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64 DESIGN007 MAGAZINE I FEBRUARY 2019 Analysis and Results To study the influence of metal plating on PCB channel loss (i.e., insertion loss or S21) and impedance (i.e., time domain reflectom- etry or TDR), four models of a one-inch long microstrip are constructed and listed in Table 2. For Model A as depicted in Figure 1, bare copper trace (i.e., 38 mils wide and 1.3 mils thick) without plating is laid out 17.5 mils above the reference plane and insulated by low-loss dielectric substrate material. Model B mimics immersion silver plating with 0.02- mil silver on top. Meanwhile, Model C mim- ics ENIG plating with nickel and gold on top. Model D has slightly thicker nickel but thin- ner gold versus Model C. All four models have the same overall trace thickness and width and dielectric substrate material and thickness but without solder mask. With reference to S21 plots depicted in Fig- ure 2, Models A and B experience almost the similar insertion loss due to slightly higher conductivity for silver versus copper. Mean- while, Model C with 0.19-mil thick nickel (i.e., much lower conductivity) in the mid layer and 0.02 mils of gold on top encounters an additional 0.06 dB attenuation at 10 GHz versus Models A and B, created by overlap- ping between skin depth with the mid nickel layer. Further decreasing the top gold coating and increasing the mid nickel layer intensifies the attenuation, resulting in extra 0.04 dB PCB loss for Model D versus Model C. Meanwhile, the TDR plots plating (i.e., base copper 1.09 mils, nickel 0.2 mils in the mid layer, and gold 0.01 mils on top) is applied, the high-frequency signal will propagate on the gold and nickel-plated layers. This signal will encounter a larger magnitude of attenuation due to the lower conductivity of nickel. Referring to Table 1, silver has the best con- ductivity, followed by copper, gold, and nickel. Nickel's conductivity is less than half that of gold. A thicker layer of gold or a better con- ductor on top (i.e., one that is enough to cover the skin depth) alleviates the attenuation. This phenomenon is proven in simulated cases to be discussed in the following section. 2 Table 1: List of metal conductivity. Table 2: List of models for metal plating on PCB microstrip. Figure 1: Microstrip Model A in Keysight EMPro.

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