Process Index
Process capability indices quantify the alignment between machine positional accuracy and component land pattern tolerance windows during high-speed surface mount assembly. A statistical measure known as cpk placement capability calculates the ratio between upper and lower specification limits and six standard deviations of placement offset error. Acceptance requires a value of at least 1.33, which corresponds to a process yield above 99.99 percent, while medical or automotive assembly mandates 1.67 or higher.
The index applies strictly to x-axis, y-axis, and rotational theta alignment on pick-and-place gantry systems during active board population. It stops applying once reflow soldering begins, because liquid solder surface tension introduces secondary self-alignment forces that mask raw mechanical placement error.
Statistical Calculation
Positional error distributions follow a normal bell curve around the intended coordinate center during automated pick-and-place operations. Evaluation of cpk placement capability requires taking at least thirty glass plate or optical test board placements to establish a statistically valid sample mean and standard deviation. Optical coordinate measuring machines record the positional deviation of each placement relative to copper pad centers.
When positional drift causes the process mean to shift away from the target coordinate, the resulting index drops sharply. High placement repeatability preserves solder joint integrity on fine-pitch quad flat packages and micro ball grid arrays.
Machine Limit
Nozzle wear and thermal expansion of gantry ball screws introduce mechanical variance into surface-mount equipment over extended production shifts. Regular calibration using fiducial glass targets maintains cpk placement capability within specified contract assembly boundaries. Machine vision systems offset baseline mechanical drift by recalculating component centerlines prior to board release.
If a pick-and-place machine falls below required statistical limits, bridging defects or tombstoning increase exponentially across fine-pitch packages. Mechanical recalibration restores axis true-position tolerance.
