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JUNE 2021 I DESIGN007 MAGAZINE 85 of the lay-up. Typically, an additional one- inch border area is required. When a 20% or less nominal impedance tolerance is specified, using controlled geometry coupons is recom- mended to ensure values are met. MIL-SPEC Coupons Any PCBs requiring MIL Spec conformance will require a MIL Spec coupon be placed on each PCB. Typically, an additional one-inch border area is required. Assembly Rails e following should be considered to maxi- mize the number up on a panel when assem- bly rails are required. A minimum of a 1.0" border typically must be maintained around the perimeter of the panel to allow for pin- ning, coupons, plat- ing clamps, and other assorted targets that are required to tool a panel. It is a common practice to allow the assembly rail (break- away) to protrude into the 1.0" border by 0.5" to keep the PCB away from the border. When the rail enters the 1.0" border of the panel, some tooling features may remain on the rail aer processing. is can include tooling holes, targets, inner layer gates, and thieving. ese additional features do not affect any assembly operations. Assembly Arrays or Sub-panels PCBs are oen required to be step-and- repeated onto an array for assembly. e assembly array database should be provided to your PCB fabricator to ensure that the correct array is provided to the assembler. e follow- Table 2: Panel sizes are subdivided into "standard" (most common) and "optional" (custom) classifications (units in inches). Figure 2: Understanding the cost drivers in PCB fabrication and early engagement between the designer and the fabricator are crucial elements that lead to cost-effective design success. Following your fabricator's DFM guidelines is the first place to start.

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