Test analytics
Monitor your test capabilities at the test phase level.
Overview
When running your tests, issues can arise: instruments may fail to connect, limits may reject good parts, or some steps may take too long.
To troubleshoot effectively, use industry-standard metrics like First-Pass Yield (FPY) and Process Capability Index (Cpk) to spot areas for improvement in your testing process.
First-pass yield (FPY)
The test procedure page shows detailed execution statistics. One key metric is First-pass yield, which tracks the percentage of parts passing without rework and indicates process efficiency.
In TofuPilot, FPY is calculated in real time for both the overall test and each step. You can select the data and time period for calculating FPY using the dropdown menu in the top right.
Process capability index (Cpk)
The Process Capability Index measures how well a test stays within its limits. It applies to test steps with numerical measurements and at least one lower or upper limit.
In TofuPilot, Cpk is calculated in real-time for each test step, showing you how capable your test is.
Understanding Cpk
A low Cpk means your test is too variable or not centered, leading to inconsistent results.
- Cpk > 1.33: test process is highly capable.
- Cpk = 1: test process is marginally capable.
- Cpk < 1: test process is not capable; improvements are needed.
A low Cpk often means your process has too much variation: limits are too strict or not centered, or test values are too close to the limits, causing inconsistent or out-of-spec results.
How-to fix low Cpk
- Identify Test Steps: Use TofuPilot's step selection to find test steps with low Cpk that are dragging down the average.
- Check Control Charts: Review historical values to see if it's a sudden drop (indicating a quality issue) or if limits were always too strict or poorly centered.
- Use Filters: Apply filters like revision, batch number, and date in TofuPilot to pinpoint the root cause of low Cpk.
Cpk Calculation
You can compute Cpk by dividing the smaller of the differences between the test mean and the specification limits (low and high) by three times the test standard deviation.
Cpk = min[(USL - μ) / (3σ), (μ - LSL) / (3σ)]
- USL = Upper Specification Limit
- LSL = Lower Specification Limit
- μ = Process Mean
- σ = Standard Deviation