Influence of Temperature on Electroless Copper Deposition from Formaldehyde-Containing Solutions Using 2-Hydroxy-1,2,3-Propanetricarboxylic Acid As Cu(II) Ligand

Electroless coating technique is one of the ways to obtain metallic coatings in which there is no usage of electric current. The industrial electroless copper plating solution containing formaldehyde as reducing agent are known from the middle of the last century and are widespread in the practice up to now. Electroless copper coatings are deposited on plastics, ceramics, polymers and other non-conductig materials. However many chemical compounds used in technological processes are hazardous for total environment, therefore the efforts are made to displace those substances with less hazardous or purely harmless compounds. Citric acid (2-hydroxy-1,2,3-propanetricarboxylic acid), forming sufficiently stable complexes with copper (II) ions in alkaline solutions, was found to be a suitable ligand for copper(II) chelating in alkaline (pH > 12) electroless copper deposition solutions.

The substrate was a smooth Pt sheet (1´1 cm) electroplated with Cu for 20 min from acid copper solution (1.0 mol l^-1 CuSO₄ + 0.5 mol l^-1 H₂SO₄) at 1.5 A dm^-2. Before the electroless plating the electroplated substrate was activated for 30 s in acid PdCl₂ solution (1 g l^-1). The main electroless copper plating experiments were carried out for 30 or 60 min. in 50 ml of the solution containing (mol l^-1): CuSO₄×5H₂O – 0.05; citrate – 0.3; formaldehyde – 0.15.

Our studies were carried out in electroless plating solutions at pH 12.0 – 13.0 and 10 – 30 °C temperature. It was found that the autocatalytic reduction of Cu(II) by formaldehyde from Cu(II)-citrate solutions containing begins at pH above 12 and accelerates with a further increase in pH and temperature. The thickness of the compact copper coatings obtained under optimal operating conditions in 1 h can reach ca. 3 μm. From Cu(II)-citrate solutions deposited copper coatings real surface areas (R_f) vary widely, i. e. from ca. 2.5 up to 15.0. The maximum R_f values were obtained at pH 12.5 solution at 20 °C temperature. Results were compared with those for systems operating with other copper (II) ligands.