Uncertainty Margin
Test threshold adjustment constitutes the mathematical offset applied to device specification limits to account for measurement uncertainty in automated test equipment. Applying a guard band calculation establishes a boundary that prevents marginally failing parts from being shipped as passing due to tester error. This offset reduces the likelihood of sending defective hardware to assembly houses.
Sizing Rule
Standard practice determines the width of this buffer by multiplying the total gauge repeatability and reproducibility value by a coverage factor based on the target quality level. When the calculation is completed, the resulting margin is subtracted from the upper limit and added to the lower limit of the device parameters. This contraction of the pass region forces some acceptable parts to be rejected but guarantees that the remaining units comply with the data sheet.
This adjustment relies on the standard deviation of multiple measurement trials on reference standards.
Process Yield
Narrowing the pass region increases the apparent reject rate at wafer sort and package test steps. High precision production environments require a balance where the loss of good die is weighed against the escape rate of defective ones. When the test equipment is calibrated to a higher precision, the required buffer shrinks, which recovers usable yield without compromising product quality.
