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This article briefly introduces different types of metal plating commonly used in PCB fabrication. 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.
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.
However, metal plating comes with disadvantages. 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 frequency 10 GHz, skin depth becomes 0.026 mils.
Equations 2, 3, and 4 indicate that attenuation 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 attenuation. For instance, skin depth becomes 0.026 mils at a signal frequency of 10 GHz. If ENIG 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.
To read this entire article, which appeared in the February 2019 issue of Design007 Magazine, click here.