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88 DESIGN007 MAGAZINE I JANUARY 2020 hole in a rigid-flex circuit. Eliminating acrylic adhesive from the plated through- hole area will greatly increase the reliability of the PTHs • Rigid sections connected by flex should be a minimum of 0.375" apart and preferably 0.5" or more • Utilize "unbonded" construction to increase flexibility. When using unbond- ed construction on impedance controlled circuits, you must ensure that signal and reference plane layers are not unbonded from each other. When the circuit is bent, the unbonded areas will buckle, which will cause an impedance mismatch if the signal and reference plane layers are not bonded together • When specifying a carrier panel or "pallet" for component installation, contact your manufacturer to make sure that the carrier panel fits efficiently on their processing panel. Failure to do this can result in a major cost increase Cost Drivers Every designer is looking for ways to de- crease costs without sacrificing performance. IPC research has shown that PCB designers drive over 75% of the circuit cost based on the decisions they make. It is imperative that the flex designer understand what features add value and what features add only cost. Designers should never sacrifice reliability to save costs, but at the same time, many flex cir- cuits are over-specified, resulting in addition- al costs that add no additional value. Here is a list of the features that drive the majority of your circuit cost. • Layer Count: As the number of layers increase, so does the cost. More layers will require additional materials and processing time. Processing high layer count flex or rigid-flex can also be very technically challenging, which may result in reduced yields • Circuit Size and Shape: Most flexible circuits are constructed in panel form. The greater the panel area a circuit occupies, the greater the cost. There are instances where even a small change in outline can result in a large cost decrease. A slight modification to the flex shape allows for a better nesting of the flexes on the panel, resulting in two more circuits per panel • Circuit Type (i.e., Type 3 vs. Type 4): Rigid-flex circuits are typically more expensive than multilayer flex with stiffeners. Scrutinize your design to determine if your applica- tion requires a rigid-flex construction or if a multilayer with stiffeners will work. If in doubt, call your flexible circuit manufacturer and ask • Circuit Class (i.e., Class 3 vs. Class 2): Class 3 circuits require additional testing, inspection, and construction require- ments, which make them more expensive. Review the requirements of your applica- tion to determine the proper class for your flex circuit Figure 6: A typical rigid-flex circuit.