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DECEMBER 2022 I DESIGN007 MAGAZINE 69 targets" need to be located near the panel edge area to assist panel alignment dur- ing the solder deposition process. Solder deposition systems use cameras to pin- point the fiducial targets, enabling precise alignment of the solder stencil to the land pattern features on the individual circuit units within the panel's central area. In addition to the globally located fiducial features, two or more fiducials will be required within the component mounting zone to facili- tate precise automated component placement. Using multiple fiducial datum features within the SMT component placement area will min- imize the effects of variable shrinkage or any process distortion in the flexible materials. e illustration in Figure 4 is an example of a flexible circuit with SMT components mounted within two zones that are separated by a nar- row interconnect section. Multiple fiducial locations will provide a tighter tolerance within each datum zone or termination area while relaxing the need to maintain a constricted tol- erance of the flexible interface section between other component termination areas. Fiducial Target Design Specification e optimum fiducial target is simply a solid fetched copper circle that is clear of surface coating or cover layer material. Coatings and cover layer film material openings surround- ing the fiducial must be adjusted to provide enough clearance around the fiducial's perim- eter to ensure that it does not overlap onto the fiducial target features during the cover layer- to-base circuit lamination process. • e optimum fiducial is a solid circular land pattern that is 0.25 to 0.50 mm (~0.010" to 0.020") in diameter. • To enable visual access for locating the fiducial targets, the solder mask or cover coat must provide a clearance 2 x R of fiducial R. • Fiducial location must be clear of the panel or circuit outline edge by a minimum of 4.75 mm (~0.187") and provide a consistent high contrast. No plating is required on the etched copper fiducial surface, but if a secondary alloy plating over the base copper is specified in the control document, the designer must ensure that flat- ness of the fiducial surface is maintained within 0.015 mm (0.006"). Final Comments When planning multiple unit circuits for panel processing, the flexible circuit designer should attempt to coordinate the final panel size and unit configuration between both those responsible for the assembly process and the fabricator designated for manufacturing the circuit. Technical Course Vern Solberg will be presenting a half-day technical course, "Flexible and Rigid-flex Cir- cuit Design for Manufacturing," on Sunday, Jan. 22, at IPC APEX EXPO 2023. e course will focus on SMT design, fabrication, and assembly process principles. DESIGN007 Vern Solberg is an independent technical consultant, specializ- ing in SMT and microelectronics design and manufacturing tech- nology. To read past columns, click here. Figure 4. Fiducial target location for two component mounting zones.