Basic knowledge of hot printing board

Basic knowledge of printed circuit board

printed circuit board (PCB) appears in almost every electronic device. If there are electronic parts in a certain device, they are also embedded on PCBs of different sizes. In addition to fixing various small parts, the main function of PCB is to provide the mutual electrical connection of the upper parts. As electronic equipment becomes more and more complex, more and more parts are needed, and the circuits and parts on PCB are more and more dense

bare board (without 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 not easy to bend. The thin circuit material that can be seen on the surface is copper foil. Originally, copper foil was covered on the whole board, but part of it was etched away in the manufacturing process, and the remaining part became a thin circuit. These circuits are called conductor patterns or wiring, and are used to provide circuit connections for parts on 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 way, 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 welded on the other side. Because of this, the front and back sides of 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 production, then the socket will be used for the installation of this part. Since the socket is directly welded to the board, the parts can be disassembled and assembled at will. The following is ZIF (zero insertion force) socket, which allows parts (here refers to CPU) to be easily inserted into the socket or removed. The fixing rod beside the socket can be fixed after you insert the part

if we want to connect the two PCBs, we usually use the edge connector commonly known as "golden finger". Gold finger contains many exposed copper pads, which are actually part of PCB wiring. Usually, when connecting, we insert the gold finger on one PCB into the appropriate slot on the other PCB (generally called expansion slot). In computers, such as display cards, sound cards, or other similar interface cards, are connected to the motherboard with a golden finger

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

single sided boards

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

double sided boards

this kind of circuit board has wiring on both sides. However, if you want to use wires on both sides, you must have appropriate circuit connections between the two sides. This "bridge" between circuits is called a via. The guide hole is a small hole filled or coated with metal on the PCB, which can be connected with the wires on both sides. Because the area of the double-sided board is twice that of the single panel, and because the wiring can be staggered (can be wound to the other side), it is more suitable for more complex circuits than the single panel

multi layer boards

in order to increase the area of wiring, multi-layer boards use more single or double-sided wiring boards. Several pieces of double-sided boards are used for multilayer boards, and an insulating layer is placed between each layer of boards and then glued (pressed). The number of layers of the board represents several independent wiring layers. Usually, the number of layers is even and includes the outermost two layers. Most of the main boards have a 4-8-layer structure, but technically they can have nearly 100 layers of PCB boards. Most large supercomputers use quite multi-layer mainboards, but because such computers can be replaced by clusters of ordinary computers, super multi-layer boards have gradually been out of use. Because the layers in the PCB are closely combined, it is generally not easy to see the actual number, but if you carefully observe the motherboard, you may be able to see it

if the via we just mentioned is applied to the double-sided board, it must be through the whole board. However, in multilayer boards, if you only want to connect some of the circuits, the pilot hole may waste some circuit space in other layers. Therefore, the mechanical properties of materials are not described in this paper. IAS and blind vias technology can avoid this problem, because they only penetrate several layers. Blind hole is to connect several layers of internal PCB with surface PCB without penetrating the whole board. The buried hole is only connected to the internal PCB, so it can't be seen from the surface alone

in Multilayer PCB, the whole layer is directly connected with ground wire and power supply. So we classify each layer into signal layer, power layer or ground layer. If the parts on the PCB need different power supplies, usually this kind of PCB will have more than two layers of power and wire

part packaging technology

through hole technology

place the parts on one side of the board and weld 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 need to drill a hole for each pin. So their pins actually take up space on both sides, and the solder joint is relatively large. On the other hand, tht parts are better connected with PCB than SMT (surface mounted technology) parts. We will talk about this later. For example, the sockets of flat cables and similar interfaces need to be able to withstand pressure, so they are usually tht packaged

surface mounted technology

for parts using surface mounted technology (SMT), the pins are welded on the same side as the parts. This technology does not need to drill holes in the PCB for the welding of each pin

surface adhesive parts can even be welded on both sides

smt is also smaller than tht parts. Compared with the PCB using tht parts, the parts on the PCB using SMT technology are much denser. SMT packaging parts are also cheaper than tht. So it's not surprising that most of today's PCBs are SMT

because the solder joint and the pin of the part are very small, it is very difficult to weld manually. However, considering that the current assembly is fully automatic, this problem will only occur when repairing parts

design process

in PCB design, in fact, there are still a long process before formal wiring. The following is the main design process:

system specification

first of all, plan out the system specifications of the electronic equipment. It includes system functions, cost constraints, size, operation, etc

system function block diagram

next, we must make a functional block diagram of the system. The relationship between blocks must also be marked

divide the system into several PCBs

if the system is divided into several PCBs, it can not only reduce the size, but also make the system have the ability to upgrade and exchange parts. The system function block diagram provides the basis for our segmentation. For example, computers can be divided into mainboard, display card, sound card, floppy disk drive, power supply and so on

decide to use the packaging method and the size of each PCB

when the technology and the number of circuits used by each PCB are determined, the next step is to decide the size of the board. If the design is too large, then the packaging technology needs to be changed or re divided. When choosing technology, we should also take into account the quality and speed of the circuit diagram

draw the circuit overview of all PCBs

the overview should show the interconnection details between the parts. PCB in all systems must be traced. Nowadays, CAD (Computer Aided Design) is mostly used. The following is an example of using circuitmakertm design

simulation operation of preliminary design

in order to ensure that the designed circuit diagram can operate normally, it must be simulated once with computer software. This kind of software can read design drawings and display the operation of circuits in many ways. This is much more effective than actually making a sample PCB and then manually measuring it

place parts on PCB

the way parts are placed depends on how they are connected. They must be connected to the path in the most efficient way. The so-called efficient wiring means that the shorter the lead and the fewer layers it passes through (which also reduces the number of guide holes), the better. However, we will mention this problem again in the real wiring. The following is how the bus is wired on the PCB. In order to make all parts have perfect wiring, the location is very important

test the possibility of wiring and the correct operation at high speed

some computer software today can check whether the positions of various parts can be correctly connected, or whether they can operate correctly at high speed. This step is called arranging parts, but we won't study these in depth. If there is a problem with the circuit design, you can also rearrange the position of parts before exporting the circuit on the spot

export the connection of the circuit

on the PCB in the overview, and now it will be made into a wiring appearance on the ground. This step is usually fully automatic, but generally speaking, some parts still need to be changed manually. The following is the conductor template of the 2-layer board. The red and blue lines represent the part layer and welding layer of PCB respectively. White words and squares represent the signs on the printing surface. Red dots and circles represent holes and pilot holes. On the far right, we can see that there are gold fingers on the welding surface of PCB. The final composition of this PCB is usually called artwork

each design must comply with a set of regulations, such as the minimum reserved gap between lines, the minimum line width, and other similar actual restrictions. These regulations vary according to the speed of the circuit, the strength of the transmission signal, the sensitivity of the circuit to power consumption and noise, as well as the material quality and manufacturing equipment. If the current intensity increases, the thickness of the wire must also be increased. In order to reduce the cost of PCB, we must also pay attention to whether these Regulations are still met while reducing the number of layers. If more than two layers of structure are needed, the power layer and ground layer are usually used to avoid the transmission signal on the signal layer being affected, and can be used as the anti-interference layer of the signal layer