Statistical Process Control Boundaries for Re-Qualifying Inline X-Ray Systems
Inline X-ray system re-qualification relies on statistical limits derived from detector drift and grey-scale stability rather than arbitrary time intervals.
Cpk Index
Process Index
Statistical metrics for manufacturing performance evaluate the ability of a production process to produce parts within specified tolerance limits. When applied to solder paste printing, the cpk index assesses how reliably the squeegee deposition maintains the required height and volume. This measure divides the distance from the process mean to the nearest specification limit by three times the standard deviation.
A low value reveals that the process lies too close to a limit, which risks bridging or insufficient solder.
Statistical Evaluation
Calculations for this metric require both the mean and the standard deviation of a series of measurements taken from a stable line. Solder paste inspection systems record thousands of points across printed boards to supply the necessary dataset. Process drift often occurs as squeegees wear or paste viscosity changes under ambient temperature shifts.
Operators monitor these shifts to adjust the printer before defects arise.
Limit Constraint
Standard assumptions for this statistical tool rely on the measured data following a normal distribution. If the printing process produces skewed height profiles due to uneven stencil release, the calculated value becomes misleading. Non-normal distributions require specialized transforms to ensure that the risk of board assembly failure is not underestimated.