PCB Classification And Characteristics

PCB classification printed circuit board (PCB) according to the nature of the substrate can be divided into two categories of rigid printed circuit boards and flexible printed circuit boards; according to the wiring level can be divided into a single panel, double-sided boards and multilayer boards. At present, the most widely used single- and double-sided board.

1. Rigid printed circuit board rigid printed circuit board has a certain degree of mechanical strength, with which it is loaded into the component has a certain degree of bending resistance, in the use of the state in the spread. General electronic equipment used in the rigid printed circuit board.

2. Flexible Printed Circuit Board Flexible Printed Circuit Board is made of soft laminated plastic or other soft insulating materials as a substrate. It is made of parts that can be bent and telescopic, and can be bent according to the installation requirements when in use. Flexible printed circuit boards are generally used for special occasions, such as: some digital multimeter display can be rotated, its internal often use flexible printed circuit boards.

3. single-panel (single-sided PCB) insulated substrate only one side of the printed circuit board with conductive graphics known as single-sided PCB. it is usually made of laminated cardboard or glass cloth board processing. Single-panel conductive graphics is relatively simple, most of the screen leakage printing method that is made of wet film process.

4. double-sided board (double-sided PCB) insulated substrate on both sides of the conductive graphics of the printed circuit board known as double-sided PCB. it is usually made of epoxy cardboard or glass cloth board processing. As both sides have conductive graphics, so the general use of perforations to make the two sides of the conductive graphics connected. Double-sided PCB board thickness: 1.6 mm, line width / line spacing: 0.8 mm / 0.3 mm, aperture: 0.6 mm, the thickness of the copper foil: 1OZ (1OZ means the weight of 1OZ of copper uniformly laid flat in 1 square foot of the area of the thickness of the arrival. It is the weight of the unit area to express the average thickness of copper foil, 1OZ copper foil thickness of about 35μm or 1.35 mil).

5. multilayer board (multilayer PCB) multilayer PCB - a printed circuit board with three or more layers of conductive graphics. Multilayer PCB inner conductive graphics and insulating bonding sheet laminated and pressed, the outer layer of the laminated foil board, pressed into a whole. In order to clamp the printed wires in the middle of the insulating substrate lead, multilayer board mounting components on the holes must be metallized holes, so that it is sandwiched in the insulating substrate of the printed wire connection.

Multilayer PCBs are characterized by:

(1) Used in conjunction with integrated circuits, it can miniaturize the whole machine and reduce the weight of the whole machine.

(2) Improved wiring density, reducing the spacing of components, shortening the signal transmission path.

(3) Reduced component soldering points, reducing the failure rate.

(4) Signal distortion of the circuit is reduced due to the addition of a shielding layer.

(5) The introduction of grounded heat dissipation layer can reduce the local overheating phenomenon, improve the reliability of the whole machine. Multi-layer PCB is suitable for a wide range of high-tech industries, such as telecommunications, computers, industrial control, digital products, scientific and educational instruments, medical equipment, automotive, aerospace defense. The actual electronic equipment used in the printed circuit board has a big difference, the simplest can be only a few soldering points or a few wires, the general number of soldering points in the printed circuit board of electronic products in the dozens of hundreds of soldering points, the number of soldering points more than 600 printed circuit board belongs to the more complex PCB, such as computer motherboards, and so on.

General Characteristics of PCB

PCB can be more and more widely used because it has a lot of unique advantages, as summarized below.

(1) High-density capable.100 Over the years, the high density of printed boards has been able to evolve with the increased integration of integrated circuits and advances in mounting technology.

(2) high reliability. Through a series of inspection, testing and aging tests, etc., can ensure that the PCB long-term and reliable work (generally 20 years of use).

(3) Designability. PCB of various properties (electrical, physical, chemical, mechanical, etc.) requirements, can be standardized through the design of standardization, specification, etc. to achieve a short period of time, high efficiency.

(4) Producibility. The use of modern management, standardization, scale (volume), automation and other production to ensure product quality consistency.

(5) Testability. The establishment of a more complete test methods, test standards, a variety of test equipment and instruments to detect and identify the qualification of PCB products and service life.

(6) can be assembled, PCB products are convenient for a variety of components for standardized assembly, but also can be automated, large-scale mass production. At the same time, PCB and a variety of components assembled parts can also be assembled to form larger parts, systems, until the whole machine.

(7) maintainability. As PCB products and a variety of components assembled parts are standardized design and scale production, and thus, these parts are also standardized. Therefore, once the system fails, it can be quickly, easily and flexibly replaced to quickly restore the system work.

PCB also has some other characteristics, such as making the system miniaturized, lightweight, high-speed signal transmission.

 Surface Mounted PCB (SMB) features SMT printed boards and traditional PCB compared to the pads, although it does not require the drilling of cartridge holes, but due to some highly integrated SMD has a large area, the number of pins, pin spacing dense, PCB wiring is dense; therefore, for the SMB, both the choice of substrate materials, or graphic design and manufacturing, have put forward a more than the through-hole insertion (THT) used PCB higher requirements. THT) PCB used for higher requirements.

First of all, for the manufacture of SMB substrate, its performance requirements than the insert PCB substrate performance requirements are much higher; Secondly, the design of SMB, manufacturing process is also much more complex, many high-tech is the manufacture of inserted PCB technology is not used at all, such as multilayer boards, metallized holes, blind holes and buried holes and other technologies, but in the manufacture of the SMB but almost all use, so the world will be SMB manufacturing capacity as a symbol of the level of PCB manufacturing. SMB has become the mainstream products of the current advanced PCB manufacturing plant, SMB and THT cartridge PCB, compared with its main features are: high density, small aperture, multi-layer number, high board thickness / aperture ratio, excellent transmission characteristics, high flat finish and dimensional stability.

(1) higher density. As some SMD devices as high as 100 to 500 pins, pin center distance has been transitioned from 1.27 mm to 0.5 mm, or even 0.3 mm, so the SMB requires a thin line, narrow spacing, line width from 0.2 to 0.3 mm narrowed to 0.15 mm, 0.1 mm or even 0.05 mm, 2.54 mm between the grid over the double line has been developed over the 3 wire, the latest technology has reached over 6 wires. Technology has reached over 6 wires, thin lines, narrow spacing greatly improves the installation density of SMB.

(2) smaller aperture. Single-sided PCB in the aperture is mainly used to insert components, and most of the metalized holes in the SMB is no longer used to insert components, but used to achieve interconnections between layers and layers of wires, small aperture for the SMB to provide more space. Currently, the hole diameters on the SMB are Φ0.46 to Φ0.3 mm and are developing towards Φ0.2 to Φ0.1 mm, while at the same time, the inner layer relay holes characterized by blind and buried hole techniques have appeared.

(3) Low coefficient of thermal expansion (CTE). Since SMD devices have many pins and are short, the CTE between the device body and the PCB is inconsistent, and damage to the device due to thermal stress often occurs. Therefore, the SMD substrate CTE should be as low as possible to adapt to the match with the device; today, CSP, FC and other chip-level devices have been used to directly mounted on the SMB, which puts forward higher requirements for the SMB CTE.

(4) high temperature performance, SMT soldering process, often need to double-sided mounting components, so the SMB is required to be able to withstand two reflow soldering temperature, and requires SMB deformation is small, no blistering; before and after the second reflow pads are still excellent weldability, the SMB surface is still a high degree of finish.

(5) higher flatness, SMB requires a high degree of flatness, so that SMD pins and SMB pads closely match the SMB pad surface coating layer is no longer used in traditional PCB manufacturing Sn / Pb used in the performance comparison of the relevant Table 8-2, Table 8-3. Table 8-2 is the error values Comparison table, Table 8-3 is the relationship between wires and pads, the table DIP for the traditional dual-row in-line package integrated circuits.