2SnOH4 SnOHii Sn 20H

This tin will codeposit with tin from the stannate ions, causing the rough spongy deposits mentioned above.

In the alkaline systems, two factors tend to restrict the usable current density range and limit the deposition rate. One factor is the solubility of the stannates in hydroxide solutions. With the sodium formula, the normal increase is not possible, because sodium stannate is one of the unusual salts that have a reverse temperature coefficient of solubility. An example of this process is given in Table 2. Less sodium stannate dissolves as the electrolyte temperature increases, which reduces the usable current density and the plating rate. Potassium stannate is more soluble with increasing temperature, but as the stannate increases, the potassium hydroxide must also increase. Stannate solubility decreases as the hydroxide content increases.

The second factor is that cathode efficiency decreases as current density increases. Eventually, a point is reached at which these factors become offset, and a further increase in current density does not increase the deposition rate. This limits the rate at which tin can be deposited.

In specialized applications, such as plating the inside of oil-well pipe, it is not possible to have an anode surface sufficient enough to avoid passivity. A higher current density can be used if insoluble anodes are utilized, but tin deposited on the cathode must then be replaced by the addition of chemicals. The addition of stannate to provide the tin cations also adds sodium or potassium hydroxide to the electrolyte. Although the resulting additional alkalinity can be neutralized by adding a calculated amount of an acetic acid, the sodium or potassium ion concentration continues to increase and the alkaline stannate solubility is reduced. This, in turn, reduces the available Sn+4 ion to a low enough concentration that the plating rate decreases rapidly, and the electrolyte must be discarded. A potassium-base composition has been developed, in which the necessary Sn+4 ions are added to the electrolyte as a soluble, colloidal, hydrated tin oxide (Ref 2). Because the potassium ion concentration builds up more slowly in this composition, electrolyte life is nearly indefinite. The throwing power of alkaline stannate solutions is quite high, allowing the coating of intricate shapes and interior parts of cathodes.

Acid Electrolytes. Several acid electrolytes are available for tin plating. Two of these--stannous sulfate and stannous fluoborate--are general systems that are adaptable to almost any application. Electrolytes such as halogen (a chloridefluoride base system) and Ferrostan (a special sulfate-base system) have been developed for tin coating cold-rolled steel strip traveling at high speed for the production of tinplate. The acid electrolytes differ from alkaline electrolytes in many respects. A stannous salt that is dissolved in a water solution of the applicable acid does not produce a smooth, adherent deposit on a cathode. Therefore, a grain-refining addition agent (such as gelatin or peptone) must be used. Usually, such materials are not directly soluble in a water solution, and a wetting-agent type of material (such as P-naphthol) is also necessary.

Organic brighteners can be added if a bright-as-coated electrodeposit is desired. This produces a coating that looks the same as a reflowed tin coating. Over time, these brighteners will decompose in the bath and must be replenished. The composition of these organic brighteners has been the subject of considerable research over the years. The earliest substance studied, in the 1920s, was wood tar dispersed with a wetting agent. Other materials were studied in later years, especially pure compounds such as cresol sulfonic acid and various aromatic sulfonates. These were seen to have more of a stabilizing effect, preventing the hydrolysis and precipitation of tin as tin(II) and tin(IV) salts. Later work has shown that a "cruder" material is more effective as a brightener. Such a material is obtained by the sulfonation of commercial cresylic acid. The implication here is that by-products of the sulfonation and not the cresol sulfonic acid itself are responsible for the brightening of the tin coating. Various proprietary brightening systems have been produced over the years. Very little of the development work on brightening agents has been published outside the patent literature. A comprehensive discussion of the topic is beyond the scope of this article. It is usually most convenient to purchase a packaged system from a plating supply house.

The organic materials will co-deposit with the tin, resulting in a higher than normal carbon content in the electrodeposit. This does not create a problem, unless the tin coating is to be soldered or reflowed. The supplier of the proprietary bath should be consulted for directions on controlling this problem.

To retard the oxidation of the stannous tin ions to the stannic form, either phenolsulfonic or cresolsulfonic acid is added to a sulfate-base system, and hydroquinone is added to a fluoboric acid-base system. Although the acid electrolytes can contain large amounts of stannic ions without affecting the operation of the system, only the stannous ions are deposited at the cathode. As a result, oxidation depletes the available stannous ions, which must be replaced by adding the corresponding stannous salt to the bath. To limit the oxidation of stannous ions, a sufficient anode area must be maintained, and the operating temperature must be kept as low as possible. In addition, one must avoid introducing oxygen into the solution, either by a filter leak or air agitation. Usually, an antioxidant is added to the solution.

In terms of operating characteristics, the basic differences between acid and alkaline electrolytes are related to the type of tin ion that is present in the electrolyte. In acid systems, the stannous ions must not be oxidized to the stannic form, and operation must occur at lower temperatures. The acid electrolytes require only half as much current to deposit one gram-molecule of tin. The tin dissolves directly from the metallic anodes, and the control of an anode film is not involved. Acid electrolytes are nearly 100% efficient, both anodically and cathodically, which avoids the necessity of regularly adding chemicals for tin. The problems of oxygen gas evolution at the anode surface and hydrogen gas at the cathode surface are reduced. Some particulate matter is produced as sludge from three sources: anode slime products, the precipitation of addition agents and their breakdown products, and basic tin compounds formed by oxidation. These materials must be removed during operation. In a still tank, the precipitates gradually settle, but agitated solutions require continuous filtration.

Acid-resistant equipment must be used. Lead-lined plating tanks were formerly used, but stoneware, rubber- or plastic-lined steel, or plastic tanks are now more common. Filtration equipment should be available, because solid particles of precipitated matter in the solution will cause deposit porosity and roughness. With still baths, suspended matter can be allowed to settle without filtration, but with agitated baths, continuous filtration is advisable. Cathode bar movement is often recommended.

The stannous sulfate electrolyte is most popular because of its general ease of operation. The rate of deposition is somewhat limited by optimum metal concentration in the electrolyte. A still bath is operated at a cathode current density of 1 to 2 A/dm2 (10 to 20 A/ft2 ). Current densities of up to 10 A/dm2 (100 A/ft2) are possible with suitable electrolyte agitation. Higher current densities will result in burned deposits. The anode surface area must be increased when higher current densities are used, otherwise the anodes will become passive. Addition agent control is not quantitative in nature, but deficiencies are easily recognized by the experienced plater. An electrolyte can be prepared from readily available chemicals, or a proprietary system can be purchased from suppliers. Most commercial bright acid tin processes and the more recent matte acid tin systems are based on the stannous sulfate solution. Precise information on operation and control should be obtained directly from the specific supplier.

The stannous fluoborate electrolyte is a good general-purpose electrolyte. It can operate at higher current densities because of the conductivity provided by the fluoboric acid. Cathode current densities of 20 A/dm2 (200 A/ft2) and higher are possible with suitable solution agitation. The need to increase anode surface area at high current densities and the control of the addition agents parallel the requirements associated with using stannous sulfate. Table 4 gives standard, high-speed, and high-throwing-power electrolyte compositions, because each meets a specific need. The solution conductivity that is lost because of the lower metal content in the high-throwing-power bath is compensated for by the higher concentration of fluoboric acid. The lower total metal in the solution reduces the variance in deposit thickness that is usually associated with varying areas of cathode current density. Boric acid is listed as a constituent of the fluoborate solutions because of its presence in the stannous fluoborate and fluoboric acid used to prepare the solutions. It is not a necessary ingredient in the electrolyte.

References cited in this section

1. F.A. Lowenheim, Ed., Modern Electroplating, 3rd ed., Wiley-Interscience, 1974

2. S. Hirsch, Tin-Lead, Lead, and Tin Plating, Metal Finishing Guidebook and Directory Issue, Vol 91 (No. 1A), Jan 1993, p 269-280

3. J.W. Price, Tin and Tin Alloy Plating, Electrochemical Publications Ltd., Ayr, Scotland, 1983 Lead Plating

Revised by George B. Rynne, Novamax Technology

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