Roughness Metric
The arithmetic average of the absolute values of profile heights and depths across a given length provides a general assessment of surface texture. Measuring surface roughness Ra allows fabricators to monitor the quality of the copper foil and the dielectric surface before bonding. This parameter is used as a baseline for ensuring that the chemical etching or mechanical abrasion steps have achieved the desired level of tooth for the resin to grab.
It is the most common industry standard for describing the finish of a material.
Adhesion Strength
Consistent texture ensures that the peel strength of the copper remains within the specified limits. When the surface roughness Ra is too low, the risk of delamination during thermal stress increases significantly. Conversely, an excessively rough surface can lead to uneven plating and copper residues after etching.
Signal Performance
High-speed digital signals are sensitive to the irregularities at the conductor interface. While a high surface roughness Ra helps with mechanical bonding, it also increases the resistive losses for signals operating in the microwave range. Designers of high-frequency boards often specify a maximum value for this metric to minimize the skin effect.
Optical profilometers or stylus-based tools are used to verify that the incoming material meets these strict requirements. Maintaining this balance is essential for both the structural integrity and the electrical performance of the finished circuit.
