A. Electrode Position
Electricity follows the path of least resistance. Consequently, areas of a part that protrude, edges, and parts closest to the anode will receive the most current. These areas tend to have the thickest deposit. It is normally desirable to have a uniform deposit thickness across the entire part. Recesses or areas farthest away from the anode, on the other hand, will tend to have the thinnest deposit. Arranging the parts so that all areas approximately the same distance from the anode will maximize current distribution provide a uniform deposit. Figures 25.3 and 25.4 show this relationship. In some cases, however, it is impossible to arrange the parts in the most desirable position. In these situations a shield or "robber" can be used to obtain the proper distribution.
A shield is an inert material that is not electrically conductive. It is placed between anode and the cathode to redirect the current flow. Figure 25.5 shows the proper of a shield.
A robber is a conductive metallic article that is plated along with the workpiece. It is positioned in such a manner as to steal current from an area where it is not wanted. A robber will receive a deposit, as will the cathodic workpiece. Figure 25.6 shows the use of a robber.
Inasmuch as robbers receive a deposit, this represents a waste of valuable plating at. They also have to be stripped occasionally so that excessive buildup is avoided. These factors normally make the use of robbers expensive and unpractical. It is for this on that shields are in greater use for parts with troublesome areas.
Fig. 25.3 Poor arrangement of electrodes favors deposition on edges.
|Fig. 25.4 Proper arrangement of electrodes favor a uniform deposition.||Fig. 25.5 The use of a sheild directs the current flow.|
Fig. 25.6 Robbers positioned near high corrent density areas will receive
a deposit and prevent unwanted buildup on edges.
B. Polarization and Current Distribution
Polarization is a term that explains the effect of a change in electrical potential of a plating solution. Normally it is not of concern to the electroplater unless a problem lops in the solution balance. Polarization is a condition caused by changes in bath concentration or changes within either the anode or cathode films. It is caused by the movement and discharge of ions. It is a force that is measured by a drop in voltage for a given ampere setting. Polarization is always present during electrolysis of a solution but is of concern when its effect becomes excessive. It can be caused by a depletion of near the electrode surface, as in concentration polarization, or resistance polarization which is caused by the formation of a diffusion layer surrounding the electrode. Polarization can be alleviated by the use of agitation in the plating solution. This maintains a uniform composition throughout the solution and consequently minimizes the effect of polarization. Current distribution at the beginning of the electroplating process prior to the onset of polarization changes during the first second or two of the current flow to the current distribution that prevails during the rest of the electroplating process. This distribution is more uniform along the cathode surface and tends to even out deposit buildup on edges and the center area of parts.
C. Throwing Power
Throwing power is the ability of an electrolyte to deposit metal of a uniform thickness across a cathode surface. This term is not to be confused with covering power, which is the ability of an electrolyte to deposit metal across an area regardless of thickness distribution. The two types of throwing power are micro and macro.
Microthrowing power is the ability of a bath to plate into small surface scratches, thereby filling them up with the deposit. A solution with good microthrowing power will to level out surface imperfections and provide a smoother surface. Microthrowing is affected by polarization. Where polarization is increased by the use of addition agents, metal will tend to deposit into surface imperfections and thereby produce a higher quality deposit.
Macrothrowing power is the ratio of deposit distribution in relation to the distance from anode to cathode. The area of a cathode that is spaced close to the anode would normally receive the thickest deposit, whereas the area farthest from the anode would receive the thinnest deposit. An electrolyte with 100% throwing power will deposit a uniform thickness regardless of anode to cathode spacing. As before, polarization is responsible for controlling this macrothrowing power.
D. Leveling and Brightening
As mentioned previously, leveling is the ability of an electrolyte to deposit metal so as to smooth out surface scratches and imperfections by filling them up with the deposit. The use of certain addition agents in the plating solution causes polarization at the peaks of these scratches. These peaks are high current density areas on a microscopic scale.
The microthrowing power is improved by these additives, and the deposit is caused to preferentially deposit in the valley of the scratches. As the plating continues these scratches become filled with metal and a leveled surface result.
Brightening is enhancing the ability of a deposit to reflect light. A deposit that reflects nearly 100% of the incident light will appear bright, whereas one that reflects less light will appear duller. Brightness is important in decorative applications and may be of no importance for functional applications. Although a deposit that is leveled will normally appear brighter, brightness is more a function of the crystalline structure of the deposit.
A deposit that has good adhesion will not peel or flake from the substrate. Good adhesion is especially important if the part is to be formed, bent, or stamped after electroplating. Adhesion is usually related to the activation prior to plating. A part that is cleaned and activated properly will exhibit good adhesion.
F. Deposit Growth
As the electroplating process continues and the deposit becomes increasingly thicker there is a tendency to produce growths around edges and other high current density areas. These undesirable growths are commonly termed "trees" or "nodules."
Trees are a deposit growth, normally forming around edges. They are quite angular and actually appear as a tree with its many branches. They are caused by local high current density accompanied by growth of the metal. A tree has a very thin structure; it will break off if touched and may cause a pinhole at its base. A nodule is a rounded bumpy kind of deposit that is usually more firmly attached due to the larger size of the base. Nodules do not usually pose a problem as regards adhesion but they are not desirable; a quality deposit should be smooth and nodule free. Trees and nodules are usually associated with thicker deposits of 0.0005 in. or more. They can, however, be present with thinner deposits if the plating electrolyte is not in proper balance. The use of certain addition agents in the plating bath is quite common to prevent trees and nodules from forming even with deposit thicknesses exceeding 0.005 in.