PCB Tech - Basic knowledge of PCB process design-PCB circuit board

Basic knowledge of PCB process design-PCB circuit board

Printed circuit board (PCB) will appear in almost every electronic device. If there are electronic parts in a certain device, they are all mounted on PCBs of different sizes. In addition to fixing various small parts, the main function of the PCB is to provide electrical connections between the upper parts. As electronic devices become more and more complex, more and more parts are needed, and the circuits and parts on the PCB are becoming more and more dense. The standard PCB looks like this. The bare board (no parts on it) is also often called "Printed Wiring Board (PWB)".

The base plate of the board itself is made of materials that are insulated and heat-insulated, and not easy to bend. The small circuit material that can be seen on the surface is copper foil. The copper foil was originally covered on the entire board, but part of it was etched away during the manufacturing process, and the remaining part became a network of small lines. . These lines are called conductor patterns or wiring, and are used to provide circuit connections for parts on the PCB.

In order to fix the parts on the PCB, we solder their pins directly on the wiring. On the most basic PCB (single panel), the parts are concentrated on one side, and the wires are concentrated on the other side. In this case, we need to make holes in the board so that the pins can pass through the board to the other side, so the pins of the parts are soldered to the other side. Because of this, the front and back sides of the PCB are called Component Side and Solder Side respectively.

If there are some parts on the PCB that need to be removed or installed back after the production is completed, then the socket (Socket) will be used when the part is installed. Since the socket is directly welded to the board, the parts can be disassembled and assembled at will. Seen below is the ZIF (Zero Insertion Force) socket, which allows the parts (here refers to the CPU) to be easily inserted into the socket or removed. The fixing rod next to the socket can be fixed after you insert the part.

If you want to connect two PCBs to each other, we generally use an edge connector commonly known as "golden finger". There are many exposed copper pads on the golden fingers, which are actually part of the PCB wiring. Generally, when connecting, we insert the golden fingers on one PCB into the appropriate slot on the other PCB (generally called the expansion slot Slot). In computers, display cards, sound cards, or other similar interface cards are connected to the motherboard by golden fingers.

The green or brown on the PCB is the color of the solder mask. This layer is an insulating protective layer, which can protect the copper wire and prevent the parts from being welded to the wrong place. A layer of silk screen will be printed on the solder mask. Usually words and symbols (mostly white) are printed on this to mark the position of each part on the board. The screen printing surface is also called the legend surface.

We just mentioned that on the most basic PCB, the parts are concentrated on one side, and the wires are concentrated on the other side. Because the wires only appear on one side, we call this kind of PCB a single-sided (Single-sided). Because single-sided boards have many strict restrictions on the design of the circuit (because there is only one side, the wiring cannot cross and must be around a separate path), so only early circuits use this type of board.

Double-Sided Boards
This kind of circuit board has wiring on both sides. However, to use wires on both sides, there must be a proper circuit connection between the two sides. This kind of "bridge" between circuits is called a via. A via is a small hole filled or coated with metal on the PCB.

It can be connected with wires on both sides. Because the area of the double panel is twice as large as that of the single panel, and because the wiring can be interleaved (it can be wound to the other side), it is more suitable for use in circuits that are more complicated than the single panel.

Multi-Layer Boards
In order to increase the area that can be wired, multi-layer boards use more single or double-sided wiring boards. The multi-layer board uses several double-sided boards, and a layer of insulating layer is placed between each board and then glued (press-fitted). The number of layers of the board means that there are several independent wiring layers. Usually the number of layers is even and contains the two outermost layers. Most motherboards have 4 to 8 layers of structure, but technically it is possible to achieve nearly 100 layers of PCB boards. Most large supercomputers use fairly multi-layered motherboards, but because these types of computers can already be replaced by clusters of many ordinary computers, super-multilayered boards have gradually ceased to be used. Because the layers in the PCB are tightly integrated, it is generally not easy to see the actual number, but if you look closely at the motherboard, you may be able to see it.

The via we just mentioned, if applied to a double-sided board, it must penetrate the entire board. However, in the multilayer board, if you only want to connect some of the lines, the vias may waste some line space in other layers. Buried vias and blind vias technologies can avoid this problem because they only penetrate a few layers. Blind holes are to connect several layers of internal PCB to the surface PCB, without having to penetrate the entire board. Buried vias only connect to the internal PCB, so they cannot be seen from the surface.

In a multilayer PCB, the entire layer is directly connected to the ground wire and the power supply. So we classify each layer as signal layer, power layer or ground layer. If the parts on the PCB require different power supplies, this type of PCB usually has more than two layers of power and wire. Part packaging technology

Through Hole Technology
Place the parts on one side of the board and solder the pins on the other side. This technology is called "Through Hole Technology (THT)" packaging. This kind of part will take up a lot of space, and a hole must be drilled for each pin. So their pins actually take up space on both sides, and the solder joints are also relatively large. But on the other hand, THT parts are better than SMT (Surface Mounted Technology, surface mount technology) parts, and the structure of the connection with the PCB is better. We will talk about this later. Sockets like flat cables and similar interfaces need to be able to withstand pressure, so usually they are all THT packages.

Surface Mounted Technology (Surface Mounted Technology)
For parts using Surface Mounted Technology (SMT), the pins are soldered on the same side as the part. This technique does not need to solder each pin, but drill holes in the PCB.

Surface-mounted parts can even be welded on both sides.
SMT is also smaller than THT parts. Compared with PCBs using THT parts, PCBs using SMT technology have much denser parts. SMT package parts are also cheaper than THT. So it is not surprising that most of the PCBs today are SMT.

Because the solder joints and parts have very small pins, it is very difficult to solder manually. But if you consider that the current assembly is fully automatic, this problem will only occur when repairing parts.
Design Flow
In the PCB design, in fact, before the formal wiring, it has to go through a very long step. The following is the main design process:
System specifications
First, we must first plan out the various system specifications of the electronic equipment. Including system functions, cost constraints, size, operating conditions, and so on. System function block diagram Next, the system function block diagram must be made. The relationship between the squares must also be marked.
Divide the system into several PCBs
If the system is divided into several PCBs, not only the size can be reduced, but also the system has the ability to upgrade and exchange parts. The system function block diagram provides the basis for our division. For example, a computer can be divided into a motherboard, a graphics card, a sound card, a floppy disk drive, a power supply, and so on. Decide on the packaging method and the size of each PCB.
When the technology and the number of circuits used in each PCB are determined, the next step is to determine the size of the board. If the design is too large, then the packaging technology must be changed or re-divided. When choosing a technology, the quality and speed of the circuit diagram must also be considered.
Draw a schematic diagram of all PCB circuits
The outline drawing should show the details of the interconnection between the parts. All PCBs in the system must be traced. Nowadays, CAD (Computer Aided Design) is mostly used. The following is an example of using CircuitMakerTM design.
PCB schematic diagram
Simulation operation of preliminary design
In order to ensure that the designed circuit diagram can work normally, this must be simulated once with computer software. This type of software can read design drawings and display the operation of the circuit in many ways. This is much more efficient than actually making a sample PCB and then manually measuring it.
Place the part on the PCB

The way the parts are placed is determined based on how they are connected. They must be connected to the path in the most efficient way. The so-called efficient wiring is that the shorter the wire and the fewer the number of layers through (this also reduces the number of vias), the better, but we will mention this issue again when we are actually wiring. The following is how the bus is routed on the PCB. In order for each part to have perfect wiring, the placement position is very important.
Test wiring possibilities and correct operation at high speed
Part of the current computer software can check whether the positions of the various parts can be connected correctly, or check whether it can operate correctly under high-speed operation. This step is called arranging parts, but we will not study them too deeply. If there is a problem with the circuit design, you can rearrange the position of the parts before exporting the circuit on the spot.
Export circuit on PCB
The connections in the overview map will now be made on the spot as wiring. This step is usually fully automatic, but in general, some parts need to be changed manually. Below is the wire template of the 2-layer board. The red and blue lines respectively represent the part layer and solder layer of the PCB. The white text and squares represent the markings on the screen printing surface. The red dots and circles represent drill holes and pilot holes. On the far right, we can see that there are gold fingers on the soldering surface of the PCB. The final composition of this PCB is usually called an artwork.
Each design must comply with a set of regulations, such as the minimum reserved gap between the lines, the minimum line width, and other similar practical restrictions. These regulations vary according to factors such as the speed of the circuit, the strength of the transmitted signal, the sensitivity of the circuit to power consumption and noise, and the quality of materials and manufacturing equipment. If the current intensity increases, the thickness of the wire must also increase. In order to reduce the cost of PCB, while reducing the number of layers, we must also pay attention to whether these regulations are still in compliance. If a structure with more than 2 layers is required, then the power layer and ground layer are usually used to prevent the transmission signal on the signal layer from being affected, and it can be used as a protective cover for the signal layer.
Circuit test after wire
In order to confirm that the circuit can function normally after the wire, it must pass the final test. This test can also check for incorrect connections, and all connections follow the outline.
Create production file
Because there are many CAD tools for PCB design, manufacturers must have files that meet the standards before they can manufacture boards. There are several standard specifications, but the most commonly used is Gerber files specifications. A set of Gerber files includes the plan view of each signal, power and ground layer, the plan view of the solder mask layer and the screen printing surface, and designated files such as drilling and picking and placing.