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All About Flex: Flexible Circuit Failure Analysis

Design reviews and early involvement by a circuit board fabrication house can minimize the possibility of field problems, but despite best efforts, there remain occasions when diagnosis of a poorly performing design is required. These instances occur most frequently in applications with unique performance requirements (mechanical abrasion, unique bending, and chemical environments are examples) and interactions among multiple features and requirements can make determination of cause and corrective action a complex analysis. Violations of standard design practice also can cause infant or long-term field failure. Solving a technical problem with a failed flexible circuit cable often requires sophisticated engineering diagnostic tools and analysis.

Failure analysis as an engineering service for flexible circuits is available from a variety of independent labs. Conclusions are formalized in written reports with engineering analysis, high magnification photography and design recommendations.

Diagnosing a premature failure of a flexible circuit can be elusive. The best analysis requires a combination of flexible circuit product expertise, sophisticated laboratory analysis technology, and formal problem-solving methodology.

The goal of most diagnostic endeavors is to:

• Identify and isolate the problem
• Identify the root cause
• Recommend the appropriate changes to address the issue

An initial analytical step is to gather as much background information as possible. An excellent tool for organizing the background information in a systematic way is the Is/Is Not table. The table asks a series of questions to help the diagnostic team gather information in a logical and systematic method while helping focus the team on higher probability causation. The questions ask Who, What, Why, When, and How, as related to the problem. Problem statements are recorded along with eliminating possible options.

Below is an example of an IS/IS NOT table:

Gathering the background information can be routine and in most cases examination of multiple parts under magnification is required. Sophisticated statistical analysis using factorial designed experiments might also be utilized and could take several days. Going through this process helps focus the diagnostic effort on the most likely causes based on the available empirical data.

The next diagnostic phase usually involves testing/analysis on the physical part. In most cases this requires destructive testing. The following are examples of analytical equipment frequently used for diagnosis:

• Cross-sectional analysis
• High-level magnification
• High-voltage electrical tester
• Time domain reflectometer
• Frequency/spectrum analyzer
• XRF (X-ray fluorescence)
• X-ray
• Infrared heat mapping

Once the failure has been physically identified, the root cause assessment can be performed. Additional analytical tools such as the Ishikawa or herringbone diagram might provide additional insight.

Documentation of the analysis performed and conclusions reached should be summarized through creation of a comprehensive report including photography, analytical tools employed, and organization of the data generated. In most cases a cause/corrective action linkage can be established with recommendations for next steps.  

Dave Becker is vice president of sales and marketing at All Flex Flexible Circuits LLC.