Modified Semi Additive Process Stackup Selection Guidelines
Modified semi additive process stackups require ultra-thin copper seeds under three microns to achieve sub-twenty-micron traces with stable impedance.
Line Width Space
Trace Geometry
Conductive tracks on a printed circuit board must comply with specific dimensional tolerances to prevent electrical shorts and ensure reliable signal transmission. The line width space defines the width of the printed copper trace and the distance between that trace and its adjacent conductor. This geometry determines the routing density that can be achieved on a single layer of the board.
Lithographic Resolution
Photolithographic processing dictates the minimum limits of these copper features by the resolution of the imaging equipment. During exposure, light passes through a photo-tool onto a light-sensitive resist, which is then developed to leave the desired circuit pattern. If the exposure is incorrect, the line width may increase while the gap between the traces decreases, which can lead to electrical bridging during the etching process.
Fine-line technologies require advanced direct imaging systems to achieve dimensions below fifty microns with high yield. This precision allows designers to fit more interconnects into a smaller surface area, which is essential for modern high-density designs.
Electrical Performance
Impedance calculations for high-frequency signals depend heavily on both the width of the conductor and the spacing to adjacent copper. Variations in these dimensions alter the characteristic impedance of the transmission line, which can cause signal reflections and degrade the signal integrity. This dimensional stability is verified using automated optical inspection after the etching step.
This inspection step ensures that any trace defects are caught before the board is laminated.