How to make PCB with blind hole

The use of blind holes and buried holes is an effective method to increase the density of multilayer boards, reduce the number of layers and board size, and significantly reduce the number of plated through-holes. Almost all BUM boards use buried and blind via structures. Buried holes and blind holes are mostly small holes with a diameter of 0.05 to 0.15 mm. Buried holes are manufactured on the inner thin laminate using the same process of manufacturing double-sided PCBs. In contrast, the manufacturing of blind holes started with a small hole CNC bed that controls the Z-axis depth, laser drilling, plasma etching, and photo-induced holes are now commonly used. Laser drilling has carbon dioxide laser machine and Nd: YAG ultraviolet laser machine. The carbon dioxide laser drilling machine of Hitachi, Japan, has a laser wavelength of 9.4 Hongm, and one blind hole is drilled three times, which can drill 30,000 holes per minute. With the development of electronic products that require high density and high precision, the same requirements are put forward on circuit boards accordingly. The most effective way to increase the PCB density is to reduce the number of through-holes and accurately set blind holes and buried holes.

There are three different methods to make the blind hole PCB:

1. Mechanical Controlled Depth Drilling

In the traditional multilayer board manufacturing process, the drilling machine is used to set the Z-axis depth for drilling, but there are some concerns:

a.Only one piece can be drilled at a time, and the output is very low.

b.The drilling machine table must be level, and the drilling depth of each spindle must be the same. Otherwise, it is difficult to control the depth of each hole.

c.Electroplating in the hole is difficult, especially if the depth is greater than the hole diameter, it is almost impossible to electroplate in the hole.

Due to the limitations of the processes mentioned above, this method is becoming less popular.

2.Sequential Lamination

Take an 8-layer PCB board as an example (see Figure )

altium blind via

The Sequential lamination can make blind and buried holes at the same time. First, make the circuit and PTH of the four inner layers (six layers + double layer, top and double bottom layer, and inner four layers). Then laminate the four pieces together into a four-layer board, then make a through-hole. This method has a long process and a higher cost, so it is not universal.

3. Build Up Process and Non-machine Drilling Method

This method is the most popular in the industry, and many domestic manufacturers have similar manufacturing experience.

This method extends the above-mentioned sequential lamination concept, adding layer by layer to the outside of the board and using non-machine-drilled blind holes as the interconnection between the layers. Here are the three main methods:

  1. Photo Defined- The photo defined is formed using a photosensitive resist, which is also a permanent medium layer. The exposed film in a specific location was then developed to expose the copper pad at the bottom to form a bowl-shaped blind hole. After etching, we can get the outer circuit and blind via. Instead of copper plating, copper paste or silver paste is filled to complete the conduction. According to the same principle, it can be added layer by layer.
  2. Laser Ablation- There are three types: CO2 laser, excimer laser, Nd:YAG laser.
  3. Plasma Etching, This is a patent of Dyconex, and the commercial name is DYCOSTRATE.

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