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/m2).
It is also in the best interest of efficiency to run these plating baths at as high a current density as possible. The higher the current density the faster the coating plates on the surface. The current is carried by the ions in these baths and each type of ion has its own specific conductance. In 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.
A variety of metals and metal alloys are commercially plated from solutions with sulfate as the primary anion. See for example U.S. Pat. Nos. 4,347,107; 4,331,518 and 3,616,306. Certain sulfate electroplating baths have limitations that can sometimes be alleviated with the addition of additives including other anions. For example the steel industry has been tin plating steel for many years from sulfuric acid/tin sulfate baths where phenol sulfonic acid is used as a special electrolyte additive which improves both the oxidative stability of the tin as well as increasing its current density range. This is known as the ferrostan process but because of environmental problems with phenol derivatives the steel industry is looking to replace this bath with one which is less harmful to the environment.
Similarly nickel sulfate is used for nickel plating but nickel chloride must also be present to provide enough conductivity and improve anode dissolution. This bath is known as the Watts bath but although economical, suffers from a number of disadvantages including a nickel plate that is highly stressed.
It is therefore worthwhile to identify other additives that can improve the performance of metal sulfate electroplating baths. There are many examples in the prior art where surfactants and other organic additives are used to provide a more desirable finish. In the case of tin the prior art also describes additives which can improve the oxidative stability of the tin and therefore provide a bath with less sludge formation. It is less common to find examples of additives which improve the plating range especially at the high current density end. Increasing the current density is a very desirable feature but it has been difficult to identify additives which can do this without creating other problems in the bath.
Many plating baths are also very sensitive to the presence of impurities and often as impurities build up in the bath they affect the quality of the deposit. Therefore either these impurities must be removed or the baths must be replaced. For example in tin plating steel, iron builds up in the bath and eventually affects the quality of the deposit and must be removed. It is very desirable to find additives that will make the bath less sensitive to these impurities.
The present invention relates to the use of alkali metal, alkaline earth metal, ammonium and substituted ammonium salts of alkyl and alkanol sulfonic acid which have been found to improve the performance of sulfate electroplating baths. When used in these electroplating baths these salt additives were found to generally increase the plating range so that these baths can be used at much higher current densities. Thus these baths can be run at greater speeds than those without these additives. Further improvements are seen in the quality of the deposits. In the case of stannous sulfate plating solutions, some improvements in the oxidative stability of the tin was also observed.
Thus, the present invention is directed to a method of improving the plating performance of an aqueous sulfate based electroplating bath comprising the step of adding an effective performance enhancing amount of a salt of an alkyl and/or alkanol sulfonic acid to said bath.
The salts used to improve the bath plating performance characteristics are particularly selected from the group consisting of alkali metal, alkaline earth metal, ammonium and substituted ammonium salts. Especially preferred are salts of 2-hydroxy ethyl sulfonic acid, especially the sodium salt (sodium isethionate).
The baths that can be improved by the present invention include tin and tin alloys, nickel and nickel alloys, copper and copper alloys, chromium and chromium alloys, cadmium and cadmium alloys, iron and iron alloys, rhodium and rhodium alloys, ruthenium and ruthenium alloys, and especially the iron/zinc and tin/zinc alloy plating baths.
Preferably, tin and tin alloy baths are improved by this invention. Examples include tin-antimony, tin-cadmium, tin-copper, tin-lead, tin-nickel, tin-niobium, tin-titanium, tin-zirconium, and tin-antimony-copper, tin-silver, tin-bismuth, tin-copper-silver, tin-copper-bismuth alloy baths. Alloy compositions comprising these metals are well known to those having ordinary skill in this art, and are the subjects of numerous patents.
The use of alkali metal, alkaline earth metal, ammonium and substituted ammonium salts of alkyl and alkanol sulfonic acids as additives in pure metal and metal alloy sulfate electroplating baths has a number of unexpected benefits including wider useful current density range, improved appearance and in the case of tin improved oxidative stability.
Thus these baths can be run at greater speeds than those without these additives. Further improvements are seen in the quality of the deposits and greater tolerance to impurities such as iron. In the case of stannous sulfate plating solutions some improvements in the oxidative stability of the tin was also observed.
Unlike phenol sulfonic acid 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 solubility, low corrosivity to equipment, good stability at high temperatures, and compatibility with many other metal salts.
These baths also contain the corresponding metal salt or metal salts if any 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 a sludge. The process adds to the inefficiency of these baths and also creates a requirement for constant filtering. Several patents, for example U.S. Pat. Nos. 4,717,460, 5,538,617 and 5,562,814, describe additives and/or processes that can decrease the amount of tin being oxidized.
The present invention will be further illustrated with reference to the following example which will aid in the understanding of the present invention, but which is not to be construed as a limitation thereof. All percentages reported herein, unless otherwise specified, are percent by weight. All temperatures are expressed in degrees Celsius.