1. Use demand of titanium anode
Based on the actual needs of users, when the copper plating process is switched from phosphor copper ball to titanium anode, the primary demand is to effectively and stably improve the electroplating uniformity, which will lead to the improvement of quality; Secondly, it is required that the quality of titanium anode is stable and can reach the expected service life and stable additive consumption level during this period to ensure that the operating cost is controllable. Therefore, in summary, the main requirements are as follows: excellent electroplating uniformity, stable service life, and controllable additive consumption level.
For anode manufacturers, how to translate customer needs into internal requirements for product design is the most important point for anode manufacturers to study and provide corresponding support. The titanium substrate and coating make up the majority of the titanium anode structure. According to the specific requirements, the electroplating uniformity requirements are mainly determined by the mechanical design of the titanium substrate, while the other two requirements are closely related to the coating design.
2. Design of titanium anode discharge uniformity
The main mechanical design of the titanium anode needs to be matched with the equipment, and the main work is completed by the equipment supplier. The anode manufacturer should give corresponding suggestions and support on how to optimize the design of titanium anode discharge uniformity, mainly from the following aspects.
i. Resistivity problem
In the design of titanium anode discharge uniformity, the first point to be concerned with is the resistivity of titanium materials. The resistivity of pure titanium is about 0.47 μ Ω · m, close to 30 times of pure copper under the same conditions. When using a phosphor copper ball, the anode current is introduced through the upper part of the titanium basket, and then conducted through the copper ball inside the entire anode (in essence, it can be considered that the current is conducted through copper). Therefore, the conductivity difference between the upper part and the lower part is very small, which can be ignored. When titanium anode is used, the conductivity of titanium is relatively poor, especially when the titanium anode is working at a high current density, and the current is transmitted from the upper part of the anode to the lower part, the resistance of the titanium itself will lead to a significant decrease in the voltage from top to bottom. In this way, the discharge current density at the bottom of the titanium anode will be significantly lower than that at the top of the titanium anode.
In anode design, the primary consideration is how to reduce the voltage drop caused by the long-distance conduction of titanium materials. It can be optimized mainly through the following two aspects: ① reduce the conductive resistivity, use wider and thicker titanium materials for current conduction, or use titanium copper composite materials to assist current conduction; ② Scatter the current conduction points, and set multiple current conduction points on the anode surface to avoid long transmission distance.
ii. Targeted optimization of anode substrate types
At present, in the design of titanium anode, there are basically two types of anode substrate: one is titanium plate, and the other is titanium mesh.
Titanium mesh is made of titanium plate by punching and drawing, and its main advantages are in two aspects: first, compared with a titanium plate, titanium material consumption can be saved; Second, because the titanium mesh is usually coated on both sides, even if it does not face the back of the product, because the mesh material is a hollow structure, the back coating can also participate in the discharge, so the effective discharge area of the entire mesh anode is larger than that of the titanium plate, which can reduce the current density under actual anode working conditions. The mechanical strength of the reticulated anode is usually worse, and its resistivity is higher than that of the plate anode. To solve the above problems, the flatness and discharge uniformity of titanium mesh anode can be greatly improved by designing a suitable frame and optimizing the position of solder joints.
The biggest advantage of using a plate anode is that the substrate of the plate anode can be reused. After the anode coating fails, the residual coating can be stripped, the substrate surface can be thoroughly cleaned, and then the coating can be recoated. In this way, the long-term use cost can be saved to a certain extent in the future application of the anode (although the one-time investment will be slightly larger). On the other hand, the thickness of the plate anode substrate is usually 2mm and 3mm, while the mesh anode is generally suitable for drawing from a 1mm titanium plate (there is a hollow in the middle), so the conductivity of the plate anode is better than that of the mesh anode. The flatness will also be better, and the plate anode's relative mechanical strength is higher than that of the mesh anode. However, this does not mean that the discharge uniformity of the plate anode is better than that of the mesh anode. In contrast, the overall mechanical design of the plate anode is simpler than that of the mesh anode (with a frame). However, there is still room for optimization of the distribution of the plate anode current access points if it is required to adapt to higher electroplating uniformity requirements.
