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FEBRUARY 2020 I DESIGN007 MAGAZINE 87 Etched in Memory Forever A multi-board circuit combines the attributes of rigid and flexible circuits. Given these attri- butes, the circuit offers the advantage of allow- ing circuits and components to fit into limited spaces. In addition, the use of rigid-flex tech- nologies reduces plug and connector compo- nents. Because multi-board circuits have appli- cations across a wide range of industries and applications, the circuits often feature dense, fine line trace width and spacing. The circuits also have complex layout options that meet the design requirements of specialized appli- cations. Satisfying those requirements requires rout- ing software that provides contour routing for curved, flexible circuits. Contour routing eliminates any imprecise spacing and allows design teams to specify uniform spacing for traces. The unique attributes of multi-board cir- cuits also require adjustments in tradition- al manufacturing processes. While rigid- flex PCBs follow most of the traditional PCB design processes in terms of the use of de- sign documentation, the printing of outer and inner layers, the substrate, and the removal of copper, the manufacturing processes can become more complex. For example, etching the conductive layer of a flexible circuit can cause the polyimide core to shrink and may force manufacturers to use materials that compen- sate for the shrinkage. Traditional etching prac- tices also offer challenges for the production of the fine lines and line width tolerances seen with multi- board designs. The need for precision requires greater attention to etch rates, the reduction of pools, and higher etching e f f i c i e n c y. W i t h t h e growth in multi-board applications and the use of rigid-flex technologies, manufacturers have incorporated features into standard mechanical PCB operations that accommodate etching of traces that have spacing as low as four mils. As processes and technologies emerge, PCB manufactur- ers have begun using other technologies such as laser machines for small flexible circuit etching. Laser technologies offer the precise position- al accuracy and depth-of-cut calibration needed for etching complex designs and or working with unique flexible polyimide materials. A 20 µm diameter focused laser beam easily handles smaller traces and spacing requirements. Laser systems use hatch and controlled delamina- tion processes for removing copper from dou- ble-sided designs. While laser etching systems place less stress on flexible boards, the adhe- sive layer of a flexible circuit absorbs the laser energy. FLEX007 Hemant Shah is group director of product management at Cadence Design Systems. This article appeared originally as a Cadence Design Systems blog item.

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