Statistical Metric
A statistical measurement represents the root-mean-square deviation of a profile from its mean line to quantify surface texture. Utilizing surface roughness Rq quantifies the height deviations of copper traces because the squaring of height values amplifies larger deviations. This parameter helps laminate manufacturers characterize the adhesion surface of copper foils with greater statistical weight than average roughness.
Electrical Impact
High-frequency circuit performance is highly sensitive to the height distribution of microscopic copper peaks. In high-speed designs, surface roughness Rq directly influences the skin effect by forcing the electrical current to travel a longer path along the rough surface profile. This increased path length raises the effective conductor resistance and leads to higher insertion loss at gigahertz frequencies.
Consequently, ultra-low-profile copper foils are developed with very low values of this metric to ensure high signal integrity, maintaining values below zero point three micrometers for demanding multi-gigabit board designs.
Characterization Method
Optical profilometers use non-contact methods like white-light interferometry to scan the copper surface and calculate the profile value. The calculation sums the squared distance of each measured point from the mean surface plane and takes the square root of the average. This metric is more responsive to isolated scratches or high spikes than average roughness.
Fabricators use these scans to monitor the quality of micro-etching processes before multilayer lamination.
