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60 DESIGN007 MAGAZINE I DECEMBER 2023 extrude 0.003" to 0.010" around the cover-coat opening. A very tight hole-to-pad ratio may cause problems meeting specifications. Terminal pad baring can be done by combination but stop on individual pads. When individual pads access holes are used, their diameter should be larger than the pad diameter. Avoid reverse pad bar- ing if possible, to reduce labor and processing costs (Figure 8). For circuits to be plated with a special finish only on the pads, all plated areas must be run to a common bus outside the cir- cuit. is connection is severed in finishing operations. Final refinement of conductor runs should result in tolerances and dimensions within the following parameters to result in the most eco- nomical and functional flexible circuit. • Minimum edge distance 0.030". Design for 0.050" if space is available. Pad diam- eter should be at least twice through-hole diameter. rough-hole should be 0.008" to 0.010" larger than component lead diameter. • Minimum trace-to-trace and trace-to-hole spacing of 0.010" • Minimum conductor width of 0.010" • Offset conductors from one side to the other to avoid an I-beam effect in fold areas Reinforcement in key areas oen is desir- able to facilitate component insertion, assure mechanical strength, or obtain the selective stiffness required in some applications. Any standard board material may be used (FR- 4, G10, polyamide glass). Added thickness of dielectric film will provide some stiffening and wear resistance; glass cloth tape also can be used as a stiffener. Hole diameters in rein- forcement materials should be at least 0.020" larger than through-holes. Adhesives for bond- ing reinforcements can be the same as used in the main circuit or transfer tape can be used. A strain-relief material should be used to form a radius between the flexible circuit and stiff- ener materials in any sections subject to flexing (Figure 9). Flexible circuits can be adapted to any con- nector design. Provide exact dimensional data for all connector types. Components can be soldered directly to a bared pad in the flex- ible circuit. Pins can be soldered into flex for direct board plug-in or for wire wraparound. Exact dimensional data is required including pin diameter, length, and spacing relative to the conductor's center. Insulation displace- ment terminals penetrate the insulation and are rolled over to mechanically grip the con- ductor. External shells plug into the crimp-on pins to serve as connectors. Wires can be wired through circuit pads or lap-soldered to circuit fingers. Gold pressure contacts or sculptured posts have limited application. Figure 9: Stiffeners and backers. Figure 8: Coverlayer pad openings.