Electroplating continued processes

First, water and often oxygen must be excluded. This can be done rather easily in continuous processes, such as plating a wire or a metal sheet. In most applications, however, electroplating is typically a batch process - parts are introduced and removed from the bath regularly. Whereas the technology to perform such operations exists, it is more expensive and much less convenient than operation in an open aqueous bath. Most nonaqueous solvents are either flammable or toxic or both. Most salts used to make up the bath are expensive, and some are quite unstable. Even a relatively inexpensive salt such as KBr can become very expensive when it must be very dry. 

The purification of electroplating baths can be carried out using a batch process or a continuous process. In the batch process, the bath solution is treated with activated carbon for 25 to 50 min and then filtered. It is ensured that it is clear and does not contain any activated carbon. The clear filtrate to be used again is the bath. The batch process can also be carried out by filtering the bath solution through a fabric coated with a suitable activated carbon. The frequency of the purification depends upon the operation of the electroplating bath, and the amount of carbon dose depends upon the degree of contamination of the bath, but usually it is 0.5 to 1.3 kg of activated carbon for each 100 liter of the bath solution. 

AppUcation of PVD coatings to coils of steel sheet in a continuous process was studied extensively in the United States during the 1970s, but the process was found to be more costly relative to other coating methods, such as electroplating and hot dipping. More recently, efforts are underway in Japan and Europe to further develop such a process. 

Steel strips are galvanized in continuous processes as shown in Figure 33.4. A zinccoating alternative for strips is continuous electroplating. Both techniques are very much utilized for modern automobile body sheets. 

The depressed prices of most metals in world markets in the 1980s and early 1990s have slowed the development of new metal extraction processes, although the search for improved extractants continues. There is a growing interest in the use of extraction for recovery of metals from effluent streams, for example the wastes from pickling plants and electroplating (qv) plants (276). Recovery of metals from Hquid effluent has been reviewed (277), and an AM-MAR concept for metal waste recovery has recentiy been reported (278). Possible appHcations exist in this area for Hquid membrane extraction (88) as weU as conventional extraction. Other schemes proposed for effluent treatment are a wetted fiber extraction process (279) and the use of two-phase aqueous extraction (280). 

Potassium cyanide [151 -50-8] KCN, a white crystalline, deUquescent soHd, was initially used as a flux, andlater for electroplating, which is the single greatest use in the 1990s. The demand for potassium cyanide was met by the ferrocyanide process until the latter part of the nineteenth century, when the extraordinary demands of the gold mining industry for alkah cyanide resulted in the development of direct synthesis processes. When cheaper sodium cyanide became available, potassium cyanide was displaced in many uses. With the decline in the use of alkah cyanides for plating the demand for potassium cyanide continues to decline. The total world production in 1990 was estimated at about 4500 t, down from 7300 t in 1976. 

There has been a continual increase in size and complexity of PCBs with a concurrent reduction in conductor and hole dimensions. Conductors can be less than 250 p.m wide some boards have conductors less than 75 pm wide. Multilayer boards greater than 2.5 mm thick having hole sizes less than 250 pm are being produced. This trend may, however, eventually cause the demise of the subtractive process. It is difficult to etch such fine lines using 35-pm copper foils, though foils as thin as 5 pm are now available. It is also difficult to electroplate holes having high aspect ratio. These factors may shift production to the semiadditive or fully additive processes. 

The isothiazole ring does not occur in nature. By far the most important synthetic isothiazole derivative is saccharin. This was the first non-carbohydrate sweetening agent to be discovered, as long ago as 1879. It is about 300 times as sweet as sucrose, and is still used in many countries as a non-nutritive sweetener. After it was found that administration of massive doses to rats caused bladder cancer, its use was banned in the New World, but the controversy continues as to whether there is any danger when it is used in small quantity. Saccharin is also used as an additive in electroplating processes (73AHC(15)233). 

The sheer amount of material deposited on a cathode is of less practical significance than the distribution of the deposit over the cathode and its thickness. Indeed, it ought to be understood that in practice metal ions cannot be expected to and do not deposit as continuous sheets from one edge of the cathode to the other. Rather, metal ions become attached to the cathode at certain favored sites. The result of this is the possible presence of discontinuities in the form of pores, cracks, or other irregularities. Thus, in electroplating, current density and its distribution play a centrally important role in determining the quality of the final deposit. Defined in terms of the actual electrodeposition setup or process, the current density is the total current divided by the area of the electrode. 

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