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 operating life.

A primary objective of the present invention is to provide a composition with reduced additive breakdown.

Another objective is to provide a composition that has an improved electroplating life.

A further objective of the present invention is to provide a method of metal plating a substrate that reduces metal plating defects.

Still yet another objective is to provide a method of plating a metal that has an improved throwing power.

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 carried by cooling air flows into electronic assemblies, both within and external to electronic article housings, where they may cause short-circuit failure.

However, such a current density is difficult to achieve.

When additives breakdown during plating, the breakdown products may result in metal layer deposit characteristics that are less than satisfactory for industry standards.

However, monitoring the concentrations of the additives that improve metal plating is still difficult because additives are present in plating baths in small concentrations, i.e., parts per million of solution.

Accordingly, the level of the additives in the bath eventually changes such that the additive concentrations are out of the acceptable range of tolerance.

If the additive concentration goes too far out of range of tolerance, the quality of the metal deposit suffers and the deposit may be dull in appearance and / or brittle or powdery in structure.

Other consequences include low throwing power and / or plating folds with bad leveling.

Many of the foregoing problems are found in reverse pulse plating baths and methods.

However, such waveforms often result in undesirable intermittent surface roughness and non-uniform surface appearance on plated metal layers, especially at current densities of 100 amps / cm2.

Another problem with reverse pulse plating baths is their short bath life, which may be in terms of a few days, i.e., two to three days, of optimum performance.

The short life of a reverse pulse plating bath is due to additive breakdown, especially due to the build-up of brightener by product.

Poor throwing power results in rough metal surfaces and non-uniform metal layers.

However, high brightener concentrations may result in high concentrations of byproducts, which may shorten the electroplating bath life.

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