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PCB Process Introduction - Blind Burial Hole

When it comes to blind/buried holes, let's start with traditional multi-layer boards. The standard multi-layer board structure includes both inner and outer layers of circuits, and utilizes drilling and metalization processes to achieve the internal connection function of each layer of circuit. However, due to the increase in circuit density, the packaging methods of parts are constantly being updated. In order to accommodate more high-performance components with limited PCB area, in addition to the narrower circuit width, the aperture of the DIP socket is also reduced from 1mm to 0.6mm of SMD, and further reduced to below 0.4mm. However, it still occupies the surface area, so there are buried holes and blind holes, which are defined as follows:

A. Buried Via

As shown in Figure 20.1, the through holes between the inner layers cannot be seen after pressing, so there is no need to occupy the area of the outer layer

B. Blind Via

As shown in Figure 20.1, the buried hole design and production process for connecting the surface layer and one or more inner layers are more complex and costly than traditional multi-layer boards. Figure 20.2 shows the differences in the production of traditional inner layers and inner layers with buried holes, while Figure 20.3 explains the laminated structure of an eight layer buried hole board Figure 20.4 shows the general specifications of buried holes, general through holes, and PAD sizes. Figure 20.2 Blind hole design and production

The board with extremely high density and double-sided SMD design will have interference between the upper and lower layers of the outer layer and the I/O guide holes, especially when there is VIP (Via in pad) design, which is even more troublesome. Blind holes can solve this problem. In addition, with the prevalence of wireless communication, the design of the line must reach the RF (Radio frequency) range, exceeding 1GHz Blind hole design can meet this requirement, and Figure 20.5 shows the general specifications of blind holes. There are three different methods for making blind hole boards, as follows

A. Mechanical depth drilling

The traditional multi-layer board manufacturing process uses a drilling machine to set the Z-axis depth for drilling after pressing, but this method has several problems

a. Only one drill per time produces very low output

b. The levelness requirement for the drilling machine table is strict, and the drilling depth setting for each spindle must be consistent, otherwise it is difficult to control the depth of each hole

c. Electroplating inside the hole is difficult, especially if the depth is greater than the aperture, it is almost impossible to do well in hole electroplating.

The limitations of the aforementioned processes have gradually led to the discontinuation of this method.

B. Sequential compaction

Taking the eight layer board as an example (see Figure 20.6), the successive pressing method can simultaneously create blind buried holes. Firstly, the four inner layers of the board are made using a typical double-sided leather circuit and PTH (other combinations may also be available; six layer board+double-sided board, upper and lower double panels+inner four layer board). Then, the four pieces are pressed together to form a four layer board, and then full through-hole production is carried out. This method has a long process and higher cost than other methods, so it is not common.

C. Non machine drilling method of Build up Process

At present, this method is most favored by the global industry, and there are also many domestic manufacturers with manufacturing experience.

This method applies the concept of sequential layering mentioned above, adding layers one by one to the outside of the board, and using non machine drilling blind holes as interconnections between layers. There are three main methods, briefly described as follows:

a. Photo Defind uses a photosensitive blocking agent, which is also a permanent dielectric layer, to expose and develop the bottom copper pad at specific locations, forming a bowl shaped blind hole. It is then fully coated with chemical copper and copper plating. After etching, the outer circuit and Blind Via can be obtained, or copper paste or silver paste can be filled instead of copper plating to complete conductivity. According to the same principle, it can be added layer by layer.

b. Laser Ablation laser hole burning can be divided into three types:; One is CO2 laser. One is Excimer laser, and the other is Nd: YAG laser. Some comparison items of these three laser hole burning methods are shown in Table 20.1

c. Plasma Etching, also known as the DYCOSTRATE method, is a patent of Dyconex company. The comparison can be found in Table 20.1. In addition to the comparison in Table 20.1, Figure 20.7 illustrates that the three commonly used blind hole processes should be visible. Chemical Etching will not be introduced here. Figure 20.8 provides a three-dimensional illustration of various drilling methods for reference.

The definition and process of blind/buried holes were explained, and Figure 20.9 is illustrated in a three-dimensional manner. The application of buried/blind hole design in traditional multi-layer boards significantly reduces the area.

The application of buried/blind holes is bound to become increasingly common, and its investment amount is very large. Large and medium-sized factories of a certain scale should aim for large-scale production and high yield, while smaller factories should act according to their own capabilities and seek niche markets in order to achieve sustainable operation