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What is a PCB? What is the PCB

Date:2019-01-21 20:18:22 Hits:614
Electronic products are usually designed with schematic circuits in mind. In a schematic circuit, various symbols are used to represent different electronic components and are connected by wires. An electronic engineer can see clearly how a circuit works and the functions of its parts through these symbols and lines.

What is the PCB

Printed Circuit Board is the abbreviation of 'Printed Circuit Board'. Its meaning is: the insulation material as the substrate processed into a certain size of the board, there is at least one conductive graphics and the design of the hole, in order to achieve the power connection between electronic components, such a board is called the printed circuit board. 2 o "u d 'q I - J8U E

Generally speaking, PCB is copper coated plate after etching treatment. Copper clad plate is composed of a base and copper foil, the base is usually made of glass fiber and other insulating materials, covered with a layer of copper foil (usually oxygen-free copper). After etching, copper foil is left with sections of twisted copper foil, which are called trace. These wires function like the wires in a schematic circuit and are responsible for connecting the pins of the components together. Copper foil is drilled with a number of holes, used to install electronic components, known as drilling. The copper foil used to weld the component pins is called Pad. Y L2} K | "Y l

Obviously, PCB can provide mechanical support for electronic components to be fixed and assembled, and can realize electric connection or insulation between electronic components. In addition, we can see that many PCBS are printed with the component number and some graphics, which provides convenience for component mounting, inspection and maintenance.

Through which pipes can components be mounted on PCB

In fact, electronic components can be packaged in many ways, and different types of components can be installed in different ways on a PCB. Conventional electronic components are larger, needle-like devices that require drilling into a PCB before they can be installed. Component pins are drilled through the hole and welded to the pad on the other side of the PCB. After welding, excess pins are cut off. But now the computer board card is more low cost, small SMD SMD surface mount components, so no need to drill in the PCB, as long as stuck to the designed position, the components can be welded on the welding pad. In addition to soldering directly to the PCB, the removable components can also be installed through a socket. For example, the well-known BIOS chip is mostly installed on the motherboard with a socket. 9 E R + z e3Z p5P8o R3 `

The concept of element faces and weld faces is often mentioned in some data. The component surface is the surface on which the electronic components are located, and the welding surface is the surface on which the pin of the component is connected with the solder plate on the PCB through solder. It is used for welding. For pin elements, solder joints and components are on two sides of the PCB, and components can only be on the component side, otherwise it will bring huge trouble to soldering. For SMD components, solder joints and components are on one side, so components can be on either or both sides of the PCB.

A multilayer PCB's

But along with the development of microelectronics technology, greatly increased the complexity of the circuit, the power efficiency is also put forward higher requirements for PCB, if still use single or double panel panel, circuit volume will be very big, wiring is difficult, in addition, electromagnetic interference between the lines is not good, then there is the laminated (layer on behalf of several independent wiring layer, is usually an even number).

Advantages of using multilayer board: high assembly density, small volume; The connection between electronic components is shortened and the signal transmission rate is increased. Convenient wiring; For high frequency circuits, a ground layer is added so that the signal lines form a constant low impedance to the ground. The mask works well. But the more layers, the higher the cost, and the longer the processing cycle.

Our common computer board CARDS usually use four or six layers, but now there are more than 100 layers of practical printed circuit board. The difference between the six and four layers is that in the middle, between the ground and the power layer, there are two more internal signal layers, which are thicker than the four layers.

The multilayer plate is actually composed of several single panels or double panels that have been etched and laminated and bonded. The two panels are easily distinguishable and, when viewed from the light, are transparent except for the alignment on both sides. For the four and six layers, because the layers in the PCB are closely connected, if there are corresponding marks on the board, there is no good way to distinguish them. ; J, M r I}

The inner and outer circuits can be connected by punching holes in the PCB insulation layer and copper plating on the hole wall. Such holes are called through holes, through holes or through holes. For multilayer panels, there are several types of through-holes: penetrative through-holes through all layers, semi-hidden through-holes that can only be seen on one surface, and all-hidden through-holes that cannot be seen.

In addition to forming holes by electroplating, a method of making holes by filling the holes with "conductive paste" has recently been popularized. Conductive paste is a paste of metal particles added to the resin, filled into the hole once solidified, the metal particles contact each other, can be connected to the circuit. Such holes are called metal conduction holes, and silver particle conductive paste is called "silver conduction hole". Recently, copper particle conductive paste has been used. 'Q' a T: j0J9p D: n M

wiring

The myth of serpentine

Many boards have trial reports that say something like this: "nice work, lots of snaking lines on the board." Generally speaking, the serpentine line refers to the continuous s-shaped change of the wiring. Intuitively, there is no obstruction between the two points of the connection, which could have been arranged as a straight line, but the actual use of serpentine wiring. In theory, the snaking wire has these effects: it creates a tiny inductance that inhibits the change in the wire's signal current; Guarantee the equal length of some lines; It can suppress crosstalk to a certain extent. It can be seen that this is only a local wiring pipeline, the designer should use according to the actual situation, and can not judge the quality of PCB wiring by the number of snaking lines.

The thickness varies


When we look at the PCB, we will find that the wiring is thick and thin. The thick part is usually the power line and ground wire, while the thin part is the data line. This is because the power cord and ground wire should be as thick as possible to carry a relatively large current. Often 囙 this, the space is covered by piece of copper foil as a ground wire. The smaller the current flowing through the data line, the finer the design, and the finer the connection, the better the wiring.


Turning also has exquisite

Walk on the PCB line can't all be straight, 囙 this will involve the issue of steering. Generally, the design requires that the turning direction of the wire should not be a right Angle, but a 45 degree Angle (refers to the clip Angle of the extension direction of the line) or so. This is because the right Angle and sharp Angle of the figure in the high frequency circuit will affect the power efficiency, and in the case of high temperature easy flaking, so usually required for the direction of the line for obtuse Angle or round Angle.

The multicolored coat of PCB

Our first impression to computer board card is probably its color, besides the most common green and brown, still have blue, gules, black, purple to wait. Let's think about why the other copper wires on the PCB don't have tin. In addition to PCB soldering pad, there is a layer of anti-wave soldering film on the surface, which can prevent the phenomenon of bridge during wave soldering, improve welding quality and save solder and other advantages. It is also the permanent protective layer of the printed board, can play a role in moisture, corrosion, mildew and mechanical abrasion. Soldering film is mostly green, so the soldering oil is often called green oil in PCB industry. The color of PCB is actually the color of soldering oil. The color of solder block oil can be changed by adding other chemicals, but the color only serves as decoration and has no effect on the performance.

Seemingly mundane details

Mounting holes

The mounting hole is the screw hole for fixing the board card. If it is not used for grounding, copper foil cannot be used within 5mm. These holes are used for grounding, so there is a ring of copper foil around them. In this way, the ground wire of the card is connected to the metal shell of the main housing through metal screws, which can serve as a cover.

benchmark

A large number of SMD components are usually very dense board components, some lsi pin arrangement is more dense, to use automation equipment PCB component placement requires very high precision. To meet this requirement, a reference point is usually designed on the PCB to help the automation equipment align with the PCB. There are usually global datum points and local datum points on PCB, two datum points seen on the diagonal of the whole PCB are global datum points, and the local datum points are seen on the diagonal of components with close spacing QFP, TSOP and BGA package. With these datum points, all the components can exactly coincide with the designed position on the PCB.