The quality of current products and manufacturing processes is dependent on their history and original design parameters. In many cases, the products and the manufacturing operations used to produce them were not created with six sigma quality in mind. It is very difficult to achieve six sigma when that was not one of the goals at the very start of the product development process.
The road to higher quality begins with understanding current quality levels, then working with a plan to incrementally increase quality until the goal is achieved. A hierarchy of tools could be used at different stages of quality. Figure 8.1 is a good example of successive quality improvements that can be used as a roadmap for improving quality in current operations. It was used to improve soldering process quality from unacceptable defect rates to six sigma quality. The progression was accomplished through several phases:
1. The TQM (total quality management) phase. This phase is shown on the left of Figure 8.1 and should be used in situations where it is obvious that the manufacturing quality is out of control. This may be due to a large influx of new production operators or a ramp-up of production volume. The goals of this phase should be to stabilize the quality of production by investing in operator training and the operational aspect of production. New support staff should be recruited, process documentation inspected and improved, and training of operators and line management increased.
2. The SPC (statistical process control) phase. In this phase, the manufacturing process is stabilized and control methods discussed in Chapter 3 are used to ensure that the process remains in control. Tools such as control charts to monitor production quality and sampling methods for incoming inspection are used to ensure that defects in material or lapses in processing methods are caught early in the manufacturing cycle and corrected promptly. The management goals in this phase are to increase the communications between the different production operations, the supply chain, and the customer This will allow for quick reaction to quality problems throughout the organization, and reveal long-term trends. The TQM efforts will continue in this phase, improving the quality and reducing defect rates incrementally. The quality levels and defect rates will continue at the same rate if no additional investments are made in materials and equipment.
3. The DoE (design of experiments) phase. In this phase, the management sets more aggressive quality improvement goals. There are greater investments in several areas, including more statistical and complex quality training tools such as DoE, DFM (design f〇r manufacture), and QFD (quality function deployment). More technical support staff, such as process engineers, are hired and trained to use these tools. Mandated quality improvement projects are prescribed, such as one DoE experiment for every team at least once per year, or performing process DFMA on every production operation. Purchasing of new equipment or materials is encouraged when economically justified. Communications loops are tightened, and reactions to quality problems are expected to be instantaneous. Examples would be the use of red lights in production to summon engineers and managers in case of a problem; production line stopping authority given to certain operators when they detect problems; quality alerts to the field and customers, and instant or 24 hour supply chain communications to share information on quality problems and design changes. The typical goals set for this phase are at four sigma quality or Cpk = 1.3. That results in a defect rate in the range of 20-200 PPM. Once this level is achieved, focused quality projects should be initiated to target specific defect problems and bring the quality closer to six sigma, as explained in the next section.