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.
Differential Etching
Subtractive Processing
Chemical subtractive processes used in high-density board fabrication allow for the precise modification of copper cladding thicknesses across the circuit board. Through differential etching, manufacturers remove a thin, uniform layer of copper from the entire board surface while leaving the thicker plated traces intact. This technique is particularly important in modified semi-additive processes where a thin seed layer of copper must be cleared without destroying the narrow traces that form the circuitry.
Etchant Dynamics
Controlling the chemistry of the etchant spray and the exposure duration allows for precise metal removal without causing excessive undercut. During differential etching, the board travels through a conveyorized spray chamber containing an acidic or alkaline solution. This solution dissolves the thin copper layer on the open areas of the board while the dry film resist or the thick plated copper of the trace absorbs the primary chemical attack.
Because the thin seed layer is typically only a few micrometers thick, the etchant only remains in contact with the board for a very short period. This rapid action minimizes the lateral etching that would otherwise reduce the width and the cross-sectional area of the trace.
Line Precision
Post-etch inspection ensures that the remaining circuit traces maintain the correct geometric profiles to meet impedance and resistance specifications. If differential etching is incomplete, thin micro-bridges of copper may remain between adjacent lines, causing short circuits that fail electrical tests. Conversely, over-etching reduces the cross-sectional area of the trace, which increases electrical resistance and alters the characteristic impedance of the transmission line.
Fabricators use automated optical inspection to verify the completeness of the copper removal before applying the solder mask.
Evaluating Semi Additive Process Yield Factors for High Density Substrates
Semi-additive yield depends on seed layer adhesion, lithographic trench verticality, and differential etch undercut control across sequential build-up layers.