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62 DESIGN007 MAGAZINE I FEBRUARY 2019 However, metal plating comes with disad- vantages. On a PCB, the current of the signal tends to propagate more closely to the surface of the trace when the frequency of the signal becomes higher. Skin depth is the parameter that determines how extensive the current of signal travels with reference to the surface of the transmission channel. The relationship between skin depth and signal frequency is governed by Equation 1. For instance, at fre- quency 10 GHz, skin depth becomes 0.026 mils. Equation 1 σ = skin depth in microns f = frequency of the signal in MHz Equations 2, 3, and 4 indicate that attenua- tion of the signal is inversely proportional to the metal conductivity. Once metal with lower conductivity is coated over a copper trace, the signal experiences a larger amount of attenu- ation. For instance, skin depth becomes 0.026 mils at a signal frequency of 10 GHz. If ENIG Article by Chang Fei Yee KEYSIGHT TECHNOLOGIES This article briefly introduces different types of metal plating commonly used in PCB fab- rication. Subsequently, the influence of metal plating on PCB channel loss (i.e., insertion loss or S21) and impedance (i.e., time domain reflectometry or TDR) is studied with 3DEM modeling using Keysight EMPro. Introduction Metal plating that serves as a protective layer is applied on top of the copper traces during PCB fabrication, thus alleviating the oxidation process of the copper. Common finishes include immersion silver (IAg), electroless nickel immersion gold (ENIG), etc. With immersion silver, nearly pure silver (i.e., ~0.02 mils in thickness) is coated over the copper traces on a PCB. Meanwhile, with ENIG, nickel (i.e., ~0.2 mils in thickness) is deposited on the copper trace followed by a coating of gold (i.e., ~0.01 mils in thickness) on top. Nickel serves as a barrier layer to prevent the migration of gold into the base copper. 3DEM Modeling: Influence of Metal Plating on PCB Channel Loss and Impedance

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