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# KCC's Quizzes AQQ244 about Wafer Processing Yield puzzle

An IC fab is producing semiconductor chips. All the circuits are to be tested...
The process line has a final yield of 95%.

The automatic test equipment (ATE) put in place has the following characteristics:
1. When an IC is OK, the ATE test confirms it at a probability of 96%
2. When an IC is BAD, ATE test rejects it with a probability of 98%

Just before shipment to customer, one circuit is randomly selected.

Questions:
1. What is the probability the ATE tester has been wrong?
2. What is the probability that an IC (tested good) is in fact bad?

Try to be among the first ones giving the correct answers!

Good luck!

P.S. If you have colleagues around you and who can be interested in such quizzes, please pass the information!

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[編集者:StephenV、編集時刻: 19 Sep 2023 日 12:59 PM 時 (GMT -4)]
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• We can assume roughly 5% of defective chips (yield), as most defects are found.

1. Of the 95% roughly good ICs, it directly finds 96%= 0.95*0.96=91.2%. Of the 5% defective is directly finds 98%. This makes up for 5%*98%=4.9%. Thus, is is wrong in 100%-4,9%-91.2%=3.9% of the results.

2. The OK test rate of 96% plays a minor role in IC test, because chips that are not OK, e.g. due to a contact failure, typically become re-tested.

With the ATE only recognizing 98% of defect ICs, this means, that 2% of the roughly 5% defective ICs are not discovered and therefore 0,02*0,05=0.1% of the ICs tested "good" still have an undiscovered fault.

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• We can assume roughly 5% of defective chips (yield), as most defects are found.

1. Of the 95% roughly good ICs, it directly finds 96%= 0.95*0.96=91.2%. Of the 5% defective is directly finds 98%. This makes up for 5%*98%=4.9%. Thus, is is wrong in 100%-4,9%-91.2%=3.9% of the results.

2. The OK test rate of 96% plays a minor role in IC test, because chips that are not OK, e.g. due to a contact failure, typically become re-tested.

With the ATE only recognizing 98% of defect ICs, this means, that 2% of the roughly 5% defective ICs are not discovered and therefore 0,02*0,05=0.1% of the ICs tested "good" still have an undiscovered fault.

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