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article an introduction to rigid-flex design best practices continues Figure 2: Fill patterns in this table's "flexible" and "rigid" columns identify the rigid and flexible areas of the board. specifying bends and folds in 2-D space. This means that every rigid-flex designer must document where critical design elements cross the boundaries between rigid and flexible sections. Keeping the Flex in Rigid-Flex: Staggered Length Circuits This design practice, also called bookbinder construction, adds a small amount of length layer by layer, moving outward from the bend radius. This method allows the circuit to bend in only one direction. A common rule of thumb is to add additional length to a subsequent layer, roughly 1.5 times the thickness of the individual layer. But that value varies depending on the tightness of the bend and the number of 30 The PCB Design Magazine • January 2014 layers. This is another instance where a paperdoll mockup can provide an informative quick check. Bookbinder construction relieves tension produced during bending and also prevents buckling of the inner layers near the bend radius. More Golden Rules • Avoid bending at the corners. Copper traces perform best when placed at right angles to the flexible circuit bend. In cases where bending is unavoidable, one alternative is to use conical radius bends. • Use curved traces. Avoid hard right angle traces and even 45° hard corners because they increase stresses on copper traces during bending.