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APRIL 2019 I FLEX007 MAGAZINE 17 and predictable bends during assembly and in the field. 3. Flexible solder mask is not intended to bend and should be well supported by stiffeners or isolated from the system. 4. The bend configurations conceived and represented by engineering design tools are idealized; the actual bends a circuit sees during assembly and application may be far different and more severe. 5. Dual-row ZIF connectors are inherently vulnerable to cracking, so it is important to protect the ZIF fingers by ensuring they are isolated from sources of strain during assembly and use. As described in this article, there are often no right answers or perfect solutions for a given FPC design challenge. Ensuring robust, reliable FPC designs requires close cross-functional col- laboration with internal electrical, mechanical, and PCB design teams as well as with the FPC fabricator and the assembly partner. Achieving success often requires some iteration, so it's best to engage with internal and external partners as early as possible in the development cycle. FLEX007 Todd MacFadden is a PCB technology engineer at Bose Corporation. • The layout of the rigid-flex board was reconfigured to accommodate moving the plated vias into the rigid areas • The assembly process was modified to avoid requiring blind insertion of the ZIF into the receptacle on the PCB side • The team developed a pre-bend fixture, providing a consistent, repeatable alterna- tive to manual bending (Figure 8) After these changes were implemented, no subsequent failures were encountered. Conclusion The three case studies presented here pro- vide an opportunity to share lessons learned by highlighting some of the common failure modes of flexible printed circuits and illustrat- ing several key concepts and best practices to ensure robust, reliable FPCs. Key takeaways include: 1. IPC-2223 bend recommendations provide a good starting place for FPC design, but they are often too conservative for many applications. 2. It is possible to exceed the bend radius recommendations, but extreme care must be taken with (a) material selection, (b) conductor design and the layout of supporting structures, and (c) the assembly process to ensure controlled Figure 8: A pre-bend fixture was developed and implemented to ensure predictable and repeatable bend of the flex tail with the ZIF fingers (L), and the rigid-flex circuit after pre-bend (R).

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