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16 The PCB Design Magazine • August 2014 5. Trace thickness, width and separation need to be above the manufacturable limits. Trace width/clearance should not go below 4/4 mils to minimize costs. 6. And most important of all, the price needs to be low. All of the above need to be considered, to es- tablish the right material without over-design. Once the ball park, virtual material numbers are established, the material needs to be select- ed for 2 GHz operation. This I suggest you do in consultation with your preferred fab shop, as choosing the materials that they stock will result in up to 5% better accuracy. Obviously, what you select is based on what is available at a reasonable price. (The ICD Stackup Planner features 8,800 materials up to 40 GHz to choose from.) Boolean searches can be done in order to reduce the select list as Illustrated in Figure 4. Look for a 2 GHz material with Er<4, Df<0.02 and Tg = >180C. In Figure 5, I have chosen ITEQ IT-180A which fits the specs. Prepreg materials are only available up to 8–9 mil thick, so in order to attain the desired thickness, multiple prepregs must be stacked to- gether to give the required 10 mils. In this case, I have used 2 x 2.8 plus a 4.6 giving 10.2 mils total. Make sure these are symmetrical, about in the center, otherwise there will be a slight offset in impedance due to the field solver seeing an imbalance in dielectric constant. In conclusion, selecting an adequate mate- rial for the project will minimize the cost. The MATERIAL SELECTIoN FoR DIGITAL DESIGN continues Figure 3: stripline configuration using "virtual materials." Figure 4: Material selection in the iCD stackup Planner Dielectric Materials library. beyond design