Issue link: https://iconnect007.uberflip.com/i/1512857
58 DESIGN007 MAGAZINE I DECEMBER 2023 increases in pad area will reduce the chance of hole breakout, prevent stress build-up from drilling, and prevent possible conductor-to- pad interface cracks. Apply minimum spac- ing requirements to adjacent conductors and pads. Maintain a minimum of 0.050" from the edge of a trace to a conductor or pad and allow 0.020" for required outline tolerances, material misregistration and process tolerance buildup. In connector areas where small center-to- center spacing between pads is used, it may be necessary to stab or truncate pads to run conductors through the pattern. Elongated pads may be used to increase solderable pad area in at least two directions and give the ability to capture the pads with the cover layer (Figure 4). Terminal pads associated with large conduc- tor areas (ground planes, voltage planes, heat sinks, etc.) should be relieved locally to facili- tate soldering (Figure 5). Specify hole sizes with standard drill tolerances. Do not specify hole-to-pad concentricity; specify annular pad. Minimum annular on single-sided circuits (unsupported pad) should be 0.015". e radius of a bend in a one-layer flexible circuit should not be less than 10 times the cir- cuit thickness. Conductors should run perpen- dicular to fold lines. Keep them neat and uni- form and add extra copper (space permitting) to help hold their folded shape (Figure 6). Kapton tensile strength is typically 25,000 pounds for 1-mil material, but tear strength is low. Copper reinforced areas (tear stops) should be used for corners of 120 degrees or less to restrict possible damage during assem- bly. Glass cloth tape and FEP Teflon can be used (Figure 7). When pads cannot have tabs or fillets, the cover-coat should lap onto the pad at least 0.005" for 270 degrees. Use slots or irregu- lar shapes as a last resort only. Adhesives will Figure 4: Pad geometry standards. Figure 5: Pad relief geometry. Figure 6: Bend and fold geometry. Figure 7: Tear stops.