Advantages and Disadvantages of Blind and Buried Via PCB Stacked Via Design

Author: Wang Huidong, member of Mr. Yibo Technology High Speed

Maomao said that his recent life is like a weather forecast, sunny, cloudy, rainy, and if it rains, it will rain. It is still a violent storm, with thunder and lightning.

This is not just the design of a second-order HDI, and it was put into the board factory for two weeks, and it will be delivered soon. The factory suddenly called and said that there was an open circuit between the blind and buried holes on the board.I'm confused, what's the problem? No, I'm going to call Ruyan from Company E and talk to her about this PCB.

Ruyan received the call and saw Maomao's design. There are blind holes and buried holes stacked on the board. Haha, Maomao, let's talk about it today, this is the cornerstone of PCB wiring.Magical technology - stacked via design of blind and buried vias.

From the picture above, can you see how many levels of HDI the PCB is?

With the development of the electronic industry, electronic products are developing towards high density and high precision, and there are relatively strict requirements for the wiring space of circuit boards.The most effective way to increase the density of PCB wiring is to reduce the number of through holes. The design of blind and buried vias is no longer out of reach, but is integrated into the PCB design bit by bit and becomes within reach.

Usually, there are two types of blind holes of second order or above. One is wrong holes. As the name suggests, wrong holes are two blind holes that are staggered.

Another design is the stacking design of blind vias, that is, two blind vias are overlapped.As shown below:

Can you tell which is the wrong hole and which is the stacked hole design?

So what are the advantages of the stacked hole design?

Blind and buried vias can provide more space and options to the PCB.The buried via design will help free up space on the board without affecting surface devices or traces on the upper and lower layers.Stacked vias of blind vias and stacked vias of blind-buried vias help free up more space.
The following is a production flow chart of a second-order HDI:

We can see from the above picture that the blind hole in the inner layer must be filled with electroplating once after the stacked hole design is made, so the flatness of the blind hole filled electroplating is more stringent.If the bottom of the blind hole is filled with uneven electroplating, after the second blind hole is drilled, there is no conduction of the copper plated layer below, and there is a floating between the layers, and an open circuit appears.

So for the blind via stack, the first blind via should be filled and electroplated, and the wrong via does not need to be done.Therefore, the process is longer, the time is longer, and the requirements for the process are also stricter.

The method of filling blind holes by electroplating is used to realize the stacking method of blind holes and blind holes. Due to its advantages of high reliability and simple process, it has become the most ideal filling method at present.Most of the manufacturing technologies of second-order or multi-level HDI boards use the methods of laser drilling blind holes and electroplating blind holes to realize the interconnection between layers. The manufacturing difficulties are blind hole processing, electroplating and blind hole filling, and alignmentAccuracy control, the blind holes are stacked and then filled and electroplated has the following advantages:

1. It saves more wiring space than wrong holes
2. It increases the reliability of heat conduction
3.Increase the current carrying capacity
4. Make the surface pad more flat and make the welding more reliable.

If the blind hole and the buried hole are stacked, a POFV process (resin plug hole electroplating and leveling) process for the inner layer is required.

Blind hole and blind hole stacking process, the flatness of the resin plug hole becomes the key point for the success or failure of the process.The key point of resin plug holes is that if the buried resin plug holes are not plated flat, everything is designed to be empty.

Ruyan said: "Maomao, there is nothing wrong with your design, but the key depends on the craftsmanship of the factory."
Maomao said I understand, next time I will go to Ruyan to make a board.

Summary:
The stacking design of blind vias and blind vias, the process increases the inner layer blind via filling and electroplating, the process increases, and the cost increases.
The stacking design of blind vias and buried vias, the process increases the POFV process of the inner layer, the process increases, and the cost increases.
The through-hole design should try not to have blind holes, and the blind holes should not be stacked as much as possible.