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Acid Copper Bath for Through-Hole Plating

IP.com Disclosure Number: IPCOM000047131D
Original Publication Date: 1983-Sep-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 3 page(s) / 32K

Publishing Venue

IBM

Related People

Bindra, P: AUTHOR [+6]

Abstract

This acid copper bath has a high throwing power suitable for through-hole plating in printed circuit boards. The acid copper plating bath contains copper sulfate, free sulfuric acid, chloride ions and an organic additive of the polyglycol family with a molecular weight greater than 600 g/mole. In prior acid copper plating baths, a bright or brilliant finish of the plated metal is obtained, with minimum control over the throwing power specified. Additives for increasing the leveling effect of acid copper plating baths have been developed. The effect of these additives on throwing power enhancement, particularly for through-hole plating, is not clear.

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Acid Copper Bath for Through-Hole Plating

This acid copper bath has a high throwing power suitable for through-hole plating in printed circuit boards. The acid copper plating bath contains copper sulfate, free sulfuric acid, chloride ions and an organic additive of the polyglycol family with a molecular weight greater than 600 g/mole. In prior acid copper plating baths, a bright or brilliant finish of the plated metal is obtained, with minimum control over the throwing power specified. Additives for increasing the leveling effect of acid copper plating baths have been developed. The effect of these additives on throwing power enhancement, particularly for through-hole plating, is not clear. With this bath, surface-active, organic additives used in conjunction with chloride ions in acid copper plating baths change the rate of one of the steps of the electrode reaction, or introduce a new step into the reaction mechanism. The total overvoltage (h) during the galvanostatic deposition of a metal may be divided into its various components by

(Image Omitted)

are the charge transfer, diffusion and crystallization overpotentials, respectively. The particular overpotential with the highest value determines the rate-controlling step. The rate- determining parameter for charge-transfer rate control is the exchange current density, io . The addition of the additive to the acid copper plating bath lowers the exchange current density. For constant current plating, this results in a substantial increase in electrode polarization. Thus, copper deposition occurs at a large overpotential, resulting in an increased nucleation rate. The formation of a dense, uniform copper film occurs. A surprising and highly beneficial aspect of the improved acid copper plating bath occurs during potentiostatic plating. A limiting current-type phenomenon is observed in substrates such as glassy carbon, gold and copper. This limiting current is significantly lower than the calculated value for a diffusion-limiting current. Under these circumstances, the plating rates on a planar surface and inside a through hole are similar. This leads to uniform plating thickness on the surface and inside the through hole. The effective additive appears to be a chloride substitute of the organic polyglycol. This effective additive is generated in situ by adding a suitable chloride salt, such as sodium chloride or hydrochloric acid, prior to or after addition of the organic glycol. The concentration of the chloride ions, which is in the range of 40 to 60 ppm, must always exceed the concentration of the organic additive. It is postulated that the effective organic molecule is generated on the plating surface by a reaction of the following type: H(R)nOH + Cl- H(R)nCl + -OH

where R = O(CH2-CH2)p The present bath is illustrated in further detail in the examples which follow: Example 1 A rotating

1

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disc system was employed wherein the substrate to be plated was centrally posit...