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Surface Finishing Tutorial

#169 Plating Thickness Variation Factors - 2

Category : Electroplating
September27, 2013

(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].

[Fig.1] Relationship of Cr plating bath temp., current density, and plating speed

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.

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