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#168 Plating Thickness Variation Factors - 1

Category : Electroplating
September13, 2013

(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.

[Fig.1] Electroplating tub layout

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.....

(1)Contact resistance between the cathode conductor bar and the top of rack's main rib
(2)Main rib material, normally flat copper is used.
(3)Cross sectional area and length of main rib
(4)Contact resistance between Main rib and Small ribs
(5)Small rib material, normally stainless steel is used
(6)Cross sectional area and length of small ribs
(7)Contact resistance of small ribs and plating objects

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
MetalResistivity
[Ω・mm2/m]
%conductivity
(%)
Silver0.016106
Copper0.0169102
Std. soft copper0.01724100
Gold0.02471.8
Aluminium0.026265.8
Tungsten0.054831.5
Nickel0.06925.0
Iron0.1017.2
Platinum0.10516.4

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