Meaning
Synthetic copolymer compounds are used as the primary suppressor or carrier additive in acid copper electroplating baths to control the rate of metal deposition. Through the use of polyalkylene glycol, plating systems achieve a uniform distribution of copper across complex substrates by inhibiting the electrochemical reaction on the outer surfaces of high-aspect-ratio features. The polymer acts by adsorbing onto the cathode surface in the presence of chloride ions.
Surface Inhibition
Adsorption of the polymer creates a resistive barrier that increases the overpotential required for copper deposition, slowing the reaction. This inhibition is strongest at the high-current-density areas, such as the surface and corners of a printed circuit board. This localized reduction in plating rate allows the uninhibited areas to catch up, improving overall uniformity.
Polymer Structure
Molecular weight and chemical composition of the copolymer dictate its adsorption strength and solubility in the acid copper bath. High-molecular-weight formulations offer stronger suppression but are more difficult to rinse off and can accumulate in the filter media. This trade-off requires careful selection of the polymer grade.
Additive Depletion
Depletion occurs through drag-out and mechanical entrapping in the deposited copper. Automated dosing systems add the polymer based on ampere-hour usage to maintain the proper concentration. This replenishment prevents drift.
