As an important electronic component, the printed circuit board is the support of electronic components. Because of its important role in the field of electronic components, it is called "electronic aircraft carrier" by many people.
Today, communication products, computers and almost all other electronic products use printed circuits. The development and improvement of printed circuit technology created conditions for the invention of the integrated circuit 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, communications, medical, electric power, automobiles, industrial control, smart phones, and wearables.
Invention of the printed circuit

The printed circuit was invented by Austrian electrical engineer Paul 8226 ; Eisler in the mid-1930s. Eisler studied electrical engineering at the Vienna Institute of Technology in his early years, and after graduating in 1930 , studied printing technology. While he was doing research on electronic circuit boards, he often went to the library to consult books and periodicals on printing technology.
After serious thinking, he came up with an idea: if the circuit of electronic equipment is printed on the circuit board one at a time, just like printing books or newspapers, there is no need to make the circuit board piece by piece by hand, and the circuit does not need to be made by people one by one. If the root 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, photoengraving techniques are usually used. That is to say, by taking pictures, the negative plate of the photographed picture is etched on the copper plate or zinc plate, and many pictures can be printed with this copper plate or zinc plate.

Eisler also tried a plate-making method similar to the printing industry when making circuit boards. 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 lines. 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 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, laying the foundation for the mechanized and automated production of electronic products. Since the 1950s, the vast advances in various electronic products, including communications equipment, have been inseparable from the adoption of printed circuit technology.

With the continuous improvement of the manufacturing level of printed circuits, the printed circuits produced can achieve high precision, thus pushing the 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 a small area, and you can get a copy of the electronic data by paying attention to the fancier's official account and replying to the information and address. Electronic circuit boards with complex circuits and high reliability. This printed circuit board is very suitable for communication equipment and computers with complex lines 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 shows that without circuit boards, without electronic circuits, flight, transportation, atomic energy, computers, aerospace, communications, home appliances... all of this cannot be achieved.

The reasoning 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 vary widely, and there are different classifications of printed circuit boards according to different standards.
1. Classification according to the distribution of printed circuits According to the approximate distribution of printed circuits, printed circuit boards can be divided into three types: single-sided, double-sided and multi-layered. Single-sided

The single-sided board is on an insulating substrate with a thickness of 0.2-5mm, and 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, and is suitable for one-place circuit requirements, such as radios, TV sets, etc.; it is not suitable for occasions requiring 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 equipment can be reduced.
• Multilayer boards

A printed board with more than 3 layers of printed circuits printed on an insulating substrate is called a multi-layer board. It is composed of several layers of thin single-sided or double-sided boards, and its thickness is generally 1.2-2.5mm. In order to lead out the circuit sandwiched between the insulating substrates, the holes for mounting components on the multilayer board need to be metallized, that is, the inner surface of the small holes is coated with a metal layer, so that it is connected to the printed circuit sandwiched between the insulating substrates.

The following figure is a schematic diagram of the structure of a multi-layer board. The components used in the multi-layer board are mostly SMD components, which are characterized by:
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. A shielding layer is added to reduce the signal distortion of the circuit;
5. The grounding heat dissipation layer is introduced, which 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 two types: rigid and flexible.

Rigid Printed Board

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

Flexible printed board

Flexible printed boards are made of soft layered plastics or other soft insulating materials. The parts it is made of 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; the display screen and buttons of mobile phones, etc.
The picture below is a 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 features of flexible printed boards are that they can be bent, curled, and folded, and can be connected to rigid printed boards and moving parts, so that three-dimensional wiring can be achieved and three-dimensional space interconnection can be realized. Small, high-density electronic products.

3. Classification according to the scope of application

According to the scope of application, printed circuit boards can be divided into low-frequency and high-frequency printed circuit boards.
The high frequency of electronic equipment is the development trend, especially in today's increasingly development of wireless networks 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 high speed. Therefore, the new generation of products developed require high-frequency printed boards, and the foil-clad substrates can be made of materials with low dielectric loss and low dielectric constant such as polytetrane, ethylene, polystyrene, and polytetrafluoroethylene glass cloth.

4. Types of special printed boards

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

Metal Core Printed Board

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

surface mount printed board

The surface mount printed board is a printed board developed to meet the needs of "light, thin, short and small" electronic products, and to cooperate with the mounting 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 printed board

The carbon film printed board is a printed board in which a layer of carbon film is printed to form a contact or jumper (the resistance value meets the specified requirements) after the conductor pattern is made on the copper-clad plate. It is characterized by simple production process, low cost, short cycle, good wear resistance and electrical conductivity, which can realize high density of single panel, miniaturization and light weight of products, and is suitable for TVs, telephones, video recorders and electronic organs. and other products.
Demonstration of the production process of printed circuit boards

Taking four layers as an example, let's take a look at how a printed circuit board is made.

• The first step, chemical cleaning In order to obtain a good quality etched pattern, it is necessary to ensure that the resist layer is firmly bonded to the substrate surface, and the substrate surface is required to be free of oxide layers, oil, dust, fingerprints and other contaminants. Therefore, before coating the resist layer, the surface of the board should be cleaned and the surface of the copper foil should be roughened to a certain degree.

Inner layer board: To start making a four-layer board, the inner layer (the second layer and the third layer) must be done first. The inner layer board is a copper sheet compounded on the upper and lower surfaces by glass fiber and epoxy resin matrix.

The second step, cutting and laminating

Apply photoresist: In order to make the shape we need on the inner sheet, we first stick a dry film (photoresist, photoresist) on the inner sheet. The dry film is composed of three parts: polyester film, photoresist film and polyethylene protective film. When sticking the film, first peel off the polyethylene protective film from the dry film, and then stick the dry film on the copper surface under the condition of 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 trigger the photopolymerization monomer to produce a cross-linking reaction, which forms a polymer structure that is 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 stay for more than 15 minutes, so that the polymerization reaction continues, and tear off the polyester film before developing.

Development: The reactive groups in the unexposed part of the photosensitive film react with the dilute alkaline solution to produce soluble substances and dissolve them, leaving the photosensitive cross-linked and cured graphic parts.

• Step 4, Etching

In the production process of flexible printed boards or printed boards, 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 preserved from etching of influence.

• Step 5, film removal, post-etch punching, AOI inspection, oxidation

The purpose of removing the film is to remove the resist layer remaining on the board surface after etching to expose the underlying copper foil. The "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 to do acid washing. After the board is cleaned, it should be completely dried to avoid water residue.
• Step 6, Laminate - Protective Film

Before entering the press, it is necessary to prepare the raw materials of each multi-layer board for the Lay-up operation. In addition to the inner layer that has been oxidized, a protective film film (Prepreg) - epoxy resin impregnated glass fiber is required. The function of the lamination is to stack the plates covered with protective film in a certain order and place them between the two layers of steel plates.
• Step 7, Lamination - 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 pressing (extrusion that needs to measure temperature and pressure for a fixed time) and then cool to room temperature, the rest It is a multi-layered board.
• Step 8, CNC drilling

Under precise conditions on the inner layer, CNC drilling drills according to the pattern. Drilling accuracy is required to ensure that the hole is in the correct position.

• Step 9, Electroplating - Through Holes

In order to make the through holes conduct between the layers (metallization of the resin and glass fiber bundles on the non-conductive part of the hole walls), the holes must be filled with copper. The first step is to plate a thin layer of copper in the hole, which is all chemical. The final plated copper thickness is 50 millionths of an inch.

• Step 10, cutting and laminating

Apply photoresist: We once applied photoresist on the outer layer.

• Step 11, Exposure and Development

Outer layer exposure and development

• The twelfth step, circuit plating

This time, it has also become secondary copper plating, the main purpose is to thicken the copper thickness of the line and the through hole.
• The thirteenth step, electroplating tin

Its main purpose is an etch resist, protecting the copper conductors it covers from being attacked by alkaline copper etching (protecting all copper traces and inside vias).

• Step 14, remove film

We already know the purpose, just chemically, the copper on the surface is exposed.

• Step 15, Etching

We know the purpose of the etching, the tinned part protects the copper foil underneath.

• The sixteenth step, pre-hardening exposure and development on the solder mask

The solder mask 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 the green oil. Proper cleaning can result in suitable surface features.

• Step 17, 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 put the printed board. The excess solder is blown off, and the excess solder in the metal hole is removed, resulting in a bright, flat, and uniform solder coating.