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Testing Flexible Circuits, Part 3: The Completed Flex Circuit

This is the final article in a three-part series relating to testing a flexible circuit.  In the first column we cited IPC-6013, “Qualification and Performance Specification of Flexible Printed Boards,” as a comprehensive specification widely accepted in the industry. Many of the tests discussed in this article are outlined in that document.

Most flex houses perform a variety of tests on completed flexible circuits. The type, frequency, and complexity of these tests vary with customer and application. Test requirements are generally defined by the customer, but input is often solicited from the supplier during the quote process.

Production Testing

Most production tests are non-destructive and performed as part of the fabrication or assembly process. Many flex circuit fabricators also perform 100% inspection on final bare circuits. Testing may be done as an automated machine optical inspection (AOI) or as an electrical test. Normal electrical testing generally verifies parts are free of opens and/or shorts, but in some cases high voltage or current may be specified. AOI is recognized as an acceptable substitute (vs. electrical test) for single- and double-sided circuits. AOI can be done by programming a set of “rules” such as minimum trace width and apply that to the circuit pattern.

Destructive testing is used to examine copper thickness thru cross sections of plated thru holes. Porosity of gold plating is another verification also done as a destructive test.

Unique customer requirements can result in a variety of strategies for testing fully assembled product. This may include “in circuit” component testing, or in some cases full or partial functional testing.  Machine optical inspection may also be done 100% with sophisticated equipment to examine for orientation and presence of components.

Other production testing involves surface cleanliness. Ionic and surface organic contamination tests are specified in IPC-TM-650. These tests are fairly common for medical and aerospace applications. 

Reliability or Performance Testing

While production testing will verify form, fit and function, other tests are used to assess the product reliability. These tests tend to involve environmentally stressing the part and can partially destroy or degrade the tested circuit. They also can be fairly expensive and time consuming.

 The following are common reliability tests:

• Long-term humidity aging
• High-temperature stress
• Salt–sulfur fog exposure
• Thermal cycling (with or without current)
• Flammability testing (UL)
• Chemical immersion

The specific parameters of each test depend on the application and customer. Application engineers work with the customer to jointly determine the exact test parameters, balancing cost with product robustness requirements. A specific performance test can be as short as a few seconds or require weeks to complete. When and how frequently reliability tests are performed is mutually determined by both the customer and the supplier. The following are typical:

• One-time testing done as part of the design qualification 
• Testing performed on a set time schedule such as weekly, monthly or quarterly
• Testing done on a sample from each lot
• Testing based on production frequency such as every 10^th lot, etc.

Many fabricators have resident capability to perform the life cycle testing required by customers, either with an in-house test facility or by outsourcing with an independent test lab.   

Fabricators focusing on a specific product niche often develop a large data base of results covering a variety of performance tests. For many applications, previous test results can be a starting point for discussions in determining what tests are needed on a new part number.  

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