Root cause identification and corrective action brought higher yields for an automotive PCB.

SigmaTron International’s Tijuana, Mexico, facility is using an old school approach to drive a broad range of continuous improvement initiatives. The Eight Disciplines (8D) problem-solving methodology is used as a framework for analyzing customer issues and defining the specific challenges to be solved.

The 8D methodology was originally developed by the US government in World War II. It was refined by Ford in the 1980s and grew in popularity. One of its advantages is the fact that it can be used to drive continuous improvement throughout an organization.

Technically there are nine disciplines: Plan was added as discipline zero to the methodology after it was named. They are:

• Discipline 0: Plan.
• Discipline 2: Define and Describe the Problem.
• Discipline 3: Develop an Interim Containment Plan.
• Discipline 4: Determine, Identify and Verify Root Causes and Escape Points.
• Discipline 5: Choose and Verify Permanent Corrections for Problem/Non-Conformity.
• Discipline 6: Implement and Validate Corrective Actions.
• Discipline 7: Take Preventive Measures.
• Discipline 8: Recognize the Team.

The 8D activity was triggered by customer feedback on a program with unacceptably high fallout rates in the customer’s final assembly process. The cause of the defects was varied, so the breadth of the 8D process was valuable in better defining areas of focus and the ultimate scope of the corrective action.

The product was an automotive-related printed circuit board assembly. SigmaTron analyzed the issues using a cause-and-effect fishbone diagram. They found several repeated defects needed to be addressed:

• Design issues with PCB.
• Contaminants on the PCBA.
• Solder bridging.
• Misaligned components.
• Problems with flex cable adhesion.

Using a Six Sigma DMAIC (Define, Measure, Analyze, Improve and Control) and statistical tools, corrective actions were developed for each of the identified areas of concern. A key part of this process involved discussion with the customer because the issues related to design, contaminants and flex cable adhesion related to processes performed at the customer’s facility. There were visits among facilities to study the processes. The design changes plated over vias that otherwise drove a masking requirement in the customer’s encapsulation process. The customer was also installing a switch on a gold pad that required a higher level of cleanliness to achieve optimum solderability. The solution to that challenge was installing modular clean room benches with HEPA filters toward the end of the line so that final processing of the PCBA eliminated all potential contaminants.

The solder bridging had several root causes. Humidifiers were installed at paste deposition, and the gap between the squeegee and the stencil was adjusted. Routine maintenance frequency was increased. Stencil cleaning frequency has been increased. Design of Experiments (DoE) was performed to determine the optimum squeegee height, and work instructions were modified to reflect the change. The misaligned components were corrected with a stencil redesign.

SigmaTron’s engineering team did a stress test on the flex cable and had discussions with the connector supplier on ways to best address the cable adhesion issue. The recommended correction was to increase the amount of adhesive used to 5 mils from 2 mils. This resulted in a 50% improvement in the adhesive peel stress test. The customer is evaluating the proposed changes.

The overall production line was evaluated for efficiency, and improvements included:

• Point of use stocking.
• A focus on minimizing changeover time.
• EEPROM programming done at in-circuit test (ICT) and label, pack and ship is now part of the final functional test.
• Final packaging changed to a plastic ESD tray, which can go straight into production at the customer’s facility.

The process evaluation also identified that production operators had varying skill levels operating the equipment and tended to rely on the expertise of the most-skilled operators. A written test was used to determine the level of each operator’s expertise. A training matrix was developed to ensure all operators, technicians and inspectors had consistent levels of knowledge. Trainers from equipment manufacturers were brought in to conduct training. Inspectors, QA auditors and engineers are now all IPC-certified. Engineers and supervisors all now go through training on 8D. Engineers are all now trained in Preliminary Failure Mode and Effects Analysis (PFMEA) and Control Plan. Inspectors and engineers undergo training on gage repeatability and reproducibility (GR&R). SMT operators now go through a certification process.

The result of this focused approach to continuous improvement is a 40% improvement in overall yields during the first six months of 2014.

The changes identified for this dedicated line are now being replicated throughout the factory. Taking a broader focus using tools such as 8D, DMAIC, GR&R and Lean philosophy resulted in improvements both within SigmaTron’s operations and in the processes used by the customer during final assembly.

Rajesh Upadhyaya is executive vice president, West Coast operations, SigmaTron International (sigmatronintl.com); raj.upadhyaya@sigmatronintl.com.

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