Finite Element Modelling of Copper Foil Loss from AFM Measurements
Copper roughness has a negative influence on insertion loss in PCBs. Different models linking copper roughness and losses exist in the literature, but all of them make different assumptions on surface geometry (triangular or rectangular asperities, spherical nodular treatment etc.). While these models are very useful in practice, the assumptions on which they are based limit their validity to some cases. In the present paper, we propose a finite element modelling approach to compute the additional losses due to roughness. First, the surface topography is measured by AFM on different kind of copper foils (with different level/geometry of roughness). Then, the results of the roughness measurements are implemented in the FEM software as an input. A magnetic field is applied on the surface of the meshed AFM data, and the Maxwell equations are solved numerically to determine the roughness additional losses as a function of frequency for each kind of foil. Finally, these results are used to compute the insertion loss of a whole track (laminate + copper foil) and compared with experimental data. This new approach is very promising as it allows to compute roughness additional loss on real surfaces by avoiding any assumption on the topography shape.
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