Yield and quality control

In control charts (Shewhart control charts) used for process control, how are the control limits generally set?

  1. ACenter line ± 1σ
  2. BCenter line ± 2σ
  3. CCenter line ± 3σ
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Answer: C. Center line ± 3σ

In Shewhart control charts, upper and lower control limits are generally drawn at ± 3σ (σ: standard deviation) from the center line. With a normal distribution, the probability that a point falls outside this range from chance variation alone is small, about 0.3%, so a point outside is taken as a sign of some abnormality in the process.

Control chart (Shewhart chart) Time (sample order) Outside the limits → investigate the cause UCL (mean + 3σ) Center line (mean) LCL (mean − 3σ)
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More Yield and quality control questions

  1. Which correctly describes the purpose of a Pareto chart used in quality control?
  2. Which formula gives the process capability index Cp (when the process mean coincides with the center of the specification)? (σ: process standard deviation)
  3. Among the causes of yield loss, what are defects called that recur in specific patterns due to, for example, interactions between design and manufacturing processes?
  4. Which diagram organizes the relationship between a problem (effect) and the factors that influence it in a shape like a fish skeleton?
  5. What is the main purpose of analyzing wafer maps, which show wafer test results as pass/fail for each chip position on the wafer?