Introduction to multilayer printed circuit boards

Many large-scale electronic devices have high functional requirements, so the requirements for circuit boards are also relatively high. Ordinary single-layer circuit boards cannot meet functional needs at all. Therefore, the circuit board design is multilayer. For example, automotive electronics, large medical devices, and satellite communications equipment are only multilayer printed circuit boards. Now I ‘ll take you to learn more about multilayer circuit boards.

1. What is a multilayer circuit board?

PCB multilayer board refers to the multilayer line board used in electrical products. The multilayer board uses a single panel or double panel wiring board. Use one double -sided as the inner layer, two single -sided as the outer layer or two double -sided as the inner layer, two single -sided outer layer printing line boards, alternate positioning system and insulating bonding material, and electrophoretic graphics . Interconnected printed circuit boards are made according to design requirements.

With the continuous development of SMT (surface mount technology) and the continuous launch of the new generation of SMD (surface mount device), such as QFP, QFN, CSP, BGA (especially MBGA), they make electronic products more intelligent and miniaturized, thus promoting major reforms and progress in PCB industry technology. Since IBM first successfully developed high density multilayer boards (HLCs) in 1991, major groups from various countries have successively developed a variety of high density interconnection (HDI) micropores. The rapid development of these processing technologies has prompted PCB design to develop in the direction of multilayer and high density wiring. The multilayer printing board is widely used in the manufacture of electronic products for its flexible, stable and reliable electrical performance and superior economic performance.

 

2. The birth of multilayer circuit boards

Due to the increased density of integrated circuit packaging, the high concentration of interconnected connections has made the use of multi-substrates a must. In the layout of the printed circuit, an unforeseen design problem occurred, such as noise, amazing capacitors, and string disturbances. Therefore, the design of the printed circuit board must be committed to a minimum signal line length and avoid parallel paths. Obviously, in single-panels and even double-panels, due to the limited number of cross-scale crossings, these needs cannot be satisfactorily answered. In the case of a large number of interconnections and cross-demand, the circuit board must achieve satisfactory performance, and the plate layer must be expanded to more than two layers, so multiple layers of circuit boards appear. Therefore, the original intention of making a multilayered circuit board was to provide more freedom for complex/ sensitive electronic circuits that are sensitive to noise. The multilayer circuit board has at least three conductive layers, two of which are on the outer surface, and the remaining layer is synthesized on the insulating board. Electrical connections between them are usually made by plating holes on the horizontal side of the circuit board. Unless otherwise explained, the multilayer printed circuit board is the same as the double panel, which is usually plated.

The multi-base board is made by stacking two or more circuits from each other. They have a reliable pre-set connection between them. Since all the layers are crushed together, drilling and electroplating have been completed, and this technology has violated the traditional production process from the beginning. The inner two layers are made up of traditional double panels, while the outer layers are different. They are composed of independent single-panels. Before crushing, the end plate will be drilled, coated with hole plating, graphic transfer, shaping, and etching. The outer layer of the drill hole is the signal layer. It is plated by a method of forming a balanced copper ring on the inner edge of the hole. Then crush each layer together to form a plurality of substrates, and the plurality of -substrate may be connected with wave welding (between components).

3. Multilayer PCB Design Requirements

Multilayer circuit boards must take into account various problems when designing, such as the placement of metal devices, line wiring, PCB layers, etc. Therefore, we must follow these requirements when designing PCBs.

1. PCB component library requirements

(1) The packaging of components used on the PCB board must be correct, including the size of the component pin, the spacing of the pins, the number of pins, the size of the border, and the direction representation.

(2) Performance components (electrolytic capacitors, diode, triode, etc.) positive and negative electrode or pin numbers should be marked on the PCB component library and PCB board.

(3) The pin number of the PCB library in the middle component and the pin number of the principle graphic device should be consistent. question.

(4) The components of the heat sink must be used as component packaging. The size of the heat sink should be considered. The components and the heat sink can be pulled together in the form of the overall package.

(5) The pins of the component and the inner diameter of the pad should be matched. The inner diameter of the pad should be slightly larger than the pin size of the component to be installed.

Essence

2. PCB component layout requirements

(1) The component is uniformly arranged, and the component of the same functional module should be as close as possible.

(2) Try to arrange components using the same type of power supply and ground network as much as possible, which is conducive to completing the electrical connection between each other through the internal electrical layer.

(3) The interface component should be placed side by side and indicates the type of interface with a string. The direction of the wiring should usually leave the circuit board.

(4) Power -changing components (such as transformers, DC/DC converters, three-terminal voltage voltage tubes, etc.) should be left with enough heat dissipation space.

(5) The pins or reference points of the component should be placed on the grid point, which is conducive to wiring and aesthetics.

(6) Filter capacitors can be placed on the back of the chip, close to the power and ground pitch of the chip.

(7) The signs of the component’s pin or the direction of the logo should be indicated on the PCB and cannot be covered by components.

(8) The label of the component should be close to the frame of the component, unified size, neat direction, and not overlapped with pads and perfusion. It cannot be placed in the covered area after the component is installed.

3. PCB wiring requirements

(1) Different voltage levels should be isolated, and the power supply wiring should not cross.

(2) The wiring uses a 45 ° corner or an arc corner, and the corner of a sharp corner is not allowed.

(3) PCB wiring is directly connected to the center of the pad, and the width of the wire connected to the pads does not allow the size of the outer diameter of the pad.

(4) The line width of the high frequency signal line is not less than 20mil, surrounded by ground lines and isolated from other ground wires.

(5) Do not wire the bottom of the interference source (DC/DC converter, crystal, transformer, etc.) to avoid interference.

(6) Bold the power cord and ground line as much as possible. With the space permit, the width of the power cord is not less than 50mil.

(7) Low voltage and low current signal line width of 9 to 30 mIL, as much as possible when space permits.

(8) The distance between the signal line should be greater than 10mil, and the distance between the power cord should be greater than 20mil.

(9) The width of the large current signal line should be greater than 40mil, and the distance should be greater than 30mil.

(10) Outdoor size is preferably 40mil, and the inner diameter is 28mil. When connecting a wire between the top and bottom layers, the pad is preferred.

(11) The signal line is not allowed to arrange the internal electrical layer.

(12) The spacing width between different areas of the internal electrical layer shall not be less than 40mil.

(13) When drawing boundaries, try not to let the boundary line pass through the area where the area is connected.

(14) Apply copper on the top and bottom layers. It is recommended to set the wire width value above the grid width, completely covering the space, and there is no dead copper. Set the safety spacing before, and change it back to the original safety spacing value after the copper is finished).

(15) After the wiring is finished, the pad is treated with tear drops.

(16) Metal shell devices and modules are grounded outside.

(17) Place the pad for installation and welding.

(18) DRC check is correct.

4. PCB layered requirements

(1) The power supply plane should be close to the ground surface, tightly coupled to the ground surface, and arranged under the ground surface.

(2) The signal layer shall be adjacent to the inner electrical layer and shall not be adjacent directly to other signal layers.

(3) Isolate digital circuits and analog circuits. If conditions permit, arrange the analog signal line and the digital signal line layer and adopt shielding measures; if you need to arrange at the same signal layer, you need to use an isolation zone and ground lines to reduce interference; the power supply of the analog circuit and the digital circuit is required. Earth should be isolated from each other and cannot be mixed.

(4) High frequency circuit interference is large, arranged separately, and the upper and lower intermediate signal layers are directly adjacent to the intermediate signal layer for use in order to use the copper film of the internal electrical layer to reduce external interference.

4. Multilayer PCB Manufacturing Process

Multilayer circuit board is a complex circuit board, which has a higher circuit density and a more complex circuit layout from multiple levels of circuit boards. Manufacturing multilayer circuit boards requires the following process:

(1) Raw material preparation: Choose suitable substrates, copper covering and chemicals to ensure that it meets production requirements.

(2) Internal manufacturing: According to the design requirements, apply copper to the base material, and use the internal graphic printer to print the circuit pattern on the copper cover layer. Then chemically etch to remove excess copper cover.

(3) Debugging and testing: After chemical etching, it is necessary to detect and debug the inner line board to ensure that there are no short circuits and opening.

(4) Outer layer fabrication: Apply copper to both sides of the inner circuit board, and use the outer graphic printer to print the circuit pattern on the copper cover layer. Then chemically etch to remove excess copper cover.

(5) Electroplating: After chemical etching, the line board needs to be placed to form good conductivity.

(6) Drilling: Use the drilling machine to drill the line board to form a line connecting different levels.

(7) Fill: Use impregnated copper filling technology to fill copper into the drilling hole to form circuit channels.

(8) Repair: Use technologies such as chemical etching and mechanical polishing to cut and repair the circuit board to form the final shape and size.

(9) Coating and inkjet: According to customer needs, organic coating and inkjet are on the circuit board to protect the circuit board and identify the function of the circuit board.

(10) Testing and packaging: Test and package the multilayer line board manufactured to ensure that it meets customer requirements and international standards.

5. Multilayer PCB Advantages

The multilayer circuit board is bonded by multiple etching single panels or double panels through layers. Compared to single-layer and double-layer circuit boards, multilayer circuit boards have many advantages, especially in small electronic products. Let’s share the advantages of multi-layer circuit boards.

(1) The multilayer circuit board has a high assembly density and is small in size. As the volume of electronic products becomes smaller and smaller, it also puts forward higher requirements for the function of PCB circuit boards, and the demand for multilayer circuit boards is also increasing.

(2)The use of multilayer PCB circuit board panels is convenient, and the length of the paving line is greatly shortened. The wiring between the electronic components is reduced, which also increases the transmission rate of the data signal.

(3)For high frequency circuits, after entering the ground floor, the signal line has a stable low characteristic impedance on the ground. The characteristic impedance of the power circuit decreases significantly, and the interception effect is significant.

(4)Electronic products with high heat dissipation functions can be set on the multilayer circuit board to set the metal core heat dissipation layer to facilitate the requirements of special functions such as shielding and heat dissipation.

6. What are the applications of multilayer PCBs?

Consumer Electronics
“Consumer Electronic Products” is a very broad term, including many electronic devices under general equipment directly related to ordinary users like you. Consumer electronics include: smartphones, smart watches, calculators, TV remote controls, MP3 music players, toys, kitchen appliances, washing machines, electric kettles, electronic cigarettes, LED bulbs and energy-saving appliances.

Computers and devices
Today, our world is highly dependent on computers and automation. Multilayer PCB is usually used in computers and related products, such as motherboards, graphics cards, EEPROM, power supplies, keyboards, computer mouse, ADC, graphics processor, image processing circuit, etc.

Large electronic equipment
Large electronic equipment has more functionality, so the requirements for circuit boards are relatively high. Generally, multilayer circuit boards are used, such as large medical equipment, industrial products, military products, etc.

7、Summarize

The production of multilayer circuit boards requires not only higher investment in technology and equipment, but also experienced production technicians. Leadsintec is a professional PCB processing manufacturer. We can provide customers with all PCB services. Please leave a message.