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26 The PCB Design Magazine • June 2014 • Copper traces should be in the "neutral" axis (i.e., equivalent amounts of dielectric on each side of the traces). This prevents the cop- per from being in either a tension or compres- sion mode during flexing. • Thinner circuits are better than thicker cir- cuits. A good design will use .5 or 1 ounce cop- per with either .5 or 1 mil polyimide. • Rolled annealed copper should be used for dynamic applications (the rigid PCB world uses electrodeposited copper) with a consideration for grain direction. The flexing motion of the circuit should be parallel to the copper grain di- rection. This is a critical consideration when the fabricator does their panelization. • A gentle bend radius is best. An accepted guideline for dynamic flexing is the bend radius should be at least 10x the thickness of the mate- rial. Since single-sided circuits are quite readily designed at .005" thickness, a flexing radius of .050" is safe. In static applications, a guideline for single-sided circuits is that it can be folded on itself three times with a radius equal to its thickness if rolled annealed copper is used. • When double-sided circuits are required, the circuits should be fabricated without elec- troplated copper plated on the surface of traces in bend regions. This practice is known in the industry as "pads only plating." Bend regions should be single layer copper to allow neutral axis flexing. • Copper features are usually modified to "flexize" the circuit design. This includes radi- using traces through corners and adding fillets to solder pads. These feature upgrades not only improve flex life, they also improve the overall reliability of the design. Expect the flex supplier to provide these recommendations and file up- grades as part of their service. The above guidelines are good rules of thumb. Some applications are only required to flex a few times, others thousands, while some might require millions. It is always a good prac- tice to flex test an individual circuit design with prototype parts with an assembly that mimics the final application. And make sure the circuit supplier knows the product is going to be used in a dynamic application so proper grain direc- tion is chosen during panelization! Flex to Install So while it is certainly true flex circuits are used successfully in a wide range of dynamic feature FLExIBLE CIRCUITRY...A 3D PACkAgINg TooL continues Figure 1: Flexized design.

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