Mistake-proof Manufacturing =Quality Manufacturing

Estimated reading time: 9 minutes

An alternative to the "training and punishment" approach to error avoidance, mistake proofing devices provide prevention vs. costly "after the fact" detection.

By Edwin B. Smith III

Electronics assemblers constantly are exhorted to be more careful and diligent in their work. However, this approach to error prevention, that of training and punishment, has been relatively unsuccessful in the long term. On the other hand, students of mistake proofing the manufacturing process believe that while humans generally are involved in some way when errors occur, the basic causes of assembly mishaps typically are beyond the individual's control. Accordingly, a mistake-proofing methodology for manufacturing is highly desirable.

Poka-yoke OriginsDr. Shigeo Shingo is one of the industrial engineers at Toyota Automotive credited with creating an approach to quality management that relies heavily on the use of poka-yoke. A poka-yoke (pronounced POH-kah YOH-kay Japanese for mistake proofing) device is any mechanism that either prevents a mistake in assembly or makes it obvious at a glance. These devices also prevent the special causes that result in defects or permit inexpensive inspection of each item for acceptability. The assembler performs self-inspection through use of the poka-yoke, and any defects are immediately apparent to that operator. Poka-yokes also ensure that the operation cannot be completed until the poka-yoke condition is satisfied.

Poka-yoke devices differ from other methods of process control, such as statistical process control (SPC), because they are capable of use on 100 percent of the product; are "transparent" to the process, permitting 100 percent inspection without extra burden for the operator; and require products to be processed through the device or the step cannot be completed. Other features of poka-yoke devices include low (sometimes zero) implementation costs and immediate feedback.

Manufacturing Environment MistakesThere are numerous definitions of human error, each serving a different purpose or providing a particular insight. A definition applicable to electronics might be: an inappropriate action, or intention to act, to reach a specific goal. As a response, mistake proofing often is integrated into production through the new product introduction (NPI) process for which the QS-9000 quality management system provides a model.

QS-9000 is a standard developed by the "Big Three" automotive manufacturers (General Motors, Ford and Chrysler) to ensure that their suppliers meet certain minimum requirements. Developers wrote the QS-9000 NPI procedure based on maximum customer involvement and use of quality tools to ensure the most rapid ramp from concept to production. The process involves cross-functional teams early in the process and minimizes unforeseen quality and safety issues. QS-9000 NPI also requires process controls through failure modes and affects analysis, process control plans, SPC and poka-yoke. The process for NPI under the QS-9000 model includes the following items related to poka-yoke:

Special Characteristics. These variables or product attributes can compromise user safety or necessary functions if not properly controlled during the assembly process. They are controlled by special measures taken during processing, typically through SPC or poka-yoke. Throughout the NPI process, team participants consider poka-yoke during the planning of processes, facilities, equipment and tooling.

Safety. Valuable topics to consider for poka-yoke include product safety, introduction of new chemicals to the workplace and operator safety.

Failure Modes and Effects Analysis (FMEA) improves the processes related to any special characteristics as well as other failure modes that could result in an unsafe product condition or loss of important product functions. Efforts are directed at improving the process to achieve defect prevention and reduction rather than defect detection through inspection. Such efforts typically include SPC and poka-yoke.

Control Plans document the required process controls, tools, set-up verification and other steps to be used at each phase of the process flow to ensure that the product meets all specifications. Control plans are developed at the system, subsystem component or material level. When a poka-yoke is specified, its tooling/process identification and its use are documented in the control plan.

A Quality System CornerstoneQS-9000 requires that all critical product characteristics the processes, facilities, equipment and tooling are governed by SPC or poka-yoke to avoid compromising product safety. Poka-yoke is more sensitive than SPC to process changes because it is used on 100 percent of the product assembly stream. Thus, it makes sense to use poka-yoke whenever possible for safety-critical product characteristics.

A systematic view of the manufacturing process aids in making mistake proofing a cornerstone of the quality system. When designing manufacturing systems, user errors typically are found. (Remember, they rarely are single, independent events but part of a larger sequence.) "Reasoning back" from the immediate cause then should be done to see if the process could be redesigned or reorganized to avoid such problems rather than "inspecting" quality into the process.

An analysis of operator tasks, workflow and information needs may point out problems in the way work is performed. The goal is to prevent errors by designing the system to handle them when they occur, e.g., "forgiveness" of user actions, redundancy in data storage, etc. The analysis may lead to a redesign of the process to include mistake proofing over traditional inspection and even SPC. (Aviation, for example, has benefited from the use of flight simulators to test new cockpit designs for weaknesses, which lead to pilot error.) Likewise, manufacturers can use simulations as an operator training tool as well as an opportunity to find potential causes of human error before the manufacturing line is commissioned.

Poka-yoke at Work: Three Case StudiesThe factory environment is rife with sources of error during the product assembly process. The following four case studies provide examples of mistake proofing using poka-yoke principles, showing the problem along with a solution as developed in the factory.

Figure 1. An integrated heat sink and "third-hand" fixture. Note that the posts of the product contact those on the tool.

Case Study 1 Cable AssemblyPROBLEM: During the soldering process assemblers must take care that all the components' operating characteristics remain intact while working at temperatures above 700°F. However, many components cannot tolerate such high heat. For example, in the case of a cable requiring a filter to be soldered at its end, heat from the soldering iron can destroy the component if permitted to rise above the filter's limit. In response, a heat-sink clip (Figure 1) is attached to the filter itself to dissipate some of the heat. This method works as long as the clip is used. However, due to the filter's round shape, it is a cumbersome and awkward job to hold the filter steady, thus discouraging the heat-sink approach. Eventually, operators deviate from the procedure and instead attempt to stabilize the filters in individual ways, even sometimes avoiding the heat-sink clip.

SOLUTION: A cross-functional team consisting of program management, quality, engineering and production meet to review the problem. The team develops a mistake-proofing concept based on poka-yoke via a two-function fixture, which holds the cable at the filter and eliminates the necessity of heat sinking. The new fixture acts as a clamp for soldering stability, i.e., its built-in heat-sink capability dissipates solder-iron heat away from the filter. Taking temperature readings of the filter lead entering the hexagonal side and between the heat-sink contact tests the new fixture. Results showed that temperatures did not exceed specifications.

It is now impossible to build the product without simultaneously heat sinking the tinned leads. Operator acceptance of the fixture is much higher than that of the heat-sinking clips. (Also helpful: no extra effort is required for its use.) Customer failures dropped to zero after implementing the fixture.

Case Study 2 System Assembly Fulfillment.PROBLEM: Software, safety warnings and user manuals bagged as a Media Kit arrives to the customer with extra or missing items. This occurs on multiple runs even after assemblers were admonished to be more careful. As a first step to defect prevention, the Media Kit production line is set up for continuous flow of the items into the bag, with discouraging results. The same types of errors continued unabated.

SOLUTION: A Media Kit board is prepared for each associate and is drawn up on a one-to-one scale with the items to be placed in the kit. Included are the outline of the parts, the part number and the unique tool number assigned to the Media Kit board. The assembler stocks the board with all required items by filling all the outlined spaces. Board contents then are placed all at the same time into the finished Media Kit bag and sealed. Mistakes are eliminated by ensuring that all spaces on the board are filled for each kit (Figure 2).

Figure 2. Media Kit tray is "mistake proofed." Parts are guided for placement by a board before the kit is sealed.

Case Study 3 Cable Harness Mistake Proofing.PROBLEM: In the cable harness industry, it is common for systems as long as 20' and 2" in diameter to be bundled into specific shapes to fit into finished equipment and to interconnect the circuitry. It is a time-consuming process marked by operator fatigue, stemming from the sheer volume of interconnects to be made. Additionally, this process opens many errors in the assembly.

Figure 3. A poka-yoke cable harness board. The boards are built exactly to scale and, after wiring, the cables can be connected to test fixtures to eliminate a rechecking step.

SOLUTION: Special harness boards are created for this situation. These poka-yoke boards contain pins that will mark the routing of the cables together with line drawings that show the circuitry of the harness plus explanatory notes (Figure 3). The harness boards are built exactly to scale with their dimensions certified by the quality department as part of a first-article NPI process. Once certified, harness cables require no rechecking. Also, after wiring, they can be connected to electrical test fixtures, eliminating the need to check the cable once it is removed from the board. Labels are placed on the harness board at the exact sites matching their spots on the harness, making it a simple task to transfer the labels on the harness while it is on the board.

ConclusionWhen a manufacturer uses poka-yoke mistake prevention as a part of its NPI processes, many positive things occur. Personnel at all levels and across all functions begin to think in a preventive mode rather than an "after the fact" detection mentality regarding processing errors. Finally, defect rates diminish because poka-yoke processes do not permit nonconforming or defective products to pass to the next step. The trend toward more stringent quality management systems (such as QS-9000 and the upcoming revisions to ISO-9000) provides the manufacturer with models on which to further implement mistake proofing.

REFERENCES1. S. Shingo, Study of Toyota Production System from an Industrial Engineering Viewpoint, Tokyo, Japan Management Association, 1981.

2. S. Shingo, Zero Quality Control: Source Inspection and the "Poka-Yoke" System, Cambridge, Mass., Productivity Press, 1986.

3. S. Shingo, The Sayings of Shigeo Shingo: Key Strategies for Plant Improvement, Cambridge, Mass., Productivity Press, 1987.

4. S. Shingo, Non-stock Production: The Shingo System for Continuous Improvement, Cambridge, Mass. Productivity Press, 1988.

EDWIN B. SMITH III is senior director, operations at K*TEC Electronics, 1111 Gillingham Lane, Sugar Land, TX 77478; (281) 243-5993; Fax: (281) 243-5893; E-mail: smithe@ktecelec.com.