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40 The PCB Design Magazine • July 2015 shop and ask what materials they have in stock. For this example, I will use Isola FR406 which is a common low-cost material. 1. Select an 8 mil prepreg. Since 8 mil is not available in the Isola FR406 range, I will select 4.2 mil and add two sheets together. Multiple prepregs are often combined to achieve the desired thickness. 2. Edit the solder mask and select a typical liquid photoimageable material. Now that all the virtual materials have been replaced with the stocked items, you can see that the single ended impedance is a bit out. The differential is 103.34 ohms. 1. Adjust the trace width to get closer to 50 ohms, then adjust the trace clearance to get 100 STACkUP PLANNING, PART 2 continues beyond design Figure 2 : isola FR406 4.2 mil prepreg. Figure 3: virtual materials replaced by isola FR406 material. Figure 4: impedance plots are used to determine correct trace width for 50 ohms.

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