Macro-throwing power

Deposit uniformity The uniformity of a deposit is an important factor in its overall corrosion resistance and is a function of geometrical factors and the throwing power of the plating solution. A distinction is made here between macro-throwing power, which refers to distribution over relatively large-scale profiles, and micro-throwing power, which relates to smaller irregularities... 

The copper cyanide bath has excellent macro-throwing power and is chosen whenever irregular-shaped parts are to be plated. The sulphate bath is not inferior when parts with very narrow recesses, i.e. with width of opening less than 6 mm, are to be plated, although its macro-throwing power is... 

Yet another, more precise, way to define this important property is as follows. The ability of a bath to produce deposits of more or less uniform thickness on cathodes having macroscopic irregularities is termed macro throwing power. 

We distinguish between macro throwing power and micro throwing power. We discuss the former first. This quantity is often measured using the Haring—Blum throwing power box (see Fig. 12.2). The throwing power is then expressed by... 

Let us try and understand this. As stated, Ni plating baths (as well as other acidic baths such as those of Cu and Zn) show poor throwing power. This is so because their CE values are =100% at the low and high current density values, and so macroscopic irregularities on a cathode will lead to nonuniform deposits. Alkaline baths, on the other hand, have a better macro throwing power. This is the case since, in order to remain in solution in such a bath, the metal ion, to be deposited, must be present in complex ions. These ions, in turn, encounter high concentration polarization. Also, in most complex baths the deposition potentials are amenable to hydrogen evolution, which competes with metal deposition such that CE falls as current density is increased. That kind of behavior results in a more uniform deposit on... 

Haring-Blum cell — A rectangular cell with two cathodes at the small ends with an anode placed between them (see Fig. 1). The two cathodes are connected electrically. Usually the ratio, L, of the two anode-cathode distances is five. Thus a high current density (hcd) and a low current density (led) edge results. The Haring-Blum cell is used to determine the macro - throwing power, TP, of an electrolyte, according to the relation TP = (L - M) / (L + M - 2) where M is the ratio of the deposited masses. 

Having defined the throwing power quantitatively, we can now proceed to discuss the physical reasons for the dependence of the throwing power on geometry and the methods available to increase the value of the T.P. to acceptable levels. This discussion refers to the so-called macro throwing power. In Section 30.3 we shall discuss the micro throwing power, which controls the smoothness of the deposit and depends on quite different factors. 

The throwing power is said to be micro (Micro-throwing Power) if the metal deposit occurs preferentially in pores and scratches and macro (Macrothrowing Power) when a dejiosit of relatively uniform thickness is produced on an irregularly-shaped cathode. 

Figure 19.3 Top view of the Haring and Blum cell for the determination of the macro-throwing power.

Eliminating the need for a counter electrode in electroless plating has a profound influence, on the current distribution. Macro-throwing power is no longer an issue, because primary current distribution is caused by the nonuniformity of the electrostatic field between the object being plated and the counter electrode. Since there is no counter electrode, uniformity of plating on complex shapes can readily be achieved. 

In contrast, during electroplating both macro-and micro-throwing power play a role. Moreover, since the plated layer is relatively thick (compared to that produced by electroless plating), the current density is higher, in order to increase production rate, and suitable additives are used to improve uniformity of thickness, as well as brightness and smoothness. 

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