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PCB Board

What is a printed circuit board (PCB board)?

PCB board is the mother of electronic products. it is an electronic component that uses electronic printing to etch on the surface of insulated and non bending copper clad laminate, leaving a network of small circuits, so that various electronic components can form a predetermined circuit connection and achieve the relay transmission function between electronic components. Most electronic equipment and products need to be equipped PCB Board, PCB Board is called "the mother of electronic products". PCB Board's quality not only directly affects the reliability of electronic products, but also affects the overall competitiveness of system products. Its development has a history of more than 100 years; its design is mainly layout design; the main advantage of using PCB board is to greatly reduce wiring and assembly errors, and improve the level of automation and production labor.

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Printed circuit boards(PCB) also known as printed circuit boards, are providers of electrical connections for electronic components.
Printed circuit boards are often referred to as "PCB", also have many peoples called "PCB boards". Since the printed circuit board is not a general terminal product, the definition of the name is slightly confusing. For example, the motherboard for personal computers is called the main board, and cannot be directly called the circuit board. Although there are circuit boards in the motherboard, They are not the same, so when evaluating the industry, the two are related but cannot be said to be the same. Another example: because there are integrated circuit components mounted on the circuit board, the news media call it an IC board, but in fact it is not equivalent to a printed circuit board. We usually say that the printed circuit board refers to the bare board-that is, the circuit board without upper components.

According to the number of circuit board layers, it can be divided into single-sided, double-layer, four-layer, six-layer and other multilayer circuit boards. And continue to develop in the direction of high precision, high density and high reliability. Continuously shrinking volume, reducing costs, and improving performance have enabled printed circuit boards to maintain strong vitality in the development of future electronic products. The future development trend of PCB boards manufacturing technology is to develop in the direction of high density, high precision, fine aperture, fine wire, small pitch, high reliability, multilayer, high-speed transmission, light weight, and thinness in performance.

According to the distribution of printed circuits

-Single-sided PCB board:

The single layer board is on an insulating substrate with a thickness of 0.2-5mm, only one surface is covered with copper foil, and a printed circuit is formed on the substrate by printing and etching. The single panel is simple to manufacture and easy to assemble. It is suitable for the requirements of a circuit, such as radios, televisions, etc.. It is not suitable for occasions requiring high assembly density or complex circuits.

-Double-sided PCB board:

The double-sided boards are printed circuits on both sides of an insulating substrate with a thickness of 0.2-5mm. It is suitable for electronic products with general requirements, such as electronic computers, electronic instruments and meters. Since the wiring density of the double-sided printed circuit is higher than that of the single-sided printed circuit, the volume of the device can be reduced.

-Multilayer PCB board:

Printed boards with more than 3 layers of printed circuits printed on an insulating substrate are called multilayer boards. It is a combination of several thin single or double panels, and its thickness is generally 1.2-2.5mm. In order to lead out the circuit sandwiched between the insulating substrate, the holes for mounting components on the multilayer board need to be metalized, that is, a metal layer is applied to the inner surface of the small holes to connect them with the printed circuit sandwiched between the insulating substrates.

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Classified by the nature of the substrate

-Rigid PCB board:

The rigid PCB board has a certain mechanical strength, and the parts assembled with it have a flat state. Rigid printed boards are used in general electronic products.

-Flex PCB board:

The flexible PCB board is made of soft layered plastic or other soft insulating materials as the base material. The parts made of it can be bent and stretched, and can be bent according to the installation requirements during use. Flexible printed boards are generally used in special occasions. For example, the display screen of some digital multimeters can be rotated, and flexible printed boards are often used inside; the display screens, buttons of mobile phones, etc.

-Rigid-flex PCB board:

The birth and development of FPC and PCB gave birth to the new product of soft and hard board. Therefore, the rigid-flex board is the flexible circuit board and the rigid circuit board. After pressing and other processes, they are combined according to the relevant process requirements to form a circuit board with FPC characteristics and PCB characteristics.

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Classified by the scope of application

PCB can be divided into low frequency PCB and high frequency PCB. High frequency of electronic equipment is the development trend, especially in today's wireless network and satellite communication, information products are moving towards high speed and high frequency, and communication products are moving towards the standardization of voice, video and data of wireless transmission with large capacity and high speed. Therefore, the new generation of products need high-frequency printed circuit board, and the foil substrate can be made up of materials with small dielectric loss and dielectric constant, such as polytetrane, polyethylene, polystyrene, polytetrafluoroethylene glass cloth.

Classified by the special of printed boards(PCB board)

At present, there are some special printed boards, such as metal core printed board, surface mounted printed circuit board and carbon film printed board.

Metal core PCB board:

The metal core PCB boards is to replace the epoxy glass cloth board with a metal plate of the same thickness. After special treatment, the conductor circuits on both sides of the metal board are connected with each other and highly insulated from the metal part. The advantage of metal core PCB is good heat dissipation and dimensional stability. This is because magnetic materials such as aluminum and iron have shielding effect and can prevent mutual interference.

Surface mount PCB:
Surface mount printed circuit board (SMB) is a kind of PCB developed to meet the needs of "light, thin, short and small" of electronic products, and with the installation process of surface mount devices with pin density and low cost. The printed circuit board has the characteristics of small aperture, small line width and spacing, high precision and high substrate requirements.

Carbon film printed board:
Carbon film printed board is a kind of printed board which is printed with a layer of carbon film after the conductor pattern is made on the copper foil to form contact or jumper wire (the resistance value meets the specified requirements). It is characterized by simple production process, low cost, short cycle, good wear resistance and electrical conductivity. It can realize high density of single panel and miniaturization and lightweight of products. It is suitable for TV, telephone, video recorder and electronic organ.

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The function of impedance control in PCB

For analog or high frequency digital circuits, it is important to ensure the integrity of the signals transmitted on the PCB. In fact, signals higher than 100 MHz are affected by the impedance of the conductor run, which, if not properly considered, can lead to inconvenient errors and are particularly difficult to analyze. Fortunately, impedance control allows PCB designers and manufacturers to manage this phenomenon.

PCB board impedance refers to the parameters of resistance and reactance, which hinders alternating current. In the production of PCB circuit board, impedance processing is indispensable. The reasons are as follows:

PCB circuit board (board bottom) needs to consider the plug installation of electronic components, after the plug in consideration of conductivity and signal transmission performance and other factors, so the lower the impedance, the better, the resistivity should be less than 1 & MES; 10-6 per square centimeter.

2. PCB circuit board in the production process has to go through copper deposition, electro tinning (or electroless plating, or hot spray tin), connector soldering process, and the materials used in these links must ensure the low resistivity, to ensure that the overall impedance of the circuit board is low, meet the product quality requirements, and can operate normally.

3. The tin plating of PCB circuit board is the most easy problem in the production of the whole circuit board, and it is the key link that affects the impedance. The biggest defect of electroless tin coating is easy to change color (easy to oxidize or deliquescence), poor solderability, which will lead to the circuit board difficult to weld, high impedance, poor conductivity or instability of the whole board performance.

4. There will be all kinds of signal transmission in the conductor of the circuit board factory. When the frequency must be increased in order to improve the transmission rate, if the circuit itself is different due to etching, stack thickness, wire width and other factors, the impedance value will change, the signal will be distorted and the performance of the circuit board will be reduced. Therefore, it is necessary to control the impedance value within a certain range.

For the electronic industry, according to the industry survey, the most fatal weakness of electroless tin coating is easy to change color (i.e. easy to oxidize or deliquescence), poor solderability leads to difficult soldering, high impedance leads to poor conductivity or instability of the whole board performance, and easy to grow tin leads to PCB circuit short circuit, burn out or fire events.

General rules for PCB lamination design:

(1). the formation and signal layer should be closely coupled, which means that the distance between the formation and the power layer shall be as small as possible, and the medium thickness shall be as small as possible to increase the capacitance between the power layer and the stratum (if it is not understood here, you can think about the plate capacitance, which is in inverse proportion to the distance).
(2). as far as possible, the two signal layers should not be directly adjacent, which will easily cause crosstalk of signals and affect the performance of the circuit.
(3). for multilayer circuit boards, such as 4-layer board and 6-layer board, signal layer is generally required to be as close as possible to an inner electric layer (stratum or power layer), so that large area copper coating of inner layer can be used to shield signal layer, thus effectively avoiding crosstalk between signal layers.
(4). for high-speed signal layer, it is generally located between two internal electrical layers. The purpose of this is to provide an effective shielding layer for high-speed signal on one hand, and on the other hand, limit the high-speed signal between two internal electrical layers to reduce interference to other signal layers.
(5). the symmetry of the laminated structure should be considered.
(6). the internal electrical layer with multiple grounding can effectively reduce the grounding impedance.