PCB printed circuit board knowledge points

PCB is short for Printed Circuit Board. In general, a conductive pattern made of a printed circuit, a printed component, or a combination of both on an insulating material according to a predetermined design is called a printed circuit. A conductive pattern that provides electrical connection between components on an insulating substrate is called a printed circuit. In this way, a printed circuit board or a printed circuit board is called a printed circuit board, which is also called a printed board or a printed circuit board.

PCBs almost all of us can see electronic devices can not do without it, from small electronic watches, calculators, general-purpose computers, to computers, communications electronics equipment, military weapons systems, as long as there is no electronic devices such as integrated circuits, they PCBs are used for electrical interconnections. It provides mechanical support for the assembly of various electronic components such as integrated circuits, wiring and electrical connection between various electronic components such as integrated circuits, or electrical insulation, and provides required electrical characteristics such as characteristic impedance. At the same time, it provides solder resist patterns for automatic soldering; it provides identification characters and graphics for component insertion, inspection and maintenance.

How is PCB made? When we open the keyboard of a general-purpose computer, we can see a flexible film (flexible insulating substrate) with a silver-white (silver paste) conductive pattern and a healthy image. Because the universal screen printing method obtains this pattern, we call this printed circuit board a flexible silver paste printed circuit board. The computer boards, video cards, network cards, modems, sound cards, and printed circuit boards on home appliances that we see at Computer City are different. The substrates used are paper-based (usually used for single-sided) or glass-based (usually used for double-sided and multi-layered), pre-impregnated phenolic or epoxy resin, with one or both sides of the surface coated with copper, and then laminated and cured. Made. This kind of circuit board copper plate, we call it a rigid board. Then we made the printed circuit board, we call it a rigid printed circuit board. Single-sided printed circuit patterns We call single-sided printed circuit boards, double-sided printed circuit patterns, and then double-sided interconnected printed circuit boards formed by metallization of holes. We call them double-sided boards. If a double-sided inner layer, two single-sided outer layers or two double-sided inner layers and two single-sided printed wiring boards are used, the positioning system and the insulating bonding material are alternated and Printed circuit boards whose conductive patterns are interconnected according to design requirements have become four-layer and six-layer printed circuit boards, also called multilayer printed circuit boards. There are now more than 100 practical printed circuit boards.

PCB production process is more complex, it involves a wide range of processes, from simple mechanical processing to complex mechanical processing, ordinary chemical reactions, photochemical electrochemical thermochemistry and other processes, computer-aided design CAM and other aspects of knowledge . Moreover, there are many process problems in the production process and they may encounter new problems from time to time. Some problems disappear when the cause is not clearly identified. Because the production process is a discontinuous assembly line, any problems in one of the links will cause a complete shutdown. Or the consequences of a lot of scrap, if the printed circuit board cannot be recycled for reuse, the work pressure of process engineers is high, so many engineers have left the industry to go to the work of printing circuit board equipment or materials vendors for sales and technical services. .

In order to further understand the PCB, it is necessary to understand the manufacturing process of usually single-sided, double-sided printed circuit boards and common multilayer boards to deepen understanding of it.

Single-sided rigid printed boards: → single-sided copper clad laminates → blanking → (brushing, drying) → drilling or punching → screen printing circuit etching patterns or using dry film → curing check plate → etching copper → resisting corrosion Printing material, drying, brushing, drying, screen printing solder mask pattern (used in green oil), UV curing, screen printing character marking pattern, UV curing, preheating, punching and appearance, electrical opening, short circuit testing, brushing, drying Pre-painted welding anti-oxidant (dry) or spray hot air leveling → inspection packaging → finished products.

Double-sided rigid printed boards: → double-sided copper clad → cutting → lamination → CNC drilling through holes → inspection, deburring and brushing → electroless plating (metallization of vias) → (full-plate copper plating) → inspection Brushing → screen printing negative circuit pattern, curing (dry film or wet film, exposure, development) → inspection, repair plate → circuit pattern plating → electroplating tin (anti-corrosion nickel / gold) → to print material (photosensitive film) → etching Copper → (retreat tin) → cleaning brush → screen printing solder mask pattern commonly used thermal curing green oil (paste sensitive dry film or wet film, exposure, development, thermal curing, commonly used photographic heat curing green oil) → cleaning, drying → screen printing Marked character graphics, curing → (spray or organic solder mask) → shape processing → cleaning, drying → electrical continuity testing → inspection packaging → finished products.

Through-hole metallurgy method to manufacture multi-layered plates → Inner-layer copper clad plate double-sided cutting → brushing → drilling positioning holes → photo-resist dry film or photoresist coating → exposure → development → etching and removal Membrane → Inner Coarsening, Deoxidation → Inner Inspection → (External Single-sided CCL Production, B-stage Bonding, Plate Bonding Inspection, Drilling Positioning) → Lamination → Number Controlled Drilling→ Hole Inspection → Pre-hole Treatment and Electroless Copper Plating → Full Plate Copper Plating → Plating Inspection → Laminate Plating Resistant Electroplating Dry Film or Coating Photoresist Plating Agent → Surface Backplane Exposure → Development, Plate Repair → Circuit Pattern Plating → Plating tin-lead alloy or nickel/gold plating → film removal and etching → inspection → screen printing solder resist pattern or photo-resistance solder pattern → printed character pattern → (hot air leveling or organic solder mask) → numerical control wash shape → cleaning , Drying → Electrical continuity test → Finished product inspection → Packing and shipping.


From the process flow chart, it can be seen that the multilayer board process is developed based on the two-face metallization process. In addition to the two-sided process, it has several unique features: metallized hole interconnections, drilling and depollution drilling, positioning systems, lamination, and special materials.

Our common computer board is basically an epoxy glass cloth based double-sided printed circuit board. One side of the board is a component foot soldering surface on the other side. We can see that the solder joints are very regular. These solder joints are The discrete soldering surface of the component leg is called a pad. Why are other copper wire patterns not tinned? Because in addition to solder pads and other parts, the rest of the surface has a wave solder mask. The surface of the solder mask is mostly green, and a few are yellow, black, blue, etc. So in the PCB industry, the solder resist oil is often called green oil. Its role is to prevent bridging during wave soldering, improve soldering quality and save solder. It is also a permanent protective layer of the printed board, can play a role in moisture, corrosion, mold and mechanical abrasion. From the outside, the surface is smooth and bright green solder mask, which is a photo-sensitive heat-cured green oil film for the film. Not only does the appearance look better, what is important is the high accuracy of its pads, which improves the reliability of solder joints.

We can see from the computer board that there are three ways to install components. A plug-in mounting process for the drive inserts electronic components into the vias of a printed circuit board. It is easy to see that the through-holes of the double-sided printed circuit board have the following types: one is a simple component insertion hole; the other is a component insertion and double-sided interconnection via hole; and the third is a simple two-sided conduction Through holes; four is the substrate mounting and positioning holes. The other two installation methods are surface mounting and direct chip mounting. Actually, direct chip mounting technology can be considered as a branch of surface mounting technology. It is a method of directly bonding a chip to a printed circuit board, and then interconnecting to India by using wire bonding or tape mounting, flip-chip mounting, and beam-type wiring. On the board. Its welding surface is on the surface of the component.

Surface Mount Technology has the following advantages:

1) Due to the large number of printed circuit boards eliminating large vias or buried interconnects, the density of the printed circuit board is increased, and the area of ​​the printed circuit board is reduced (typically one of the three stages of the plug-in installation). It can also reduce the design layer and cost of printed boards.

2) Reduced weight, increased seismic performance, using colloidal solder and new welding technology to improve product quality and reliability.

3) As the wiring density is increased and the lead length is shortened, parasitic capacitance and parasitic inductance are reduced, which is more conducive to improving the electrical parameters of the printed board.

4) It is easier to implement automation than cartridge-type installation, increase installation speed and labor productivity, and reduce assembly costs accordingly.

From the above surface security technology, it can be seen that the improvement of the circuit board technology is improved with the improvement of chip packaging technology and surface mounting technology. Now we see the computer board card surface adhesion rate is constantly rising. In fact, this type of circuit board re-use screen printing line graphics can not meet the technical requirements. Therefore, an ordinary high-precision circuit board, the circuit pattern and solder resist pattern basically uses photosensitive lines and photosensitive green oil production process.

With the development trend of high-density circuit boards, the production requirements of circuit boards are getting higher and higher, and more and more new technologies are applied to the production of circuit boards, such as laser technology, photosensitive resins, and so on. The above is only a superficial introduction of some of the surface, there are many things in the production of circuit boards due to space limitations did not explain, such as blind buried hole, coiled plate, Teflon plate, photolithography technology and so on. If you want to conduct in-depth research, you need to work hard.

Source: Bison