PCB Tech - Basic knowledge of PCB design for novices

Basic knowledge of PCB design 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.

Basic knowledge of PCB design for novices

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.

Single-Sided Boards 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 type 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, which can be connected to the wires on both sides. Because the area of the double-sided board is twice as large as that of the single-sided board, 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-sided board.

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 a 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 used 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.

Surface Mounted Technology (Surface Mounted Technology) Parts using Surface Mounted Technology (SMT), the pins are soldered on the same surface 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. However, if you consider that the current assembly is fully automatic, most of this problem will only occur when repairing parts.

Design process-In the PCB design, in fact, before the formal wiring, it has to go through a long process. The following is the main design process: System specifications First of all, we must first plan out the various system specifications of the electronic equipment. Including system functions, cost constraints, size, operating conditions, and so on.

Manufacturing process The manufacturing process of the PCB is started with a "substrate" made of glass epoxy (Glass Epoxy) or similar materials (forming/wire making). The first step of the manufacturing process is to establish the wiring between the parts. We use Subtractive Transfer to express the working film on the metal conductor. This technique is to spread a thin layer of copper foil on the entire surface and eliminate the excess. Additive pattern transfer is another method that less people use. This is a method of laying copper wires only where needed, but we won’t talk about it here.

The method of automatically welding SMT parts is called Over Reflow Soldering. The paste solder containing flux and solder is processed once after the parts are mounted on the PCB, and then processed again after the PCB is heated. After the PCB is cooled, the soldering is completed. The next step is to prepare for the final test of the PCB. A method to save manufacturing costs. In order to make the cost of the PCB as low as possible, there are many factors that must be considered.