Functional Test Fixtures Owned by the Factory or the Buyer
Buyer-owned test fixtures require explicit PO line items, complete source files, and calibrated limit audits to prevent factory lock-in.
Test Limit Drift
Measurement Instability
Statistical control charts track the variance of measurement values against a stable reference point to identify systemic changes. Test limit drift occurs when the results from a piece of testing equipment slowly shift away from the original calibration over time. This can cause the tester to pass boards that are actually defective or to fail boards that are perfectly good.
It is measured through periodic calibration checks and stops when the equipment is adjusted back to the standard.
Calibration Decay
The causes of this shift are often physical, such as the wearing down of pogo pins or the aging of electronic sensors. As test limit drift progresses, the distribution of the test results will show a steady movement towards the upper or lower pass-fail boundaries. If this movement is not caught, it will eventually result in a loss of yield or a defect escape.
Technicians monitor the mean and standard deviation of the test data to catch these trends before they affect the product quality. Replacing worn parts and recalibrating the software are the standard ways to resolve the issue.
Yield Threshold
Early detection of these changes prevents the production line from generating inconsistent results. Monitoring for test limit drift ensures that the quality of the inspection remains constant throughout the manufacturing run. This practice maintains the integrity of the testing process.