Extracting In-Plane Dielectric Permittivity Values from Clamped Stripline Measurements
Clamped stripline measurements extract true in-plane dielectric permittivity when analytical models eliminate air gap capacitance errors.
Copper Foil Surface Roughness
Material Profile
Microscopic tooth structure on a printed circuit board conductive layer determines the adhesion of metal to the underlying dielectric substrate. Copper foil surface roughness affects signal loss at high frequencies due to the skin effect, which forces current to travel along the outer boundary of the conductor. In high speed digital design, this profile governs the trade off between peel strength and insert loss up to the frequency limits where dielectric loss becomes dominant.
Loss Mechanism
As signal frequencies rise above a few gigahertz, the depth of current penetration decreases until it is less than the peak to valley height of the metal profile. When this condition occurs, the electrical path length of the signal increases as the current must navigate the irregular contours of the foil. This added path length increases the resistive losses of the transmission line, which degrades the rise times of high speed digital signals.
If left unchecked during high speed layout, the attenuation from these resistive losses will close the eye diagram at the receiver, resulting in bit errors.
Measurement Practice
Profilometers use optical or tactile sensors to measure the average roughness parameters on a test coupon before board lamination. Accurate values are necessary for electromagnetic simulators to predict board behavior, especially for sub five micron heights. Profiling scans typically record roughness on both the drum side and the treated side of the foil to ensure uniformity across the entire laminate sheet.