Printed circuit board – the core of electronic components
As an important electronic component, printed circuit board is the support of electronic components. Because of its important role in the field of electronic components, it is considered by many to be the mother of electronic components.
Today, communication products, computers and almost all other electronic products use PCBs. The development and improvement of PCB technology has created conditions for the invention that changed the face of the world – the advent of integrated circuits.
With the development of science and technology, PCBs are widely used in high-tech fields such as military industry, communications, medical treatment, electric power, automobiles, industrial control, smart phones, and wearables. But how much do you know about PCBs? Let’s take a look.
Invention of the PCB
Printed circuit boards were invented by Austrian electrical engineer Paul Eisler in the mid-1930s. Eisler studied electrical engineering at the Vienna Academy 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.
In the printing industry, in order to print pictures on paper, photolithography is usually used. That is to say, by taking pictures, the base plate of the photographed pictures is etched on the copper plate or zinc plate, and many pictures can be printed out with this copper plate or zinc plate.
When Eisler made circuit boards, he also tried using a plate-making method similar to the printing industry. He first drew the electronic circuit diagram, and then etched the circuit diagram on an 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, each electronic component is 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 the 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, and lays the foundation for the mechanization and automation of electronic products. Since the 1950s, the substantial progress made in various electronic products, including communication equipment, is inseparable from the use of PCB technology.
With the continuous improvement of the manufacturing level of printed circuits, the printed circuits produced can achieve 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 circuit boards, it is also possible to reduce the electronic circuit diagram to make a plate, making it an electronic circuit board with a small area, complex circuits and high reliability. This printed circuit board is very suitable for communication equipment and computers with complex circuits and high reliability requirements. The development of PCB technology laid the necessary technical foundation for the subsequent invention of integrated circuits.
Printed circuit boards has a history of more than 60 years since its invention. History shows: Without printed circuit boards, without electronic circuits, flight, transportation, atomic energy, computers, aerospace, communications, home appliances…all of these cannot be realized.
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 a circuit board.
Types of PCBs
The printed circuit boards used in actual electronic products vary widely, and there are different classifications of printed circuit boards according to different standards.
Classification according to the layers of PCBs
According to the layers of PCBs, PCB can be divided into three types:
Single sided PCB
The single sided PCB 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 sided PCB is simple to manufacture and easy to assemble. It is suitable for one-stage circuit requirements, such as radios, televisions, etc.; it is not suitable for occasions that require high assembly density or complex circuits.
Double layer PCB
The double layer PCB is to print 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, etc. Since the wiring density of the double layer PCB is higher than that of the single-sided PCB, the volume of the device can be reduced.
A printed circuit board that prints more than 3 layers of printed circuits on an insulating substrate is called a multilayer PCB. It is made of several thinner single or double boards, and its thickness is generally 1.2-2.5m. In order to lead out the circuit sandwiched between the insulating substrates, the holes for mounting components on the multilayer PCB need to be metallized, that is, to coat the inner surface of the small hole with a metal layer to connect it with the printed circuit sandwiched between the insulating substrates.
Most of the components used in multilayer PCBs are SMD components, which are characterized by:
1. Used in conjunction with integrated circuits, it can make the whole machine miniaturized and reduce the weight of the whole machine;
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, and the signal distortion of the circuit is reduced;
Classified by the nature of the substrate
According to the nature of the substrate, PCB can be divided into rigid and flexible PCB:
The rigid PCB has a certain mechanical strength, and the parts assembled with it have a flat state. Generally, rigid PCBs are used in electronic products.
Flexible PCBs are made of soft laminated plastic or other soft insulating materials. The parts it makes can be bent and stretched, and can be bent according to the installation requirements when in use. Flexible PCBs are generally used in special occasions. For example, the display screens of some digital multimeters can be rotated, and Flexible PCBs are often used inside; the display screens and buttons of mobile phones, etc.
Mobile phone Flexible PCB, its substrate is polyimide, and the surface has been treated with anti-oxidation, the minimum line width and line spacing is set to o. 1mm. The outstanding features of flexible PCBs are that they can be bent, curled, folded, and connected to rigid PCBs and moving parts, so that they can be wired three-dimensionally and realize three-dimensional space interconnection.
It is small in size, light in weight, and easy to assemble. It is suitable for space Electronic products with small size and high packing density.
Classified by scope of application
According to the scope of application, PCBs can be divided into low-frequency PCB and high-frequency PCB.
The high frequency of electronic equipment is a development trend, especially in today’s increasingly developed wireless network and satellite communication, information products are moving towards high speed and high frequency, and communication products are moving towards high-capacity and fast wireless transmission of voice, video and data standardization.
Therefore, the development of a new generation of products requires high-frequency PCBs, and their foil-coated substrates can be made of polytetrane, polyethylene, polystyrene, polytetrafluoroethylene glass cloth and other materials with low dielectric loss and low dielectric constant.
Types of special PCBs
At present, some special PCBs such as metal core PCB, SMT PCB and carbon film PCB have also appeared.
Metal core PCB
The metal core PCB is to replace the epoxy glass cloth board with a metal plate of comparable thickness. After special treatment, the conductor circuits on both sides of the gold board are connected to each other and highly insulated from the metal part. The advantages of metal core PCB 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.
The SMT PCB is a printed board developed to meet the needs of “light, thin, short, and small” electronic products, and to cooperate with the installation process of surface mount devices with high 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.
Carbon film PCB
The carbon film PCB is a printed board in which a layer of carbon film is printed to form a contact or a jumper (the resistance value meets the specified requirements) after the conductor pattern is made on the copper-plated foil board. It is characterized by simple production process, low cost, short cycle, good wear resistance and electrical conductivity, can realize high density of single panel, miniaturization and light weight of products, and is suitable for TVs, telephones, video recorders and electronic pianos and other products.
PCB manufacturing process
Taking a four -layer PCB as an example, let’s take a look at how a PCB is made.
Step 1: chemical cleaning
In order to obtain good-quality etching patterns, it is necessary to ensure that the resist layer is firmly bonded to the surface of the substrate, and the surface of the substrate is required to be free of oxide layer, oil, dust, fingerprints and other dirt. Therefore, before coating the resist layer, it is first necessary to clean the surface of the board and make the surface of the copper foil reach a certain degree of roughness.
Inner layer board: start to make a four-layer PCB, and the inner layer (second and third layers) must be done first. The inner plate is a copper sheet laminated with glass fiber and epoxy resin matrix on the upper and lower surfaces.
Step 2: cutting board , lamination
Apply photoresist: In order to make the shape we need on the inner sheet, we first apply a dry film (photoresist, photoresist) on the inner sheet. The dry film is composed of polyester thin film, photoresist film and polyethylene protective film. When pasting the film, first peel off the polyethylene protective film from the dry film, and then paste the dry film on the copper surface under heat and pressure.
Step 3: 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 then initiate the polymerization and crosslinking reaction of the photopolymerizable monomer, and form a polymer structure insoluble in dilute alkali solution after the reaction. The polymerization reaction will continue for a period of time.
In order to ensure the stability of the process, do not tear off the polyester film immediately after exposure, but should stay for more than 15 minutes to allow the polymerization reaction to continue, and tear off the polyester film before developing.
Development: The active groups in the unexposed part of the photosensitive film react with dilute alkali solution to produce soluble substances and dissolve, leaving the photosensitive cross-linked and cured graphic part.
Step 4: etching
In the production process of flexible PCB or PCB, the unnecessary part of the copper foil is removed by chemical reaction to form the required circuit pattern, and the copper under the photoresist is kept from being etched of the influence.
Step 5: film removal, post-etch punching, AOI inspection, oxidation
The purpose of film removal is to remove the resist layer remaining on the PCB surface after etching to expose the underlying copper foil. “Membrane slag” filtration and waste recovery must be properly handled. If the water washing after removing the film can be completely cleaned, it may be considered not to do pickling. After the PCB surface is cleaned, it should be completely dried at the end to avoid moisture residue.
Step 6: laminated board – protective film
Before entering the press machine, the raw materials for each multilayer PCB need to be prepared for the Lay-up operation. In addition to the inner layer that has been oxidized, a protective film (Prepreg)-epoxy resin impregnated glass is still required fiber. The function of lamination is to stack the PCBs covered with protective film in a certain order and place them between the two layers of steel plates.
Step 7: laminated board – copper foil and vacuum lamination
Copper foil – cover the current inner layer with a layer of copper foil on both sides, and then perform multilayer pressing (extrusion that needs to measure temperature and pressure within a fixed time) and cool to room temperature after completion, the rest It’s a multiayer PCB.
Step 8: CNC drilling
Under the precise conditions of the inner layer, CNC drilling drills holes according to the pattern. Drilling requires high precision to ensure that the holes are in the correct position.
Step 9: electroplating – through hole
In order to make the through hole conduct between layers (to metallize the resin and glass fiber bundles on the non-conductive part of the hole wall), the hole must be filled with copper. The first step is to plate a thin layer of copper in the hole, a process that is purely chemical. The final plated copper is 50 millionths of an inch thick.
Step 10: cutting board lamination
Apply photoresist: We once applied photoresist on the outer layer.
Step 11: exposure and development
Outer layer exposure and development.
Step 12: line plating
This time it also be called secondary copper plating, the main purpose is to thicken the line copper and through-hole copper.
Step 13: electroplating tin
Its main purpose is an etch resist, protecting the copper conductors it covers from attack by alkaline copper etching (protecting all copper lines and inside vias).
Step 14: remove the film
We already know the purpose, we just need to chemically expose the copper on the surface.
Step 15: etching
We know the purpose of the etch, and the tinning partially protects the underlying copper foil.
Step 16: pre-hardening exposure development on the solder mask
The solder resist layer is used to expose the pads, which is commonly referred to as the green oil layer. In fact, it is to dig holes in the green oil layer to expose the pads and other places that do not need to be covered by green oil. Proper cleaning will result in the proper surface characteristics.
Step 17: surface treatment
The hot air leveling solder coating process is to dip the printed board with flux first, then dip it in the molten solder, and then pass between two air knives, and use the hot compressed air in the air knife to clean the printed board. The excess solder is blown off, and the excess solder in the metal hole is excluded at the same time, so as to obtain a bright, flat and uniform solder coating.