Foil Structure
Electrodeposited copper foils manufactured for high-speed circuit boards undergo specialized electroplating treatments applied to their outer drum-side surface rather than the matte dielectric side. Reverse treated copper features a smooth dielectric contact surface combined with a micro-roughened outer profile that faces the prepreg layer during lamination. This foil construction minimizes high-frequency conductor loss caused by the skin effect at the copper-dielectric interface.
At frequencies above 10 GHz, electrical current concentrates within the thin outer boundary of the conductor, making interface smoothness critical for low-loss transmission.
Profile Depth
Traditional electrodeposited copper foils possess deep dendritic roughness on the matte side to achieve mechanical peel strength, but these deep profile valleys degrade high-speed signal integrity by forcing current to travel longer paths around profile peaks. Reverse treated copper presents a low profile, typically under 2 micrometers root-mean-square roughness, to the underlying core laminate where high-frequency signal fields are concentrated. The treated outer surface provides sufficient mechanical tooth for adhesion to the bonding prepreg during lamination without disturbing the smooth interface on the signal side.
Substrate fabricators specify reverse treated foil to reduce conductor attenuation in high-speed digital backplanes while maintaining acceptable peel strength.
Adhesion Mechanism
Chemical treatment layers applied to the outer profile of reverse treated copper promote chemical bonding with resin systems. Peel strength testing confirms that mechanical lock and organosilane coupling prevent delamination during thermal stress testing.
