1. Definition of insoluble anode

Anodes that do not dissolve themselves and only undergo oxidation reactions when current passes through during the electroplating process are collectively called insoluble anodes. Insoluble anode materials in electroplating include lead, carbon, platinum, graphite, nickel, stainless steel, titanium plated with platinum, iridium-plated tantalum, ruthenium-plated iridium, rhodium, etc.

The scenarios where insoluble anodes are used in the circuit board industry mainly include electroplating copper, electroplating gold, electroplating silver, and environmentally friendly treatment of ammonia nitrogen and COD, etc. The most widely used HDI board copper-plated insoluble iridium and tantalum anode on circuit boards has high requirements on the consumption of light agent. The stability of electroplating light agent during the electroplating process has a crucial impact on the quality of electroplating.

The insoluble anode plays an anode conductive role during the entire electroplating process and precipitates oxygen or oxidized metal ions. There are two main effects of using insoluble anodes on circuit boards on the bath liquid. First, the evolution of oxygen on the surface of the anode will also cause additional loss of electroplating brightener. What occurs on the surface of the anode catalytic coating is a direct oxidation reaction. Its main reaction The hydroxide in the bath liquid is catalyzed by the precious metal catalytic coating to lose an electron at a lower potential and become oxygen. At the same time, the organic matter in the bath liquid also has the opportunity to be oxidized by discharge on the anode. The key point in the anode manufacturing process is to control the oxidation. Oxygen potential, do not allow the organic matter in the bath liquid to have the opportunity to be oxidized by direct discharge on the anode. The second affects the dissolved oxygen content of the bath liquid. The oxygen generated on the anode surface needs to escape from the anode and run out of the bath as quickly as possible to reduce its residence time in the bath. (The reaction mechanism of pulse iron-containing ions is different and very little oxygen is released).

2. Insoluble anodes have the following advantages over copper balls in use:

01. The anode current is not limited and can break through the current density bottleneck of the anode copper ball 4.2asd (the anode film is easy to fall off and passivate if the current density is too high), increase the production speed and increase the production capacity, especially for FPC production lines, the continuous production of RTR has already There are insoluble anodes throughout the factory.

02. During the electroplating process, the anode undergoes an oxidation reaction to obtain electrons from hydroxyl radicals to generate oxygen. Without anode mud, only oxygen is released to maintain the metal ion concentration distribution in the electroplating solution at a stable level; (when solving the problem of pulse impact on anode life) After the impact, it has great benefits for the pulse production line, which can significantly improve product quality, reduce maintenance costs, and increase product utilization rate).

03. The size of the anode is stable, and the anode area does not change during the electroplating process. The primary current density distribution can be considered as a constant state, which is of great benefit to improving the current density distribution, especially for HDI fine lines, hole filling, and pulses. .

3. Insoluble anode process requirements

The difference between the insoluble anodes commonly used in circuit board electroplating and ordinary insoluble anodes is the loss of organic matter. This depends on the composition and structure of the precious metal coating, that is, the catalyst layer.

In the production process, two aspects need to be ensured. First, ensure a strong bonding force, which refers to the bonding force between the coating and the titanium substrate. Second, ensure the conversion rate of the precious metals in the coating into effective catalysts.

In the first aspect, ensuring the bonding force requires: 1. A clean surface; 2. Appropriate surface roughness; 3. The crystal structure of the catalyst layer is similar to the crystal structure of the bottom layer (rutile structure). According to the principle of similar dissolution, the formation of a riveted structure can greatly enhance the bonding force.

The second aspect is the conversion rate of the precious metals in the coating. This requires a lot of measured data on the formula and production process to find the best formula and production process.

IV. Introduction to the production process of insoluble anodes

Hot rolling/cold rolling of titanium ingots Sponge titanium becomes titanium plates of different thicknesses:

Punching (cutting) Titanium plates are made into titanium meshes of different meshes:

Pretreatment Clean the surface of the titanium mesh to obtain a clean titanium mesh surface:

After the above steps, we strictly follow the production quality control to produce high-performance and low-cost anodes, providing customers with high-quality anodes.

Design principle of titanium anode:

1. Use requirements of titanium anode

Based on the actual needs of users, when the copper plating process is switched from phosphor copper balls to titanium anodes, the first requirement is to be able to effectively and stably improve the uniformity of electroplating, thereby improving the quality; secondly, the quality of titanium anodes is required to be stable, to achieve the expected service life and a stable level of additive consumption during this period, so as 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 level of additive consumption.

For anode manufacturers, how to convert customer needs into internal requirements for product design is the most important area for anode manufacturers to study and provide corresponding support. The structure of titanium anode mainly consists of two parts: titanium substrate and coating. According to the specific requirements, the electroplating uniformity requirement is mainly determined by the mechanical design of the titanium substrate, while the other two requirements are closely related to the design of the coating.

2. Titanium anode discharge uniformity design

Since the main mechanical design of the titanium anode needs to match the equipment, the main work is completed by the equipment manufacturer. Faced with the design problem of how to optimize the discharge uniformity of titanium anodes, anode manufacturers should provide corresponding suggestions and support, which can mainly be considered from the following aspects.

1. Resistivity problem

When designing titanium anode discharge uniformity, the first thing that needs to be paid attention to is the resistivity of titanium material. The resistivity of pure titanium is about 0.47 μΩ·m, which is close to 30 times that of pure copper under the same conditions. When using phosphor copper balls, the anode current is introduced through the entire titanium basket, and the conduction resistance inside the anode is basically negligible through the copper balls. When using a titanium anode, due to the relatively poor electrical conductivity of the titanium material, especially when the titanium anode works at a higher current density and the current is conducted from the upper part of the anode to the lower part, the resistance of the titanium material itself will cause the voltage to flow from top to bottom. There is a significant reduction. This will cause the discharge current density in the lowermost part of the titanium anode to be significantly lower than that in the uppermost part of the titanium anode.

When designing an anode, the primary consideration is how to reduce the voltage drop problem caused by long-distance conduction of titanium. It can be optimized mainly through the following two aspects: ① Reduce the conduction resistivity, use wider and thicker titanium materials for current conduction, or use titanium-copper composite materials to assist current conduction; ② Disperse current conduction points and set them on the anode surface Multiple current conduction points to avoid long transmission distances.

2. Targeted optimization of anode substrate types

In the current design of titanium anodes, there are basically two types of anode base materials: one is titanium plate and the other is titanium mesh.

Titanium mesh is punched and stretched from titanium plates. Its main advantages are that titanium mesh can only be coated on both sides, even if it is not facing the back of the product. Since the mesh material has a hollow structure, the back coating It can also participate in 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. Mesh anodes tend to be less mechanically strong and have higher resistivity than plate anodes. In response to the above problems, designing a suitable frame and optimizing the position of the solder joints can improve the flatness and discharge uniformity of the titanium mesh anode.

The biggest advantage of using plate anodes is that the base material of the plate anode can be reused. After the anode coating fails, the residual coating can be peeled off and the substrate surface can be re-coated after thorough cleaning. This can save long-term use costs 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 base material is usually 2mm and 3mm, while the mesh anode is generally suitable to be drawn from a 1mm titanium plate (with a hollow in the middle), so the conductivity of the plate anode is better than that of the mesh anode. At the same time, the relative mechanical strength of the plate anode is stronger than that of the mesh anode, and the flatness will be better. But this does not mean that the discharge uniformity of the plate anode is necessarily better than that of the mesh anode. In comparison, the overall mechanical design of the plate anode will be simpler than the mesh anode (with frame), but if it is to adapt to higher electroplating uniformity requirements, the distribution of the current access points of the plate anode should still be optimized. spatial.

3. Effect of bubbles on conductive uniformity

During the use of the titanium anode, the anode reaction will produce oxygen. Therefore, the generation of oxygen will form a shielding effect between the cathode and the anode and have a certain impact on the uniformity of discharge. Since the generated oxygen bubbles will float, the amount of oxygen bubbles accumulated in the upper and lower parts of the anode forms a certain gradient, resulting in a shielding effect and a certain gradient effect. The impact of the shielding effect of oxygen bubbles on plating uniformity needs to be balanced.

3. Design of catalytic coating

The design of anode coating is the core value of anode manufacturers. Titanium anode is a highly customized product. Its high customization is not only reflected by the variable processing shape of the substrate, but more importantly, it is based on the needs of the client and selects the appropriate coating formula design to ultimately meet the personalized needs of customers. Anode coating design is usually considered from two aspects: the content of precious metals and the design of coating structure. For anodes suitable for PCB copper plating process, the precious metal content mainly refers to the content of iridium metal; while the coating structure includes the selection of specific raw material types of coatings, the adjustment of coating ratios, the change of coating coating sequence and other processing and manufacturing links.

First, the design of anode coating needs to be adapted to specific electroplating conditions. PCB copper plating conditions are divided into DC electroplating and reverse pulse electroplating, and the coating designs adapted to the two copper plating conditions are completely different. If the wrong coating design is selected, not only will the final requirements of the electroplated product not be met, but the life and performance of the anode will also have serious problems.

Secondly, how the anode coating can meet the life requirements mainly depends on the actual use conditions and the customer's expected life requirements. The determination of the precious metal content is not just a simple conversion based on the amount of anode overcharge, but also needs to be determined based on the use conditions, such as the amount of organic matter in the potion, whether there are substances that seriously affect the life of the anode, whether the equipment has design defects that cause the anode to fail to work properly, and many other factors. If the coating structure design is optimized, the precious metal consumption rate can also be reduced to a certain extent. Choosing a suitable coating design scheme is more practical and more important than simply stipulating the precious metal content.

The most important thing for the design of electroplating anode coating is the control requirement for the consumption of additives, which is the core part of the anode coating design. Simply put, to control the consumption of additives, it is necessary to shield the coating with high catalytic activity to reduce its direct contact with the additives. Usually, we call this special coating a barrier coating. At the same time, according to the additives of various manufacturers and the different properties of the additives, we also need to optimize and adapt the coating design accordingly. By changing the properties of the coating (such as surface roughness, surface energy, charge properties, etc.), it is possible to make targeted adsorption or repulsion for certain additives, thereby adjusting the consumption level of certain additives to a certain extent. In summary, the design of the coating truly reflects the high degree of customization of the anode and the core professional capabilities and competitiveness of the manufacturer.

IV. Summary

With the improvement of PCB product requirements and the improvement of PCB factory automation, titanium anodes will gradually replace phosphor copper balls in the PCB copper plating process with their excellent performance. With the improvement of equipment automation, especially in the field of pulse electroplating, the trend of insoluble anodes replacing copper balls is more obvious. At the same time, the emergence of some new applications has also put forward new requirements for the product development of titanium anodes. This is both an opportunity and a challenge for the design and development of titanium anodes.