Surface Finish Layer Thickness Limits for High Density Circuit Boards
High density circuit board surface finishes require tight thickness boundaries to prevent nickel hyper-corrosion, black pad, and solder joint embrittlement.
Skin Effect Attenuation
Resistive Loss
Electromagnetic phenomena where alternating current tends to flow primarily near the surface of a conductor lead to increased signal loss at high frequencies. This skin effect attenuation reduces the effective cross-sectional area of a copper trace as the frequency of the signal increases. It is a fundamental concern in the design of transmission lines for high-speed digital and rf circuits.
Frequency Dependency
As the frequency rises, the depth of current penetration, known as skin depth, decreases. In the context of skin effect attenuation, the resistance of the conductor increases because the current is confined to a thin layer near the outer edge. For a copper trace at ten gigahertz, the skin depth is approximately point-six-six microns.
This means that a large portion of the copper thickness is not used for current conduction.
Surface Quality
Roughness of the copper foil surface increases the magnitude of this loss. When the surface profile is larger than the skin depth, skin effect attenuation is worsened because the current must travel a longer path over the peaks and valleys of the copper. Treatments designed to improve adhesion between copper and resin often conflict with the need for smooth surfaces to minimize these losses.