Experimental work was undertaken to determine the mechanism controUing the co-deposition of aluminas with copper during electrode-position of copper. The aluminas were present in the copper plating electrolytes as an insoluble disperse phase.

Three possible mechanisms for co-deposition of aluminas were studied, They were mechanical inclusion, electrophoretic deposition, and adsorption.
The results of experimental studies established that mechanical inclusion was not a significant factor in the mechanism controlling co-deposition of aluminas with copper.

Theoretical considerations of the compositions of the copper plating electrolytes indicated that co-deposition of aluminas by an electrophoretic mechanism was highly unlikely. The conclusions arrived at by theoretical considerations were substantiated by experimental measurements of the zeta potential of the aluminas.

The alumina content of the copper electra deposits was also studied as a function of the pH of the plating bath, The results of these tests in conjunction with sedimentation studies demonstrated the absence of an isoelectric point for the aluminas over the pH range studied.

The presence of thiourea in the electrolytic plating baths (a sub stance known to be adsorbed on a copper cathode during electrodeposition tion), profoundly affected the amount of alumina in the electrodepasit, however, no adsorption of thiourea on aluminas in aqueous dispersions was detected.