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82 DESIGN007 MAGAZINE I SEPTEMBER 2019 Where α C = conductor loss, α D = dielec- tric loss, α R = radiation loss, and α L = leak- age loss. Leakage loss (α L ) is typically ignored when using high-frequency substrates due to the very high-level volume resistivity of the circuit ma- terial. If the material is semiconductor grade where volume resistivity is not high, leakage losses may be a concern. Leakage loss is more of a concern for certain appli- cations, although those are typically high-power circuits. The evaluations in this article are for low-power cir- cuits. Radiation loss (α R ) can be difficult to assess because it depends on many factors. It will be ignored in the pres- ent article because the circuits under study are relatively low in radiation loss. However, readers interested in ra- diation loss can learn more from a pre- vious IPC paper. [2] To evaluate the effects of final plated finish on PCB performance, this study will focus on conductor loss and di- electric loss as are graphically depict- ed in Figure 1. As Figure 1 shows, a circuit using a thinner substrate will have higher in- sertion loss than a circuit on a thick- er substrate, largely due to conductor loss. For the 30-mil circuit material at the far right (the thickest substrate), the insertion loss is rela- tively low, and the dominant loss component is dielectric loss, largely due to the dissipation factor (Df) of the circuit material. In general, circuits fabricated on thinner substrates are more sensitive to differences in the conductor. Plated finishes will have an impact on a cir- Figure 1: These plots of insertion loss versus frequency compare three sets of circuits on three different thicknesses of the same circuit material. Figure 2: These plots of loss versus frequency for the same circuit material with different thicknesses and with and without ENIG finish show that circuits on thinner substrates are more impacted by loss than circuits on thicker substrates.

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