Quasi-Optical Transmission Loss Extraction for High-Frequency Stackup Design
Quasi-optical extraction isolates intrinsic substrate loss from copper roughness, requiring explicit z-axis anisotropy conversion for accurate mmWave stackup design.
Surface Finish Loss
Conductive Plating Effect
Conductive plating effect quantifies the reduction in electrical power caused by the metal finish applied to the outer layers of a circuit board. The presence of surface finish loss is most pronounced in high frequency designs where the skin effect forces current to flow through the outermost metal layers. Finishes like electroless nickel immersion gold introduce higher resistance than the underlying copper.
This effect reduces the overall efficiency of the transmission line.
Differences in Resistivity
Variation in the electrical conductivity of different plating metals determines the severity of the attenuation. While gold is highly conductive, the nickel layer required for adhesion in many processes is relatively resistive. Other options like immersion tin or immersion silver provide lower surface finish loss by maintaining a conductivity closer to that of pure copper.
Designers must weigh the electrical performance against the shelf life and solderability of the finish.
Modern Accuracy
Simulation tools incorporate the conductivity and thickness of the plating layers to predict the total insertion loss of a trace. Failure to account for surface finish loss during the design phase can lead to unexpected operational failure in the final hardware. Fabricators use specialized test coupons, such as the SET2D02 or similar structures, to measure the actual loss of the finished board.
This verification ensures that the manufacturing process does not introduce more resistance than the link budget allows.