Speaking of PCB board, many friends will think that it can be seen everywhere around us, from all household appliances, all kinds of accessories in the computer, to all kinds of digital products, as long as electronic products almost all use PCB board, so what is PCB board? A PCB is a PrintedCircuitBlock, which is a printed circuit board for electronic components to be inserted. A copperplated base plate is printed and etched out of the etching circuit.

PCB board can be divided into single layer board, double layer board and multi layer board. Electronic components are integrated into the PCB. On a basic single-layer PCB, the components are concentrated on one side and the wires are concentrated on the other. So we need to make holes in the board so that the pins can go through the board to the other side, so the pins of the parts are welded to the other side. Because of this, the positive and negative sides of such PCB are respectively called ComponentSide and SolderSide.

A double-layer board can be seen as two single-layer boards glued together, with electronic components and wiring on both sides of the board. Sometimes it is necessary to connect a single wire from one side to the other side of the board through a guide hole (via). Guide holes are small holes in the PCB filled or coated with metal that can be connected to wires on both sides. Now many computer motherboards are using 4 or even 6 layers of PCB board, while graphics cards generally use 6 layers of PCB board. Many high-end graphics cards like nVIDIAGeForce4Ti series use 8 layers of PCB board, which is called multi-layer PCB board. The problem of connecting lines between layers is also encountered on multi-layer PCBS, which can also be achieved through guide holes.

Because it is a multi-layer PCB, sometimes the guide holes do not need to penetrate the entire PCB. Such guide holes are called Buriedvias and Blindvias because they only penetrate a few layers. Blind holes connect several layers of internal PCBS to surface PCBS without penetrating the entire board. Buried holes are only connected to the internal PCB, so light is not visible from the surface. In a multilayer PCB, the entire layer is directly connected to the ground wire and the power supply. So we classify the layers as Signal, Power or Ground. If the parts on the PCB require different power supplies, they usually have more than two power and wire layers. The more layers you use, the higher the cost. Of course, the use of more layers of PCB board to provide signal stability is very helpful.

The process of making a professional PCB board is quite complicated. Take a 4-layer PCB board for example. The PCB of the main board is mostly 4 layers. When manufacturing, the middle two layers are rolled, cut, etched, oxidized and electroplated respectively. The four layers are component surface, power layer, stratum and solder lamination respectively. The four layers are then pressed together to form a PCB for the main board. Then the holes were punched and made. After cleaning, the outer two layers of the line is printed, copper, etching, testing, welding resistance layer, screen printing. Finally, the whole PCB (including many motherboards) is stamped into PCB of each motherboard, and then vacuum packaging is carried out after passing the test. If the copper skin is not well coated in THE process of PCB production, there will be poor adhesion phenomenon, easy to imply short circuit or capacitance effect (easy to cause interference). The holes on PCB must also be taken care of. If the hole is punched not in the middle, but on one side, it will result in uneven matching or easy contact with the power supply layer or formation in the middle, resulting in potential short-circuiting or bad grounding factors.

Copper wiring process

The first step in fabrication is to establish an online wiring between parts. We use negative transfer to express the working negative on a metal conductor. The trick is to spread a thin layer of copper foil over the entire surface and remove any excess. Appending transfer is another less used method, which is to apply copper wire only where it is needed, but we won’t talk about it here.

Positive photoresists are made from photosensitizers that dissolve under illumination. There are many ways to treat photoresist on copper, but the most common way is to heat it and roll it over a surface containing photoresist. It can also be sprayed in liquid form, but the dry film provides higher resolution and allows for thinner wires. The hood is just a template for making PCB layers. A hood covering the photoresist on the PCB prevents some areas of the photoresist from being exposed until the photoresist is exposed to UV light. These areas, which are covered with photoresist, will become wiring. Other bare copper parts to be etched after photoresist development. The etching process may involve dipping the board into the etching solvent or spraying the solvent onto the board. Generally used as etching solvent using ferric chloride etc. After etching, remove the remaining photoresist.

1. Wiring width and current

General width should not be less than 0.2mm (8mil)

On high density and high precision PCBS, pitch and line width are generally 0.3mm (12mil).

When the thickness of copper foil is about 50um, the wire width is 1 ~ 1.5mm (60mil) = 2A

The common ground is generally 80mil, especially for applications with microprocessors.

2. How high is the frequency of high-speed board?

When the rise/fall of the signal time “3~6 times the signal transmission time, it is considered as high speed signal.

For digital circuits, the key is to look at the edge steepness of the signal, the time it takes to rise and fall,

According to a very classic book “High Speed Digtal Design” theory, the signal from 10% to 90% of the time is less than 6 times the wire delay, is high-speed signal! — — — — — – namely! Even 8KHz square wave signals, as long as the edges are steep enough, are still high-speed signals, and transmission line theory needs to be used in wiring

3.PCB stacking and layering

The four – layer plate has the following stacking sequence. The advantages and disadvantages of different lamination are explained below:

The first case should be the best of the four layers. Because the outer layer is the stratum, it has a shielding effect on EMI. Meanwhile, the power supply layer is reliable and close to the stratum, which makes the internal resistance of the power supply smaller and achieves the best suburbs. However, the first case cannot be used when the board density is relatively high. Because then, the integrity of the first layer is not guaranteed, and the second layer signal is worse. In addition, this structure can not be used in the case of large power consumption of the whole board.

The second case is the one we usually use the most. From the structure of the board, it is not suitable for high-speed digital circuit design. It is difficult to maintain low power impedance in this structure. Take a plate 2 mm as an example: Z0=50ohm. To line width of 8mil. Copper foil thickness is 35цm. So the signal layer and the middle of the formation is 0.14mm. The formation and power layer are 1.58mm. This greatly increases the internal resistance of the power supply. In this kind of structure, because the radiation is to the space, shielding plate is needed to reduce EMI.

In the third case, the signal line on layer S1 has the best quality. S2. EMI shielding. But the power supply impedance is large. This board can be used when the power consumption of the whole board is high and the board is an interference source or adjacent to the interference source.

4. Impedance matching

The amplitude of the reflected voltage signal is determined by the source reflection coefficient ρ S and the load reflection coefficient ρL

ρL = (RL-z0)/(RL + Z0) and ρS = (rS-z0)/(RS + Z0)

In the above equation, if RL=Z0, the load reflection coefficient ρL=0. If RS=Z0 source-end reflection coefficient ρS=0.

Because the ordinary transmission line impedance Z0 should usually meet the requirements of 50 ω 50 ω, and the load impedance is usually in thousands of ohms to tens of thousands of ohms. Therefore, it is difficult to realize impedance matching at the load side. However, because the signal source (output) impedance is usually relatively small, roughly in the tens of ohms. It is therefore much easier to implement impedance matching at the source. If a resistor is connected at the load end, the resistor will absorb part of the signal to the detriment of transmission (my understanding). When the TTL/CMOS standard 24mA drive current is selected, its output impedance is approximately 13 ω. If the transmission line impedance Z0=50 ω, then a 33 ω source-end matching resistor should be added. 13 ω +33 ω =46 ω (approximately 50 ω, weak underdamping helps signal setup time)

When other transmission standards and drive currents are selected, the matching impedance can be different. In high-speed logic and circuit design, for some key signals, such as clock, control signals, we recommend that the source matching resistor must be added.

In this way, the connected signal will be reflected back from the load side, because the source impedance matches, the reflected signal will not be reflected back.