Topic: Six Sigma Process Quality

In 1985, Bill Smith at Motorola demonstrated a correlation between how often a product was repaired during manufacture and its life in the field. Defect levels in the parts per million (ppm) rather than in parts per hundred (%) were needed to improve the reliability of semiconductors and electronic products in order to compete with the Japanese. Hence, the development of the Motorola Six Sigma quality program with its landmark quality level of 3 ppm defects.

Six Sigma was intended to improve the quality of processes that are already under control -- major special causes of process problems have been removed. The output of these process usually follows a Normal distribution with the process capability defined as ± 3 sigma.

The process mean will vary each time a process is executed using different equipment, different personnel, different materials, etc. The observed variation in the process mean was ± 1.5 sigma. Motorola decided a design tolerance (specification width) of ± 6 sigma was needed so that there will be only 3.4 ppm defects -- measurements outside the design tolerance. This was defined as Six Sigma quality.