Meaning
Quantitative performance indices evaluate both process centering and process dispersion relative to defined upper and lower tolerance limits. Process capability cpk calculates the minimum distance between the process mean and the nearest specification limit, normalized by three standard deviations of process output. The metric governs quality control in surface-mount component placement and solder paste printing, applying only to stable processes that exhibit normal distribution curves.
Mathematical Structure
Calculations evaluate upper and lower capability values separately, selecting the smaller result to capture worst-case risk relative to specification boundaries. A process with tight dispersion but an off-center mean yields a low index score, identifying systemic offset errors. Target values of one point three three indicate acceptable safety margins, while values above one point six seven signify high-reliability performance required for automotive cellular modules.
Adjusting machine offsets moves the mean back toward nominal centers, directly restoring index scores without altering overall process spread.
Production Impact
Low capability values alert line supervisors to impending out-of-spec production before defective modules are assembled. Corrective actions restore process centering before product quality degrades.
Handover Requirement
Production line acceptance handovers require demonstrated capability indices across three consecutive trial runs. Customer quality audits mandate documented index records prior to granting mass production sign-off.
