One picture shows the internal structure of a multi-layer PCB! 3D perspective in-depth analysis of high-end PCB board design process

For hardware engineers who are new to multi-layer PCBs, they are often confused by their complex structures. After all, the circuits of a PCB with eight or ten layers are intricate and complex, like a spider web.

However, today I drew several internal structure diagrams of multi-layer PCBs. Through three-dimensional graphics, I clearly show the internal structure of PCB diagrams of various stacked structures. I hope it can help everyone better understand the internal structure of multi-layer PCBs.

一、The core of high-density interconnect (HDI) is vias

The circuit processing technology of multi-layer PCB is not essentially different from that of single-layer and double-layer PCB. The biggest difference is reflected in the via process.

The circuits are formed by etching, and the vias are drilled first and then copper plated. Professionals engaged in hardware development are familiar with this, so I will not elaborate on it in detail.

Common types of multi-layer circuit boards include through-hole boards, first-order boards, second-order boards, and second-order stacked boards. High-order circuit boards such as third-order boards and arbitrary-layer interconnection boards are not only rarely used in daily life, but also expensive, and are not within the scope of this discussion.

In actual applications, 8-bit single-chip microcomputer products generally use 2-layer through-hole boards; 32-bit single-chip microcomputer-level smart hardware usually uses 4 to 6 layers of through-hole boards; smart hardware based on Linux and Android systems mostly uses 6-layer through-hole boards to 8-layer first-order HDI boards; and compact products such as smart phones that have extremely high requirements for space layout generally use 8-layer first-order boards to 10-layer second-order boards.

二、The most common through-hole

In the field of printed circuit boards (PCBs), there is a type of board called a through-hole board. Its notable feature is that there is only a single type of via, which starts from the first layer of the board and goes straight through to the last layer. Whether it is a line distributed outside the board or a line hidden inside, the via involved is a completely through-hole form.

The characteristics of through-hole boards are not related to the number of layers of the circuit board. The two-layer circuit boards we come into contact with in daily life are mostly through-hole boards; in some professional fields, such as switches and circuit boards for military purposes, even if the number of layers is as high as 20, through-hole design is still used.

When making a through-hole board, a drill is needed to drill through the circuit board, and then copper plating is performed in the drilled hole to form an electrical path.

It is worth noting that the common inner diameter specifications of through holes are 0.2mm, 0.25mm and 0.3mm. Among them, the cost of through holes with an inner diameter of 0.2mm is much higher than that of 0.3mm. This is because the use of too fine a drill is very easy to break during the drilling process, and the drilling speed is slow. The resulting extended processing time and increased drill bit loss are ultimately reflected in the increase in the price of the circuit board.

三、Laser holes for high density boards (HDI boards)

This picture shows the stacked structure of a 6-layer 1-stage HDI board. Both of its surface layers use laser holes with an inner diameter of 0.1mm. The inner layer is a mechanical hole. The overall structure is equivalent to a 4-layer through-hole board with two additional layers on the outside.

Laser processing technology has unique characteristics. It can penetrate glass fiber boards but cannot penetrate metal copper. For this reason, when laser drilling is performed on the outer surface of the board, it will not cause any impact on other internal circuits.

After the laser drilling is completed, the copper plating process is carried out immediately. In this way, the laser via is successfully formed, and a stable and reliable electrical connection between different layers is achieved.

四、2-stage HDI board, two-layer laser hole

This picture shows a 6-layer 2-step staggered hole HDI board. In actual applications, people rarely use the 6-layer 2-step design, and most start with 8-layer 2-step. In fact, the design principle of the 2-step staggered hole HDI board with more layers is no different from that of the 6-layer one.

The “2-step” here refers to the presence of 2 layers of laser holes on the board. And “staggered holes” means that the positions of the two layers of laser holes are staggered.

So why use a staggered design? This is because during the copper plating process, the hole cannot be completely filled with copper, and there will be a certain gap. If you punch holes directly on the holes that are not fully filled, it may cause unstable connections or other electrical problems. Therefore, it is necessary to stagger a certain distance and then punch holes on the upper layer.

From a structural point of view, the construction of the 6-layer 2-step HDI board is equivalent to stacking 2 layers on the basis of the 4-layer 1-step HDI board. Similarly, the 8-layer 2-step HDI board is based on the 6-layer 1-step HDI board, with an additional 2 layers.

五、Stacked plate has complex process and higher price

If the two layers of laser holes of the staggered hole plate are overlapped, the circuit layout will be more compact.

However, this design requires the inner layer laser holes to be filled by electroplating before the outer layer laser holes can be processed. Due to the more complicated process, its production cost is higher than that of conventional staggered hole plates.

六、Super expensive arbitrary layer interconnection board Multi-layer laser stacking

In the arbitrary layer interconnection board, each layer is equipped with laser holes, which allows the layers to be freely connected. For layout engineers, this means almost unlimited design freedom, whether it is routing planning or punching layout, they can do whatever they want. Such design flexibility can make engineers excited just thinking about it, and they no longer have to worry about design problems.

However, for procurement personnel, arbitrary layer interconnection boards bring huge cost pressure. The price of such boards is more than 10 times higher than that of ordinary through-hole boards. The high cost makes most manufacturers stay away.

For this reason, only products such as iPhone that pursue extreme craftsmanship and performance on the market are willing to use arbitrary layer interconnection boards. As for other mobile phone brands, it is rare to hear of cases of using such high-end boards.

HXPCB focuses on the research and development of ultra-large size circuits board, multi-layer PCBs, metal-based PCBs, rigid-flex high-frequency PCBs, HDI PCBs, and related processes. We offer comprehensive services, including PCB design, prototyping, processing, and assembly.