sure I follow. The 99.8% is just an example number. If one machine messed up really bad and had just 50% yield, all other machines would need to do far better in terms of overall quality to achieve 36% yield. Of course every single machine isn't at the same quality level.
At the same time, when wafers get scrapped, they do justify why they were scrapped. There are metrology processes and tests performed as you go to ensure you don't run ruined wafers through additional manufacturing steps.
At the same time, when wafers get scrapped, they do justify why they were scrapped. There are metrology processes and tests performed as you go to ensure you don't run ruined wafers through additional manufacturing steps.