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September 2016 • The PCB Magazine 27 FLEX CIRCUIT SPECIFICATIONS FOR COMMERCIAL AND MILITARY APPLICATIONS What are the differences between IPC 6013 Class 3 and MIL-PRF-31032? IPC 6013 IPC 6013 specifies performance and qualifi- cation requirements for a flexible printed circuit board. The document specifies: • Physical and electrical characteristics • Performance requirements • Raw material requirements • Inspection methods • Acceptance criteria • Sampling plans • Tests methods IPC 6013 specifies requirements for three classes of products. Class 1 and Class 2 would normally be applicable for most commercial products; Class 3 is often required for military/ aerospace products and other high reliability applications such as medical. When a supplier and customer adopt IPC 6013, they agree the characteristics and performance of the given part numbers in their contract will meet that standard. A flexible circuit supplier can produce products in accordance with IPC 6013 require- ments, but there is no "certification process" re- quired to produce parts per IPC 6013. MIL-PRF-31032 MIL-PRF-31032 also specifies the features and performance requirements for flexible cir- cuits. The requirements are generally similar to IPC 6013 Class 3, with some minor differ- ences. With very few exceptions, a flexible cir- cuit that meets the performance requirements of IPC 6013 Class 3 would also meet the per- formance requirements of MIL-PRF-31032. But MIL-PRF-31032 goes further and details require- ments for a supplier's quality management sys- tem. This requirement is similar to the quality management aystem as defined by ISO 9000. To become certified a suppler one is required to im- plement a quality management plan, establish a self-assessment system, have an on-site audit performed and perform periodic self-audits. A summary of the difference might be IPC 6013 applies to products; MIL-PRF-31032 ap- plies to the quality management system and the products. A company does not become cer- tified or qualified to IPC 6013 as the quality of the end product governs compliance with the specification. But a fabricator does need to be certified to MIL-PRF-31032, by the Department of Defense DLA (Defense Logistics Agency). Approval of a supplier's documentation and an on-site audit approval are required to build product that meets the requirements of MIL-PRF-31032 or MIL-PRF-50884. The OEM generally defines the specification determin- ing whether a supplier must be certified to MIL-PRF-31032, however for some military programs, the DOD may require an OEM only use a supplier that is MIL- PRF-31032 certified. Differences in performance and features in IPC 6013 Class 1, 2, and 3 are often slight and in general there is little reliability sacrifice among the three classes. The following categories in- clude cases where Class 3 differs from Class 1 or Class 2. • Design features/characteristics: There are a number of instances where a Class 3 circuit has more stringent design requirements, either directly stated or implied, based on manufac- turing tolerance specs or performance in test- ing. Some examples include: Gold-plated fin- gers must be plated thicker for Class 3, or larger annular rings must be designed around vias. • Manufacturing tolerances: Class 3 circuits have tighter tolerance requirements for a number of features such as conductor width and spacing reduction, copper thickness, sol- der wicking, soldermask adhesion and stiffener registration are examples. At least 30% of the specific features and criteria documented in IPC 6013 have tighter manufacturing tolerances for Class 3 than for Class 1. • Testing or Stress performance: Most of the performance testing criteria are the same between Class 2 and Class 3 with a couple of exceptions: – Dielectric withstand voltage – Insulation resistance after exposure to moisture • Materials: IPC 6013 has specific require- ments for all materials for a flexible circuit, but there are no materials uniquely specified for a given class. In most cases material sets that