Solution Equilibria and Electroless Copper Deposition in Alkaline Formaldehyde-Containing Solutions

The industrial electroless copper plating solutions containing formaldehyde as reducing agent are known from the middle of this century and are widespread in the practice up to now. However, the investigation of this system remains actual because of its complexity, e. g.alkaline medium, hydrated form of formaldehyde (methanediol), Cu(II) complex compounds with different ligands, additives which improve the properties of the coatings obtained and/or protect from the reduction of copper(II) in the solution bulk. Moreover, various equilibria are possible including deprotonation of methanediol, Cu(II) complex formation, additional interactions between components of the solution.

The aims and tasks of this work were to investigate or define more precisely equilibria in conventional alkaline solutions of electroless copper deposition as well as influence of the nature of Cu(II) ligands on the copper deposition process.

This research report deals with the results obtained by means of dc-polarography, vis-spectrophotometry, voltammetry, potentiometry, pH-metry, ¹H and ¹³C NMR spectroscopy, scanning electron microscopy, X-ray diffractometry and gravimetry.

The composition, stability and the values of the diffusion coefficient of Cu(II) complex compounds with ligands (EDTA, DTPA, pyridine-2,6-dicarboxylic acid and 4-hydroxypyridine-2,6-dicarboxylic acid, Quadrol, L(+)-, D(–)- and DL()-tartrate, glycerol, saccharose and OH^- ions) which are used or usable in alkaline electroless copper deposition solutions were established. The formation of mixed complex compounds  between Cu(II) complexonates and K₄Fe(CN)₆, as well as between Cu(II) tartrate complexes and methanediol was investigated. The interaction of methanediol with L(+)-tartrate alongside with the influence of the ionic strength and cation nature on the deprotonation of methanediol in alkaline solutions were studied.

The kinetic data on electroless copper deposition from solutions containing above-mentioned ligands is presented and the influence of the ligand nature on the process of electroless copper deposition is discussed.