Simple fixes are often the best solution for small variances.

In a perfect world, manufacturing process setup should eliminate the potential for mistakes. In practice, however, process complexity and the impact of system variation makes that impossible. Consequently, organizations committed to the efficiencies of Lean manufacturing often use a range of tools to identify and eliminate defect opportunities from their process.

SigmaTron International’s Tijuana, Mexico, facility uses a number of these tools in this process. During project launch, advanced product quality planning (APQP) failure mode and effects analysis (FMEA) is used to set up the most efficient, defect-free process. The product part approval process (PPAP) is used on automotive projects to validate the process, while customer-specific validation processes are used for projects in other industries. Once production is ongoing, statistical process control (SPC) and other forms of quality data collection and monitoring are utilized to monitor processes and track defects. When defects occur, a kaizen event is scheduled, and tools such as 8D problem-solving, Six Sigma’s Define-Measure-Analyze-Improve-Control (DMAIC) and poka yoke are applied to analyze and correct the root cause.

In this month’s column, we focus on the poka yoke process. Poka yoke is Japanese for mistake-proofing. It was introduced by Shigeo Shingo as part of the Toyota Production System (TPS). When applied, the resulting process improvement eliminates a specific defect. Its focus on simplicity is its strength, in that simple fixes are often the best solution for small variances in the manufacturing process. Simple fixes are also easy to rapidly implement and encourage production operator involvement in the pursuit of product perfection.

In one example, an automotive product printed circuit board assembly (PCBA), which included a flex cable, was experiencing intermittent failures following in-circuit test (ICT). SigmaTron’s Tijuana team scheduled a kaizen event to analyze the test process and determine the root cause of this potential failure mode. The team determined that in some cases, the flex cable could catch in gaps in the test fixture bed as it was being removed, creating an undetected defect. The poka yoke solution was fixture redesign that placed ESD-safe plastic over the openings in the fixture bed that created the issue.

As a result, the defect opportunity was eliminated with no changes to the product design and minimal fixture modification cost.

Poka yoke begins with an awareness/identification process where a team looks at what has gone wrong. In the example above, the issue was intermittent failures following ICT. The poka yoke development process utilizes a series of five questions or five “whys” to analyze the problem. The goal is to ensure the team looks beyond the most obvious causes of the problem and analyzes it thoroughly enough to determine the actual root cause. To do that they followed the poka yoke step of going to the gemba, or work floor, to watch the actual process. In the example above, the team was able to drill down to the operator level. They found that when removing a flex cable panel from the test fixture, occasionally one of the cables would catch on an opening in the test fixture, which strained its connection to the PCBA and resulted in intermittent failures. Throughout this process of analysis, they were also looking for a solution that fixed the problem. In this case, that was covering holes in the test fixture with an ESD-safe plastic.

Poka yoke focuses on three types of solutions:

• Contact type – eliminate defects caused by variation by making variation impossible.
• Constant number/visual management type – an error signal is triggered if a specific number of movements or steps are not completed.
• Performance sequence/fail-safe – if steps are not completed in a specific order, the operator is locked out.

The example above illustrates a contact type of poka yoke. The ESD-safe plastic eliminates the ability of the cable harness to slip through the holes in the test fixtures. Other contact examples include a mechanical design where parts can only be inserted one way based on the alignment of the part with the hole shape.

The reality is poka-yokes are all around us. Many current car models use visual management signals to inform the driver of dangers in abrupt lane changes or an impending collision when the vehicle is operated in a potentially unsafe manner. Most vehicles have a lockout “fail-safe” feature that prohibits starting unless the transmission is in park.

In today’s competitive, often mission-critical electronics market, it isn’t good enough to just detect and correct defects through test and inspection. It is necessary to determine the root cause and then eliminate the opportunity for those defects to exist. Training production teams in the use of tools such as poka yoke force-multiplies the resources of a Lean enterprise because often the individuals most aware of hard-to-pinpoint defect root causes are the production operators who notice subtle variations. The emphasis on identifying simple solutions helps ensure these mistake-proofing solutions are rapidly implemented.

Juan Jose Morales Ibarra is advanced product quality planning (APQP) manager at SigmaTron International; juan.morales@sigmatronintl.com.

Filemon Sagrero Quirino is quality assurance engineer at SigmaTron; filemon.sagrero@sigmatronIntl.com.

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