This question first appeared December 1, 2016 on ProductsFinishing.com in the Plating Clinic. By Derek Vanek.
The key to uniform thickness distribution is uniform current distribution. Assuming 100% efficiency, fundamental laws of electrochemistry (i.e. current distribution) do not always allow for a uniform deposit. Direct current always seeks the path of least resistance from the anode to the cathode (substrate/work piece). As a result, paths of least resistance such as sharp edges or protrusions will receive a heavier deposit, while areas such as internal corners/radii receive a significantly less amount of deposit. The goal of the plater and designer is provide for the least amount of thickness variation across a workpiece. Design considerations take into account several variables: anode design (geometry, masking, and tool movement), work piece (masking and thieving), bath variables (current density, temperature, additives, and flow distribution) to name a few. Here will focus primarily on anode design.
Selective (brush) plating is a well engineered method of electroplating controlled thicknesses of deposits such as copper, cadmium, cobalt, gold, nickel, silver, tin, as well as alloys that include babbitt, cobalt-tungsten, nickel-tungsten, and zinc-nickel onto all commonly used base materials for industrial components.
As the name implies, the process is focused on a specific “select” area of a component. The area to be plated, as well as adjacent areas to be masked are first cleaned with a suitable solvent. The part is then masked to isolate the area to be plated and to protect the adjacent areas from the effects of the chemical processes. Typical masking materials include aluminum and vinyl tapes, masking paints, and special fixtures.
The actual selective (brush) plating process consists of several preparatory steps in which the work area is electrochemically prepared to receive an adherent final deposit, the thickness of which is controlled by ampere-hours (Factor x Area x Thickness = Ampere Hours).
- The factor is a well-established plating rate that is specific to a plating solution. It is the ampere-hours required to deposit the volume of metal equivalent to one inch thickness onto one square inch of area.
- The area is the total surface area to be plated.
- The thickness is the desired deposit thickness after plating
Uniform distribution of the deposit is primarily achieved by selection, proper design, and use of the plating tool as well as by proper masking for the application.
Covering the full length of an OD, ID, or flat surface with a tool makes it relatively easy to obtain a uniform thickness. When the tool does not cover the full length, problems arise. Take for example, the case of attempting to plate an OD 3 in. long with a tool that will cover 2 in. of the length. If the tool is moved as shown in Sketch #1 on top, center of Figure 1, the center 1 in. is always covered. At the ends there is less coverage time. A deposit distribution as shown at the bottom results. The alternative to this is to move the tool as shown in Sketch #2 on the left of Figure 1. An even deposit distribution is obtained, but now some time is wasted with the tool off the part. This motion, also, may not be practical if there is a shoulder at one side. The same situation applies to ID and flat surfaces. Summarizing, always try to have the tool cover the full length of OD or ID or the full length or width of a flat surface Sketch #3. The anode can further be masked along the outside perimeter with slight overlap onto the work surface to minimize the deposit build-up along the edges of the work piece.
When the tool is moved as shown top center, more plating is obtained in the center and less at the ends. When the tool is moved as shown on the lower left, a uniform deposit is obtained, but much time is wasted with the tool off the part.
Figure 1: Difficulties encountered when a plating tool does not cover the full length of an OD.
Another consideration for deposit uniformity is ensuring an even distribution of plating solution over the area being plated. For best results, the plating solution should be pumped to the work area through the plating tool – and be uniformly distributed over the work area. An uneven distribution of fresh solution over the work area will result in an uneven deposit thickness.
Here are some generalizations:
- The thicker the deposit, the more difficult to plate a tight tolerance
- It is easier to accurately plate on a small area than a large area
- It is easier to hold tight tolerances on simple shapes with no interruptions than complex shapes or shapes with interruptions or a large percentage of high current density edge area
- Mechanical movement of the part or the anode is going to produce more consistent results than hand movement
- It is easier to accurately plate a low thickness on a small area than to plate high thickness on large area