Surface Dimension
Measurement of the top surface of a conductive trace provides the effective area for electrical contact and signal transmission. A crown width specifies the narrowest portion of the copper feature in a standard subtractive etching process. This value dictates the resistance of the trace and the performance of high speed signals.
Trapezoidal Geometry
Etching chemicals typically narrow the upper surface of a circuit line more than the bottom area. Because of this taper, the crown width is always smaller than the base measurement in conventional pcb fabrication. When the etch process is not controlled, the top of the trace can become too thin to support reliable soldering or wire bonding.
Precise control of the spray nozzles and chemical concentration ensures the top surface remains within the design limits. Capacitive coupling between adjacent traces on the same layer is also affected by this narrow surface area.
Optical Measurement
Automated inspection systems use light to detect the edges of the copper at the highest point of the trace. The measured crown width allows the system to calculate the etch factor and predict the electrical performance of the finished board. Inconsistent top dimensions indicate problems with the photoresist adhesion or the etching chemistry.
