Sidewall Integrity
Degree of verticality of the edge of a conductive trace defines the shape of its cross section. The sidewall slope angle is measured from the base of the trace to the top edge, where a ninety degree angle represents a perfectly rectangular profile. In standard subtractive etching, the angle is typically less than ninety degrees because the chemical etchant attacks the copper from the top and sides simultaneously.
Electrical Impact
Signal performance at high frequencies is sensitive to the shape of the trace because of the skin effect. If the sidewall slope angle is too low, the trapezoidal shape of the trace changes the characteristic impedance of the circuit. This can lead to signal reflections and loss of data integrity in high speed digital designs.
Manufacturers optimize the etching process and the type of photoresist used to keep the walls as vertical as possible. A steeper angle also allows for tighter spacing between traces, which increases the overall density of the board. Cross sectional imaging is used to measure this angle and verify that the fabrication process is under control.
Process Variation
Consistency in the trace profile ensures that the electrical behavior of the board is predictable across different production runs. Reliable geometry is a primary factor in the design of controlled impedance circuit boards.
