Metal alloy sulfonic acid electroplating baths

Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Depositing predominantly alloy coating

Reexamination Certificate

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C205S252000, C205S254000, C205S261000, C205S302000, C106S001250

Reexamination Certificate

active

06183619

ABSTRACT:

BACKGROUND OF THE INVENTION
Electroplating solutions are usually aqueous. Every plating solution contains ingredients to perform at least the first, and usually several, of the following functions: (1) provide a source of ions of the metal(s) to be deposited; (2) form complexes with ions of the depositing metal; (3) provide conductivity; (4) stabilize the solution against hydrolysis or other forms of decomposition; (5) buffer the pH of the solution; (6) regulate the physical form of the deposit; (7) aid in anode corrosion; and (8) modify other properties peculiar to the solution involved.
The present invention improves the plating performance of the solution, particularly by increasing the useful current density over previously accepted norms. The current density is the average current in amperes divided by the area through which that current passes; the area is usually nominal area, since the true area for any but extremely smooth electrodes is seldom known. Units used in this regard are amperes per square meter (A/m
2
).
It is generally in the best interest of efficiency to run electroplating baths at as high a current density as possible. The higher the current density, the faster the coating plates on the surface. As the current density increases, the thickness of the coating on the surface likewise increases. The current is carried by the ions in these baths and each type of ion has its own specific conductance. In a plating bath however, ionic conductance is only one variable that must be considered in choosing an electrolyte. The final criterion is the quality of the coating at the desired current density.
SULFONIC ACID BATHS
In the last decade the commercial use of sulfonic acid metal plating baths has increased considerably because of a number of performance advantages. See for example U.S. Pat. Nos. 5,750,017; 4,849,059; 4,764,262 and 4,207,150. This growth has slowed dramatically in the last few years because of large increases in the cost of the alkyl sulfonic acid. The preferred sulfonic acid used has been methane sulfonic acid (MSA) although the prior art includes examples of other alkyl and alkanol sulfonic acids. These other alkyl or alkanol sulfonic acids are more expensive than methane sulfonic acid and are therefore not competitive with methane sulfonic acid.
Several manufacturers produce salts of 2-hydroxy ethyl sulfonic acid (isethionic acid) commercially on a large scale but it is not commonly available in the free acid form. These salts are considerably less expensive than methane sulfonic acid but in the present plating technology only the acid form of the alkyl or alkanol sulfonic acid is used in the bath.
The performance advantages of alkyl sulfonic acid baths include low corrosivity, high solubility of salts, good conductivity, good oxidative stability of tine salts and complete biodegradability. The predominant metals plated in these sulfonic acid baths are tin, lead and copper as well as alloys of these metals with each other.
SUMMARY OF THE INVENTION
The present invention relates to the use of salts of alkyl and alkanol sulfonic acid which were found to improve the performance of sulfonic acid, especially alkyl sulfonic acid electroplating baths. Advantageously the salts are selected from the group consisting of alkali metal, alkaline earth metal, ammonium and substituted ammonium salts of 2-hydroxy ethyl sulfonic acid (isethionic acid).
When used in electroplating baths such as MSA, these salt additives were found to generally increase the plating range so that the baths can be used at much higher current densities. Thus these baths can achieve greater speeds than baths without these additives can. Further improvements are seen in the quality of the deposits. In the case of stannous alkyl sulfonate plating solutions some improvement in the oxidative stability of the tin was also observed.
As an added benefit, these salts are not harmful to the environment, they are completely biodegradable and the products of the biodegradation are common ions and molecules found in the environment. In addition they have a number of other advantages including high solderability, low corrosivity to equipment, good stability at high temperatures, and compatibility with many other metal salts.
Generally these baths will also contain the corresponding metal salt or metal salts if an alloy plate is required, and various additives to control the quality and appearance of the plated surface and the stability of the bath solution. Typical additives include a surfactant such as an ethoxylated fatty alcohol, a brightening agent if required and an antioxidant such as hydroquinone or catechol, if tin is one of the metals being plated.
The tin in these baths is in the stannous or reduced form. If oxidation occurs the tin will be converted to the stannic or oxidized form which then commonly precipitates to form sludge. This process adds to the inefficiency of these baths and also creates a requirement for constant filtering. Prior art patents, for example U.S. Pat. Nos. 4,717,460, 5,538,617 and 5,562,814, describe products that can decrease the amount of tin being oxidized.
Another advantage of using the salts of alkyl or alkanol sulfonates is that they are much less expensive than their corresponding acid. Currently the only bulk commercial alkyl/alkanol sulfonic acid suitable for electroplating is methane sulfonic acid and the only bulk commercial alkali/alkaline earth/ammonium alkyl/alkanol sulfonate salt suitable for electroplating is sodium isethionate. When comparing the price of these two large commercial products the sodium isethionate is less than half the price of the methane sulfonic acid either on a mole basis or on a weight basis.
DETAILED DESCRIPTION OF THE INVENTION
There has been little previous use of alkali/alkaline earth/ammonium alkyl/alkanol sulfonate salts in electroplating, and when used, the salts were first converted to acids. The present invention thus is directed to the direct use of these salts in electroplating. The use of such salts will enable the viability of inexpensive production technology such as the Steckler process to produce these salts. For example:
CH
3
Cl+Na
2
SO
3
−CH
3
SO
3
Na+NaCl
In this reaction, the sodium chloride can be crystallized out and the resulting sodium methane sulfonate can then be used in an electroplating bath.


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Meibuhr et al.,Noble Metal Resistors in Microcircuits, “The Mechanism of the Inhibition of Stannous-Ion Oxidation by Phenolsulfonic Acid”, vol. 2, No. 9-10, Sep.-Oct. 1964 pp. 267-273.

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