1 Preface

The uniformity of the distribution of the metal coating on the cathode is an important factor to determine the quality of the coating. In the electroplating production, people always hope to get a uniform coating on the surface of the plated parts. Since the functional part of the socket contact in the connector is the inner surface of the socket, if the coating on the inner and outer surfaces of the plated part can be distributed uniformly, the production cost can be minimized. But in fact, no matter what kind of plating solution is used, there is always the phenomenon of uneven coating thickness. According to Faraday's law, in the process of electroplating, when the current passes through the electroplating solution (electrolyte solution), the amount of substance precipitated on the cathode is directly proportional to the amount of electricity passing through. From this point of view, the distribution of the coating on the part surface depends on the current distribution on the cathode surface, so all factors affecting the current distribution on the cathode surface affect the distribution of the coating on the cathode surface [1]. In addition, in the electroplating process, the reaction on the cathode is often not a simple metal precipitation, accompanied by the metal precipitation, there are often hydrogen evolution reaction or other side reactions, which shows that the coating distribution is also affected by the solution performance, but also involves the current efficiency. In the daily production of contact body gold plating, the author found that the uniform distribution of the coating on the cathode is not only related to the properties of the solution, but also closely related to the factors such as the shape of the plating piece, the selection of the plating method, the selection of the plating power supply, the selection of the current density range and the loading capacity of the plating piece.

2 factors affecting the distribution of the coating on the cathode surface

2.1 current density

Any plating bath has a range of current density to obtain a good coating, and gold plating bath is no exception. When the current density exceeds the upper limit of the process range, coarse crystalline particles will be formed, and the coating obtained on this basis is relatively coarse; while the coating obtained under low current density is relatively fine. For roll plating or vibration plating, due to the low concentration of gold in the plating solution (generally 2-6 g / L), the current density between 0.1-0.4 A / DM2 can obtain a good coating. However, when the upper current density is used, the [Au (CN) 2] - near the cathode will be deficient, which will intensify the hydrogen evolution reaction on the cathode and reduce the current efficiency. Therefore, the current density of 0.2 A / DM2 and 0.1 A / DM2 are not simple multiple relations in production time.

In the process of low-speed gold plating by roll plating and vibration plating, if higher current density is used, the possibility of tip effect will increase. Especially in the vibration electroplating, because the tip of the plated part is always facing the anode (the anode ring is outside the vibrating screen), the tip effect is more obvious. The coating at the edge of the plated part or the tip of the pin and the socket is thicker, while the coating at the low end is relatively thinner, resulting in the uneven distribution of the coating thickness on the surface of the parts. Therefore, in the application of low-speed gold plating process, for the elongated pinhole contact body, the lower limit of current density range in the process is generally used for operation, and the plating method of small current and long time is used to obtain relatively uniform coating thickness.

2.2 electroplating power supply

In the current connector electroplating industry, there are three kinds of electroplating power supply commonly used: DC power supply, pulse power supply and bidirectional pulse power supply. At present, DC power supply is the most widely used. In order to make the thickness of the gold plating layer in the hole meet the requirements of the drawing, if the traditional DC power supply is used, the thickness of the gold plating layer outside the hole will be thicker than that inside the hole, especially for many small hole parts in the contact body, the thickness difference between the coating inside and outside the hole is more obvious. When the periodic reverse pulse power supply is used, in the process of gold plating, when the positive current is applied, the gold is deposited on the surface of the plating piece as the cathode, and the convex part of the plating piece is a high current density area, so the deposition is fast; when the reverse current is applied, the coating on the surface of the plating piece dissolves, and the original high current density area dissolves quickly, so more plating can be removed at the convex part The thickness of the coating is uniform.

The production practice shows that the periodic reversing pulse power supply can not only improve the distribution of gold plating layer in the inner and outer surface of the contact body hole, but also improve the uniformity of the whole bath plating. Table 1 is the coating thickness data measured after vibration gold plating of two different electroplating power sources with a cathode current density of 0.1 A / DM2 according to the requirements of 1.3 μ m thickness (1.27 μ m as specified in the drawing) of contact parts with a hole diameter of 1 mm and a hole depth of more than 3 mm (called wiring conduit).

2.3 loading capacity of plated parts

It is also very important for the gold plating layer to be evenly distributed on the plating piece whether the loading capacity of the plating piece is appropriate or not. No matter vibration plating or roll plating is adopted, if the number of plating parts is less than the lower limit of loading, the plating parts are easy to be affected by poor conductivity in the plating process, and the uniformity of coating will also be significantly affected. Some accompanying plating parts must be added to ensure that the plating parts will not be cut off in the middle of the power supply, and at the same time, the plating parts will be uniformly turned over. When the load of plated parts is large