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.
Stripping Chemistry
Solvent Dissolution
Aqueous or semi-aqueous media dissolve organic polymers from a printed circuit board surface during the reprocessing phase. Stripping chemistry acts by breaking the chemical bonds of photoresist layers or cured solder masks to lift them away from the copper substrate. Selective formulations target specific cross-linked networks without attacking the underlying laminate or the metallic circuit traces.
Chemical performance degrades over time as the concentration of dissolved polymer solids increases within the bath.
Bath Maintenance
Effective process control monitors the specific gravity and the refractive index of the solution to track the load of removed material. Operators adjust the temperature and the flow rate to maintain constant removal speeds as the active ingredients deplete. Constant agitation prevents the redeposition of semi-dissolved flakes onto the board features.
Titration confirms the molar concentration of the active agents remains within the operating window defined by the technical data sheet.
Contamination Prevention
Incomplete removal of residues leaves ionic tracks that cause dielectric breakdown or electrochemical migration between fine-pitch conductors. Residual chemistry trapped under surface mount components promotes corrosion during subsequent environmental exposure. Rigorous rinsing steps after the chemical bath remove both the dissolved polymer and the active agents themselves.
High pressure water sprays force particles out of blind vias and tight spaces to ensure a clean finish prior to further assembly operations.