The industry standard that establishes the specific requirements for the design of flexible and rigid-flexible printed boards, IPC-2223 ‘Sectional Design Standard for Flexible Printed Boards’ was recently updated to its C revision. The standard now includes new design guidance and requirements for preformed bends, irregular folds and creases, staggered flexible layer bands as an alternative to ‘bookbinder’ designs, and depanelisation methods for flexible and rigid-flex array sub-pallets, as well as information on the use of adhesiveless materials.
Most significant, IPC-2223C includes, as Appendix A, a new design tutorial with guidance on material selection, size and shape of flex circuits and fabrication allowances. According to Mark Finstad, senior applications engineer with Flexible Circuit Technologies and vice chair of the IPC D-11 Flexible Circuits Design Subcommittee that developed the standard, the new tutorial makes the standard easier to understand for designers who do not work with flexible circuits every day.
“Very few users get deeply immersed in flex. It takes five to seven years working with it every day before you understand all the aspects of flexible circuit designs,” he said.
The new version of the standard has been updated to address technology that has matured in the years since the last revision. IPC-2223C includes plenty of information on adhesiveless materials, which were not included in earlier documents. As adhesive quality has improved, many manufacturers have moved away from substrates with adhesives.
“We tried to emphasise adhesiveless materials,” said William Ortloff, engineering fellow at Raytheon Company, and chair of the D-11 subcommittee. “Adhesiveless materials are better for soldering; they do not have the lower temperature requirements common to adhesive based substrates, which can cause problems with glues.”
While the standard has new sections, its size was kept down by avoiding redundancy and focusing only on aspects that are distinctly tied to flexible circuitry. Technologies such as blind, buried and microvias that are the same in flex and rigid printed boards, are not included in IPC-2223C. Instead, users are referred to other standards such as IPC-2221 and IPC-2222 that address these technologies.
However, one aspect of rigid printed board technology that is included in the standard is the connection between rigid and flex printed boards. As Ortloff describes, making this connection correctly is a key point for reliability. “The document explains that you should not run the adhesive coated coverlays of the flex portion through the rigid printed board; you want to keep the flex adhesive away from vias and plated through-holes (PTH) to increase the reliability of the vias and PTH.”
Finstad adds that IPC-2223C also includes sections on strain relief fillets applied at the rigid-flex interface and other factors that are unique to printed boards that have as many mechanical factors as electrical design points. “It is always difficult to get the best of both worlds. Flex boards are not much good if they cannot bend.”
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