Micro-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... 

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... 

We can see intuitively that cell geometry has little to do with micro throwing power. If the amplitude of roughness is of the order of 0.1 pm or less and the distance between the anode and the cathode is a few centimeters, the variation of solution resistance at crests and valleys on the surface must be negligible. Hence primary current... 

There is great commercial incentive to produce smooth and bright deposits. Consequently, there is a vast choice of additives for use to improve micro throwing power, making deposits smoother and more uniform. 

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. 

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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