Multilayer Printed Circuit Board Technology

Multilayer printed circuit boards are more than two layers, such as 4 layers, 6 layers, 8 layers and so on. Of course, some designs are 3-layer or 5-layer circuits, also called multilayer printed circuit boards. The conductive wiring pattern is larger than the 2-layer board, and the layers are separated by insulating substrates. After each layer of wiring is printed, each layer of wiring is overlapped by pressing. Then drill the holes, and realize the conduction between the lines of each layer of the multilayer printed circuit board through the holes.

The advantage of the multilayer printed circuit board is that the lines can be distributed in multiple layers, so that more sophisticated products can be designed. Or smaller products can be realized through multilayer printed circuit boards. Such as: mobile phone circuit boards, miniature projectors, recording pens and other relatively large products. In addition, multilayer printed circuit boards can increase the flexibility of design, better control differential impedance and single-ended impedance, and better output of some signal frequencies.

The Emergence of Multilayer Printed Circuit Boards

Multilayer printed circuit board is the inevitable product of the development of electronic technology in the direction of high speed, multi-function, large capacity and small volume. With the continuous development of electronic technology, especially the extensive and in-depth application of large-scale and ultra-large-scale integrated circuits, multilayer printed circuit boards are rapidly developing in the direction of high density, high precision, and high-level digitization. Hole buried hole high PCB thickness to diameter ratio and other technologies to meet the needs of the market.

Due to the needs of high-speed circuits in the computer and aerospace industries. It is required to further increase the packaging density, coupled with the reduction of the size of the separated components and the rapid development of microelectronics, the electronic equipment is developing in the direction of reducing the volume and reducing the quality; Single and double-sided printed boards are no longer possible due to the limited of available space. A further increase in assembly density is achieved. Therefore, it is necessary to consider using more printed circuits than double-sided board layers. This creates conditions for the emergence of multilayer printed circuit boards.

Multilayer Printed Circuit Board Principle

With the rapid development of SMT (Surface Mount Technology), and the continuous release of a new generation of SMD (Surface Mount Components), such as QFP, QFN, CSP, BGA (especially MBGA), electronic equipment is more intelligent, Miniaturization, thus promoting important innovations and developments in the technological level of PCB industrial production. Since IBM first successfully developed a high-density multilayer printed circuit board in 1991, various companies around the world have also successively developed a variety of high-density interconnect (HDI) microvias.

With the rapid development of these processing technologies, PCB design is gradually developing towards double-layer, high-density wiring. Double-layer printed circuit boards are widely used in the manufacture of electronic equipment for their flexible design, stable and reliable electrical performance and excellent economic performance.

The biggest difference between a multilayer printed circuit board and a double-sided board is that the internal power layer (protecting the internal power layer) and the grounding structure are enhanced. Switching power supply and ground wire system key is routed on the power layer. The wiring of the multilayer printed circuit board is mainly based on the upper layer and the bottom layer, and is filled by the middle wiring layer. Therefore, the design method of the multi-layer board is basically the same as that of the single-sided board. The key is how to improve the wiring of the internal electrical layer, so that the wiring of the multilayer printed circuit board is more effective, and the electromagnetic compatibility testability is stronger.

Multilayer Printed Circuit Board Construction

The number of layers of multilayer printed circuit boards is not limited. At present, there are already more than 20 layers of PCBs, and 4-layer and 6-layer boards are common.

The simple 4-layer multilayer printed circuit board increases the power layer and ground layer on the basis of Top Layer and Bottom Layer, which greatly solves the problem of interference signals, improves the stability of the operating system, and can also improve the routing of power lines rate, reducing the total area of the multilayer printed circuit board.

Most of the 4-layer and 6-layer multilayer printed circuit boards are used on computer motherboards. It is produced by bonding 4 or 6 layers of composite resin together. The electronic components on the computer motherboard are connected according to the copper wires inside the PCB. , to transmit the data signal. Taking a 4-layer multilayer printed circuit board as an example, the uppermost and lowermost double layers are "signal layers", and the middle double layers are "grounding structure" and "power layer". Putting the two "data signal layers" on both sides of the power layer and the connection application can not only avoid the influence of each other, but also facilitate the adjustment of the power line. In addition, some computer motherboards also use a 6-layer multilayer printed circuit board design, which can be regarded as an improved solution for a 4-layer PCB board, which can further reduce the mutual influence between the data signal layers.

Nowadays, multilayer printed circuit boards are used on motherboards and graphics cards, which greatly increases the area that can be routed. Multilayer printed circuit boards use a large number of single or double-sided wiring boards, and put a layer of cable sheath between each multilayer board and press them together. The number of superimposed layers of a multilayer printed circuit board indicates how many separate wiring layers there are. Usually, the number of superimposed layers is even, and includes the double layers on both sides. Common multilayer printed circuit boards generally have 4 to 8 layers structure.

Requirements for Layering of Multilayer Printed Circuit Boards

1. The power plane should be close to the ground plane, closely coupled with the ground plane, and arranged under the ground plane.

2. The signal layer of a multilayer printed circuit board should be adjacent to the internal electrical layer, and should not be directly adjacent to other signal layers.

3. The multilayer printed circuit board isolates the digital circuit and the analog circuit. If conditions permit, arrange analog signal lines and digital signal lines in layers and adopt shielding measures; if they need to be arranged on the same signal layer, isolation strips and ground lines need to be used to reduce interference; power supplies for analog circuits and digital circuits Ground and ground should be isolated from each other and cannot be mixed.

4. The high-frequency circuit has a lot of external interference, so it is best to arrange it separately. The multilayer printed circuit board uses the intermediate signal layer with the inner layer directly adjacent to the upper and lower sides to transmit, so that the copper film of the inner layer can be used to reduce external interference.

Multilayer Printed Circuit Board Layering Standard

The setting of the number of superimposed layers of the multi-layer board is very flexible, and the designer can carry out scientific and reasonable setting according to the specific situation. So, what are the layered standards for multilayer printed circuit boards? The settings of various layers should meet the following aspects:

1. The plane of the switching power supply of the multilayer printed circuit board should be close to the ground plane, closely coupled with the ground plane, and distributed under the ground plane. The lower part of the component layer is the ground wire layer, which gives the components a shielding layer and a reference layer for the high-level wiring;

2. The data signal layer of the multilayer printed circuit board should be adjacent to the internal electrical layer, not directly adjacent to other data signal layers, and all data signal layers should be as close as possible to the grounding layer;

3. The multilayer printed circuit board protects digital circuit design and digital integrated circuits. If the situation permits, lay out the surface layer of the pulse signal line and data power line, and adopt shielding measures; if it must be laid out on the same data signal layer, you must use isolation fences and ground wire frames to reduce the impact; digital integrated circuits and digital circuits The designed switching power supply and ground should protect each other and cannot be used in combination.

4. When the multilayer printed circuit board is formulated, each layer should maintain symmetry, and it is best to have an even number of copper layers. If it is asymmetrical, it will easily cause distortion.

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