“As an important Electronic component, the printed circuit board is a support for electronic components. Because of its important role in the field of electronic components, it is called “electronic aircraft carrier” by many people.
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As an important electronic component, the printed circuit board is a support for electronic components. Because of its important role in the field of electronic components, it is called “electronic aircraft carrier” by many people.
Now, communication products, computers and almost all other electronic products use printed circuits. The development and improvement of printed circuit technology have created conditions for the advent of integrated circuits, the invention that changed the face of the world. With the development of science and technology, printed circuit boards are widely used in high-tech fields such as military industry, communications, medical treatment, electric power, automobiles, industrial control, smart phones, and wearables.
The invention of the printed circuit
The printed circuit was invented by Austrian electrical engineer Paul Eisler in the mid-1930s. Eisler studied electrical engineering at the Vienna School of Engineering in his early years. After graduating in 1930, he studied printing technology. When he was doing research on electronic circuit boards, he often went to the library to check books and periodicals on printing technology.
After serious thinking, he came up with an idea: If you print the circuits of electronic devices on the circuit boards at a time, like printing books or newspapers, you don’t need to make the circuit boards piece by piece by hand, and there is no need for people to make the circuit boards one by one. Once the ground is welded, the production efficiency and reliability of electronic products can be greatly improved.
In the printing industry, in order to print pictures on paper, photo-engraving technology is usually used. That is, by taking a photo, the photo master is etched on a copper plate or zinc plate. With this copper plate or zinc plate, many pictures can be printed.
When Eisler manufactures circuit boards, it also uses a plate-making method similar to that of the printing industry. He first drew the electronic circuit diagram, and then etched the circuit diagram on the insulating board covered with a layer of copper foil, so that the unnecessary copper foil was etched away, leaving only the conductive circuit. In this way, the various electronic components are connected to each other through the circuit formed by the copper foil on this board. This kind of printed circuit can not only improve the reliability of electronic products, but also greatly improve production efficiency, and has great value and potential for the development of new electronic products.
The use of printed circuit technology makes the mass production of electronic equipment simple and easy, laying the foundation for the mechanized and automated production of electronic products. Since the 1950s, the substantial progress made in various electronic products, including communication equipment, is inseparable from the use of printed circuit technology.
With the continuous improvement of the manufacturing level of printed circuits, the printed circuits produced can reach high precision, thus pushing the production and manufacturing of circuit boards to a new stage. When making plates in the printing industry, a large picture can be reduced to a certain size by shooting.
When manufacturing printed circuits, the electronic circuit diagram can also be reduced to make a plate, making it an electronic circuit board with a small area, complex circuits and high reliability. This kind of printed circuit board is very suitable for communication equipment and computers with complicated circuits and high reliability requirements. The development of printed circuit technology laid the necessary technical foundation for the subsequent invention of integrated circuits.
The circuit board has a history of more than 60 years since its invention. History has shown that: without circuit boards, without electronic circuits, flying, transportation, atomic energy, computers, aerospace, communications, home appliances… all of this cannot be achieved.
The reason is easy to understand. Chips, ICs, and integrated circuits are the food of the electronic information industry. semiconductor technology reflects a country’s industrial modernization level and guides the development of the electronic information industry. The electrical interconnection and assembly of semiconductors (integrated circuits, ICs) must rely on circuit boards.
Types of printed circuit boards
The printed circuit boards used in actual electronic products are very different, and there are different classifications according to different standard printed circuit boards.
1. Classified according to the distribution of printed circuits
According to the approximate distribution of the printed circuit, the printed circuit board can be divided into three types: single-sided board, double-sided board, and multi-layer board.
• Single panel
The single-sided 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 that require high assembly density or complex circuits.
• Double panel
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 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 thinner 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 the 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.
The following figure is a schematic diagram of the structure of the multilayer board. The components used in the multilayer board are mostly SMD components. Its characteristics are:
1. Used in conjunction with integrated circuits, the whole machine can be miniaturized and the weight of the whole machine can be reduced;
2. Improve the wiring density, reduce the spacing of components, and shorten the signal transmission path;
3. Reduce the welding points of components and reduce the failure rate;
4. The shielding layer is added to reduce the signal distortion of the circuit;
5. The introduction of a grounded heat dissipation layer can reduce local overheating and improve the reliability of the whole machine;
2. Classification according to the nature of the substrate
According to the nature of the substrate, printed circuit boards can be divided into rigid and flexible.
Rigid PCB
The rigid printed 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.
Flexible printed board
The flexible printed 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 when in 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; mobile phone displays, buttons, etc.
The picture below shows the mobile phone flexible printed board. Its base material is polyimide, and the surface is treated with anti-oxidation. The minimum line width and line spacing are set to 0.1mm. The outstanding feature of the flexible printed board is that it can be bent, curled, and folded, and can be connected to rigid printed boards and movable parts, thereby enabling three-dimensional wiring and realizing three-dimensional interconnection. It is small in size, light in weight, easy to assemble, and suitable for space Small, high-density electronic products.
Three, according to the scope of application
The high frequency of electronic equipment is the development trend, especially in today’s wireless network and satellite communications, information products are moving towards high speed and high frequency, and communication products are moving towards the standardization of voice, video and data for wireless transmission with large capacity and speed. Therefore, the development of a new generation of products requires high-frequency printed boards, and the foil substrate can be composed of materials with low dielectric loss and dielectric constant such as polytetrane ethylene, polyvinyl ethylene, polystyrene, polytetrafluoroethylene glass cloth, etc.
At present, some special printed boards such as metal core printed boards, surface-mounted printed boards, and carbon film printed boards have also appeared.
The metal core printed board is to replace the epoxy glass cloth board with a metal plate of equivalent thickness. After special treatment, the conductor circuits on both sides of the gold board are connected to each other, and the metal part is highly insulated. The advantages of metal core printed boards are good heat dissipation and dimensional stability. This is because magnetic materials such as aluminum and iron have a shielding effect and can prevent mutual interference.
Surface mount printed board
The surface mount printed board is a printed board developed to meet the needs of electronic products for “light, thin, short, and small” and to match the mounting process of surface mount devices with pin density and low cost. The printed board has the characteristics of small aperture, small line width and spacing, high precision, and high substrate requirements.
The carbon film printed board is a printed board in which a layer of carbon film is printed to form a contact or a jumper (resistance value meets the specified requirements) after the conductor pattern is made on the copper-plated board. It is characterized by simple production process, low cost, short cycle, good wear resistance, electrical conductivity, can realize high density of single panel, miniaturization and light weight of products, suitable for televisions, telephones, video recorders and electronic organs And other products.
Taking four layers as an example, let’s take a look at how the printed circuit board is made.
• The first step, chemical cleaning
In order to obtain good-quality etching patterns, it is necessary to ensure that the resist layer is firmly combined with the substrate surface, and the substrate surface is required to be free of oxide layers, oil stains, dust, fingerprints and other contaminants. Therefore, before coating the resist layer, the surface of the board must be cleaned and the surface of the copper foil must reach a certain degree of roughness.
Inner board: start to make four-layer board, the inner layer (the second and third layer) must be done first. The inner plate is a copper sheet compounded on the upper and lower surfaces by glass fiber and epoxy resin.
The second step, cutting the board and pressing the film
Coating photoresist: In order to make the shape we need for the inner plate, we first paste a dry film (photoresist, photoresist) on the inner plate. The dry film is composed of polyester film, photoresist film and polyethylene protective film. When attaching the film, first peel off the polyethylene protective film from the dry film, and then paste the dry film on the copper surface under heating and pressurizing conditions.
• The third step, exposure and development
Exposure: Under the irradiation of ultraviolet light, the photoinitiator absorbs the light energy and decomposes into free radicals, and the free radicals initiate the polymerization and crosslinking reaction of the photopolymerized monomers, and form a polymer structure insoluble in dilute alkali solution after the reaction. The polymerization reaction will continue for a period of time. To ensure the stability of the process, do not tear off the polyester film immediately after exposure. It should stay for more than 15 minutes so that the polymerization reaction will continue. Tear off the polyester film before developing.
Development: The active groups in the unexposed part of the photosensitive film react with the dilute alkali solution to produce soluble substances and dissolve, leaving the pattern part that has been photosensitive, cross-linked and cured.
• The fourth step, etching
In the production process of flexible printed boards or printed boards, the unnecessary part of the copper foil is removed by a chemical reaction method to form the required circuit pattern, and the copper under the photoresist is retained and not etched Of the impact.
• The fifth step, film removal, punching after etching, AOI inspection, oxidation
The purpose of film removal is to remove the resist layer remaining on the board surface after etching to expose the copper foil underneath. “Membrane residue” filtration and waste liquid recycling must be properly handled. If the water washing after removing the film can be completely cleaned, you can consider not pickling. After cleaning the surface of the board, it must be completely dried to avoid residual moisture.
• The sixth step, stacking-protective film film
Before entering the laminating machine, the raw materials for each multilayer board must be prepared for lay-up operations. In addition to the oxidized inner layer, a protective film (Prepreg)-epoxy resin impregnated glass fiber is still needed. The function of the laminate is to stack the boards covered with the protective film in a certain order and place them between the two-layer steel plates.
• The seventh step, laminate-copper foil and vacuum lamination
Copper foil-Cover the current inner sheet with a layer of copper foil on both sides, and then perform multi-layer pressure (extruding that requires temperature and pressure to be measured within a fixed period of time) and cool to room temperature after completion, and the rest It is a multi-layer board.
• The eighth step, CNC drilling
Under the precise conditions of the inner layer, CNC drilling is drilled according to the pattern. The drilling accuracy is very high to ensure that the hole is in the correct position.
• The ninth step, plating-through hole
In order for the through holes to be conductive between the layers (to metalize the resin and glass fiber bundles on the non-conductor part of the hole wall), the holes must be filled with copper. The first step is to plate a thin layer of copper in the hole. This process is completely chemical. The thickness of the final plated copper is one millionth of 50 inches.
• The tenth step, cutting the board and pressing the film
• The eleventh step, exposure and development
• The twelfth step, line plating
This time it has also become a secondary copper plating, and the main purpose is to increase the copper thickness of the circuit and the through-hole.
• The thirteenth step, tin electroplating
Its main purpose is to etch resist to protect the copper conductor it covers from being attacked when alkaline copper is etched (to protect all copper lines and the inside of through holes).
• The fourteenth step, remove the film
We already know the purpose, and only need to use chemical methods to expose the copper on the surface.
• The fifteenth step, etching
We know the purpose of the etching, the tinned part protects the copper foil below.
• The sixteenth step, pre-hardening, exposure and development, solder mask
The solder mask is used for exposing the pads, which is commonly referred to as the green oil layer. In fact, holes are dug in the green oil layer to expose the pads and other areas that do not need to be covered with green oil. Proper cleaning can get the proper surface characteristics.
• The seventeenth step, surface treatment
The hot air leveling solder coating process is to first dip the printed board with flux, then dip it in the molten solder, and then pass between the two air knives, and use the hot compressed air in the air knife to press the printed board The excess solder is blown off, and the excess solder in the metal hole is removed at the same time, so as to obtain a bright, smooth and uniform solder coating.
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