A method of removing a metal skin from a through-hole surface of a copper-Invar-copper (CIC) laminate without causing differential etchback of the laminate. The metal skin includes debris deposited on the through-hole surface as the through hole is being formed by laser or mechanical drilling of a substrate that includes the laminate as an inner plane. Removing the metal skin combines electrochemical polishing (ECP) with ultrasonics. ECP dissolves the metal skin in an acid solution, while ultrasonics agitates and circulates the acid solution to sweep the metal skin out of the through hole. ECP is activated when a pulse power supply is turned on and generates a periodic voltage pulse from a pulse power supply whose positive terminal is coupled to the laminate and whose negative terminal is coupled to a conductive cathode. After the metal skin is removed, the laminate is differentially etched such that the copper is etched at a faster rate than the Invar. To prevent the differential etching, a copper layer is formed on a surface of the substrate with an electrical resistance R1 between the copper layer and the positive terminal of the pulse power supply. Additionally, an electrical resistance R2 is formed between the laminate and the positive terminal of the pulse power supply. Adjustment of R1 and R2 controls the relative etch rates of the copper and the Invar.
Dr. Madhav Datta, Galasco, R. T., Lehman, L. P., Magnuson, R. H., Susko, R. A., and Topa, R. D., “Removal of metal skin from a copper-Invar-copper laminate”, U.S. Patent US 09/347,5812001.