5 why analysis

**5 Why Analysis** is a **root cause investigation technique that drills down to the fundamental cause of a problem by repeatedly asking "why" each cause occurred** — a simple but powerful method that peels away layers of symptoms to reveal the true systemic root cause, typically reached within five iterations of questioning. **What Is 5 Why Analysis?** - **Definition**: An iterative interrogative technique where each answer to "why did this happen?" becomes the subject of the next "why?" question — continuing until the root cause is identified (typically 3-7 iterations, not strictly limited to 5). - **Origin**: Developed by Sakichi Toyoda and used within Toyota Motor Corporation as a core component of the Toyota Production System (TPS). - **Application**: Used extensively in semiconductor manufacturing for yield excursion analysis, equipment failure investigation, process deviation root cause, and customer complaint resolution. **Why 5 Why Analysis Matters** - **Simplicity**: Requires no statistical tools, software, or specialized training — any team can apply it immediately. - **Depth**: Forces investigators past superficial symptoms to underlying systemic causes that, when addressed, prevent recurrence. - **Speed**: Provides rapid root cause identification for straightforward problems — can be completed in minutes to hours. - **Cost**: Zero-cost method that can be combined with more sophisticated tools (fishbone, FMEA, DOE) for complex problems. **5 Why Example — Semiconductor Yield Excursion** - **Problem**: Wafer lot failed final test — parametric yield dropped to 60% (target: 90%). - **Why 1**: Why did parametric yield drop? Gate oxide thickness was 10% below specification. - **Why 2**: Why was oxide thickness low? Furnace temperature was 15°C below setpoint. - **Why 3**: Why was temperature low? Thermocouple gave inaccurate readings. - **Why 4**: Why were readings inaccurate? Thermocouple was past its calibration due date. - **Why 5**: Why was calibration overdue? PM tracking system failed to generate the calibration work order. - **Root Cause**: Calibration management software had a scheduling bug for new equipment entries. - **Corrective Action**: Fix software bug + add manual backup check for new equipment calibration scheduling. **Best Practices** - **Focus on Process, Not People**: Ask "why did the process fail?" not "who made the mistake?" — blame prevents honest root cause identification. - **Verify Each Level**: Each "why" answer must be factually verified, not assumed — otherwise the analysis diverges from reality. - **Multiple Branches**: Complex problems may have multiple root causes — branch the 5-Why tree when an answer has multiple contributing factors. - **Stop at Actionable Causes**: Continue asking "why" until you reach a cause that can be directly addressed with a corrective action — don't go too abstract. 5 Why analysis is **the most accessible and widely used root cause tool in semiconductor manufacturing** — its simplicity makes it the go-to first step for investigating any quality problem, equipment failure, or process deviation across every level of the organization.