August 2013 Archives

Let us give some thoughts to the conditions required to obtain desired plating thickness.

(4) Cathode current density settings

The cathode current density (A/dm2) to be applied will depend on experimental results. Normally in electroplating, glossiness, flatness, plating thickness, and cathode current efficiency in relation to cathode current density can be measured by Hull Cell Test, and the test results are applied to the plating bath to be used. Recommended values are typically provided by additive agent manufacturers.
For an example for glossy nickel plating bath, 2〜5A/dm² seems to be adopted.

(5) Plating thickness settings with variations in consideration

Suppose that a thickness of 10μm is required. Since the plating thickness will vary depending on the product shapes and energizing methods, the target thickness is 11μm which is 10% over (when the variation is small)

(6) Plating time settings

Again as the nickel plating as an example, based on the current density of 3A/dm2 since the plating speed is 0.205μm/minute at 1amp-minute/dm2, the time required to deposit 11μm plating can be obtained by the following formula.

【表1】各種金属めっき用諸係数

(7) Adjustment of plating time

All of the above is based on an ideal condition where the cathode current efficiency is at 100%. Some of the factors that may contribute on creating a less than ideal condition are non-uniform current flow on the anode or parts of the plated product due to poor energizing practices, and/or variances of anode surface area, bath temperature, bath composition, etc.
In any case, the amount of electricity determines the thickness of the plating, the most important point is to assure the uniform current flow. The above mentioned plating time is to be divided by cathode current efficiency to obtain the actual plating time.

(3) Theoretical precipitation amount of electroplating

Plating precipitation amounts and film thickness when cathode electrical current efficiency is 100% is shown in [Table 1]. Cathode electrical current efficiency will vary largely depending on the plating bath types and plating conditions.

[Table 1] Various coefficients for various metal plating

(2) Faraday's law

In electroplating, there is none other than Faraday's law as important and used as the foundation of various calculations. The law is composed of the following two sections.

Faraday discovered this law in 1834. He has defined 1 ampere per 1 second of electricity as 1 Coulomb, an amount of electricity needed to generate 1 chemical equivalent of various atoms or group as 96,540 Coulombs, and named this amount of electricity 1 Faraday.
The chemical equivalent is a product of dividing the plating metal atomic weight by the valence of the metal ions in the plating bath. In case if bright nickel in Watts bath, this will be 58.71/2＝29.36（g）. In other words, 1 Faraday of electricity value, 29.36g of nickel will be precipitated with this nickel plating.

Calculation example 1

With silver nitrate solution, how much silver will be precipitated by 1 hour electrolysis at 1 ampere?

　AgNO₃ →　Ag+ + NO^3-
Since Ag is monovalent and the atomic weight is 107.88, the amount of precipitation will be
(107.88 x 60 sec. X 60 min.) / (96,540 x 1 valency) = 4.023g

Calculation example 2

How much nickel will precipitate on cathode by electrolyzing Nickel sulfate solution ( NiSO4) at 1 ampere for 1 hour?

　NiSO₄　→ Ni²+ + SO₄^2-
Ni is divalent, and the atomic weight is 58.71, therefore...
(58.71 × 60sec. × 60min.) / (96,540×2)＝1.095g

When this is expressed in a plating thickness, since specific gravity of nickel is 8.9g/cm3, the thickness per 1dm2 is...
　　1.095 (g/10¹⁰μm²) / 8.9 (g/10¹²μm³)
　　＝0.123×10² (μm/Ah・dm²)
　　＝12.30 (μm/Ah・dm²)

When this is converted into per one minute (AM),
it will be 12.3/60=0.205 (μm/AM・dm²)

In general, a concept of "Current density" is used to determine the strength of the electrical current. This indicates the strength of current applied per unit area of the product, and typically represented with A/dm2 (Amperes per 1 square decimeter 100x100mm).
Therefore, surface area of the product to be plated is calculated, and the applicable current density multiplied constitutes the current (A) to be applied per one product.