Reverse pulse plating composition and method

Abstract

A composition and method for electroplating a metal on a substrate. The composition has a chloride to brightener concentration ratio of from 20:1 to 125:1. The method of electroplating, which employs the composition, employs pulse patterns that improve physical properties of metal surfaces.

Description

BACKGROUND OF THE INVENTION The present invention is directed to a reverse pulse plating composition and method. More specifically, the present invention is directed to a reverse pulse plating composition and method that reduces brightener decomposition and reduces defects of an electroplated metal layer. Numerous compositions and methods for electroplating articles with metal layers or coatings are employed in many industries. Such methods may involve passing a current between two electrodes in a plating composition or solution where one of the electrodes is an article to be metal plated. Using an acid copper plating solution for purposes of illustration, a plating solution may contain (1) dissolved copper (cupric ions), usually copper sulfate, (2) an acid electrolyte such as sulfuric acid in an amount sufficient to impart conductivity to the solution, and (3) additives to improve efficiency of the plating reaction and the quality of the metal deposit. Such additives include, for...

AI Technical Summary

Benefits of technology

Advantageously, the compositions and methods prevent or at least reduce dendrite or whisker formation on metal plated substrates, reduce dog boning as well as intermittent surface roughness, and provide a uniform metal layer on the substrates. Other advantages include improved leveling performance, improved throwing power and reduced corner cracking. Also additive decomposition is reduced to provide electroplating baths having a longer op

Problems solved by technology

However, as printed circuit boards became more complex and as industry standards became more rigorous, solutions used for decorative plating were found to be inadequate for printed circuit board fabrication.

One serious problem encountered using electrolytic metal plating solutions involved coatings of uneven thickness on the walls of the through-holes with metal deposits thicker at the tope and bottom of the through-holes and thinner at the center, a condition known in the art as “dog boning”.

The thinner deposit at the center of the through-hole may lead to circuit defects and board rejection.

As the aspect ratio and the thickness of the board increase, dog boning becomes more severe due to a voltage drop between the surface of the board and the center of the through-hole.

Multi-layer circuit fabrication results in an overall increase in the thickness of the board and a concomitant increase in the length of an interconnection passing through the board.

This means that increased circuit densification results in increased aspect ratios and through-hole length and an increase in the severity of the dog boning problem.

Another problem encountered in metal electroplating are defects such as intermittent surface roughness and non-uniform surface appearance of the plated metal.

Intermittent surface roughness and non-uniform surface appearance are believed to be caused by non-uniform current distribution across the surface of the printed wiring board that is being plated.

The non-uniform current distribution results in an unequal or uneven deposit of metal on the board surface resulting in the surface roughness and non-uniformity of plated metal layers.

Another defect, which is often observed, is the formation of dendrites or “whiskers”.

For example, whiskers are readily detached and carr

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