Establishing Statistical Process Control Limits for Stencil Nano Coating Degradation Rates
Track solder paste transfer efficiency standard deviation to set statistical process limits that trigger stencil re-coating before yield collapses.
Paste Volume Cpk
Capability Index
Statistical process capability indices measure the consistency of solder paste deposition relative to specified engineering limits. The paste volume cpk quantifies how well the printing process meets the volume specifications for high-reliability assembly. A value above one point three three indicates a stable, centered process.
Calculation Method
Derived from the mean and the standard deviation of measured paste volumes, the formula evaluates the distance from the process average to the nearest specification limit. The calculation of the paste volume cpk assumes a normal distribution of the volume data collected by solder paste inspection systems. When the process mean shifts away from the nominal center, the resulting value decreases even if the overall variation remains small.
This index is therefore more informative than simple variation measurements alone.
Production Influence
Maximizing this index in high-volume production minimizes the occurrence of solder defects during reflow. A low paste volume cpk correlates with increased incidences of insufficient solder and solder bridging, which require manual repair or board scrapping. By using the index to evaluate the performance of different stencil designs and squeegee materials, engineers optimize the print parameters for each product run.
The ongoing tracking of this capability metric is standard practice in automotive and aerospace electronics assembly where high reliability is mandatory.