September 2013 Archives

(2) Plating current density

In the plating bath, the current that flows from the anode to the cathode is not uniform. The current density will vary depending on electrode shapes, surface areas, racking heights, polar distances, plating bath flow conditions and temperature distribution.
And the cathode current density (A/dm²) has the largest effect on plate coating deposition speed. The relationship of various factors are shown on [Fig.1].

As the nature of electricity, the current concentrates on sharp features (high current density), the current flow is low, or not even flow at all, on concave features. This results in the plate coating thickness variations on parts of the plated products. Plating will not take place where there is no current flow.

The factors we have discussed about in the previous session volumes such as per item required current x number of items, and plating time are for the entire plating bath, and the plating thickness of each item or parts of the items will depend on the current strengths (cathode current density) of specific locations. The only way to verify the cathode current density distribution is to actually measure the plating thickness of the finished items.

Therefore, to manage the plate coating thickness in actual practices, it is important to have the understandings on the coating thickness variations by once measuring all the pertinent surfaces for the plate coating thicknesses.
Normally, in order to avoid these adverse effects, auxiliary cathodes are placed on the sharp features to lower the currents, or auxiliary anodes are placed on features where current flow tend to be lower. In other words, measures to equalize the current flow on all the parts of the plated items are devised.

At times, continuous DC flow plating is replaced by pulse current plating where large currents are applied intermittently to improve deposition uniformity.

(1) Electrical resistance in racking

Electroplating methods can be broadly divided into Racking and Barrel (tumbling). With the Racking method as an example, factors that cause the variations will be explained.

Normally, electroplating is configured as shown in [Fig.1], where objects to be plated (products) are racked on the fixture and made as the cathode, and the plating metal is made as the anode, all submerged in the tub of plating bath. A DC current (from a rectifier source, etc.) is applied between the anode and the cathode to conduct plating.

In the figure above, the DC current flows from the anode to the rack fixture through the plating objects. Here, the total current needed per rack is "Current needed per plating item" x "Number of racked items". The issue here is that if a rack has enough electrical capacity required to conduct enough current per rack.

The rack is composed of Main rib(s) with the largest cross sectional areas, and Small ribs connected to the Main rib(s). The plating objects are fixed to the Small ribs to be energized.
When this Rack structure is analyzed electrically.....

The total of above is the electrical resistance R per one rack, and a typical plating tub contains more than 10 racks (n), resulting in R x n (pcs.) connected in parallel.

Of all the resistance types above, (7) has the largest influence on plating layer thickness variations. As the ribs are used several times, ribs accumulate plating, and changing contact pressure causing the contact resistance to increase. Hence the current flowing through individual rack varies, causing the plating thickness to also vary.
Therefore, it is important to periodically remove the accumulated plating from the racks, and to perform maintenance on the insulating material.
[Table 1] below shows % conductivity in relation to the copper as 100, and resistivity (specific resistance) of various metals.

[Table 1] Resistivity, % Conductivity of metals