Basic knowledge: a brief introduction to the introduction of PCB printed circuit boards
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PCB is the abbreviation of Printed Circuie Board printed circuit board. A conductive pattern which is formed on a dielectric material in a predetermined design to form a printed circuit, a printed component, or a combination of the two is generally referred to as a printed circuit. A conductive pattern that provides electrical connection between components on an insulating substrate is referred to as a printed circuit. Thus, the finished board of printed circuit or printed circuit is called printed circuit board, also known as printed board or printed circuit board.
PCBs are almost indispensable to the electronic devices we can see. From electronic watches, calculators, general-purpose computers, computers, communication electronic devices, military weapon systems, as long as there are electronic devices such as integrated circuits, they are PCBs are used for electrical interconnections. It provides mechanical support for fixed assembly of various electronic components such as integrated circuits, realizes wiring and electrical connection or electrical insulation between various electronic components such as integrated circuits, and provides required electrical characteristics such as characteristic impedance. At the same time, it provides solder mask pattern 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 soft film (flexible insulating substrate) printed with silver-white (silver paste) conductive patterns and graphic patterns. Since the universal screen printing method obtains such a pattern, we call this printed wiring board a flexible silver paste printed circuit board. The printed circuit boards on the various computer motherboards, graphics cards, network cards, modems, sound cards and home appliances that we see in Computer City are different. The substrate used is paper-based (usually used for single-sided) or glass-based (usually used for double-sided and multi-layer), pre-impregnated with phenolic or epoxy resin, and coated on one or both sides of the surface. Made. This kind of circuit board copper-clad sheet, we call it a rigid board. To make a printed circuit board, we call it a rigid printed circuit board. Printed circuit graphics on one side We call a single-sided printed circuit board, printed circuit patterns on both sides, and printed circuit boards formed by double-sided interconnection through metallization of holes. We call it a double-sided board. If a double-sided inner layer, two single-sided outer layers or two double-sided inner layers and two single-sided outer printed circuit boards are used, the positioning system and the insulating bonding material are alternately The printed circuit boards in which the conductive patterns are interconnected according to the design requirements become four-layer and six-layer printed circuit boards, which are also called multi-layer printed circuit boards. There are now more than 100 layers of practical printed circuit boards.
PCB production process is more complicated, it involves a wide range of processes, from simple machining to complex machining, common chemical reactions and photochemical electrochemical thermochemistry, computer-aided design CAM and other aspects of knowledge . Moreover, there are many technical problems in the production process and new problems will be encountered from time to time. Some problems will disappear if the cause is not found out. Because the production process is a non-continuous pipeline form, any problem will cause the whole line to be discontinued. Or the consequences of a large number of scraps, if the printed circuit board can not be recycled and reused, the work pressure of the process engineer is large, so many engineers left the industry to transfer to the printed circuit board equipment or materials business to do sales and technical services. .
In order to further understand the PCB, it is necessary to understand the production process of the single-sided, double-sided printed circuit board and ordinary multi-layer board to deepen its understanding.
Single-sided rigid printed board: → single-sided copper clad plate → blanking → (brushing, drying) → drilling or punching → screen printing line anti-etching pattern or using dry film → curing inspection and repairing → etching copper → removing corrosion Printing, drying → brushing, drying → screen printing solder mask graphics (commonly used green oil), UV curing → screen printing character marking graphics, UV curing → preheating, punching and shape → electrical opening, short circuit test → brushing, drying → Pre-coated soldering anti-oxidant (dry) or spray hot air flattening → inspection packaging → finished product delivery.
Double-sided rigid printed board: → double-sided copper clad plate → blanking → stacking plate → CNC drill through hole → inspection, deburring brushing → electroless plating (via metallization) → (full plate electroplating thin copper) → inspection Brushing → Screen printing negative circuit pattern, curing (dry film or wet film, exposure, development) → inspection, repairing → line pattern plating → electroplating tin (resistance nickel/gold) → de-printing (photosensitive film) → etching Copper → (returning tin) → cleaning and brushing → screen printing solder mask graphics commonly used heat-curing green oil (adhesive dry film or wet film, exposure, development, heat curing, commonly used photothermal curing green oil) → cleaning, drying → screen printing Mark character graphics, curing → (spray tin or organic solder mask) → shape processing → cleaning, drying → electrical continuity test → inspection packaging → finished product delivery.
Through-hole metallization method for manufacturing multi-layer board process→ double-layered material for inner layer copper clad plate→brushing→drilling positioning hole→sticking photoresist film or coating photoresist→exposure→development→etching and going Film→inner layer roughening, deoxidation→inner layer inspection→(outer single-sided copper clad laminate production, B-stage bonding sheet, sheet bonding sheet inspection, drilling positioning hole)→lamination→number control drilling→ Hole inspection→Pore pretreatment and electroless copper plating→full plate copper plating→plating inspection→adhesive plating resistant dry film or coating photo-resistant plating agent→surface layer exposure→development, repairing→line pattern plating→ Electroplating tin-lead alloy or nickel/gold plating→de-filming and etching→checking→screen printing resistive pattern or photo-resistive soldering pattern→printing character pattern→(hot air leveling or organic soldering film)→CNC wash shape→cleaning , dry → electrical continuity test → finished product inspection → packaging factory.
It can be seen from the process flow chart that the multi-layer board process is developed from the double-face metallization process. In addition to the double-sided process, it also has several unique contents: metallized hole inner layer interconnection, drilling and de-epoxy drilling, positioning system, lamination, special materials.
Our common computer board is basically epoxy resin glass cloth double-sided printed circuit board, one of which is the other side of the component and the other side is the soldering surface of the component foot. It can be seen that the solder joints are very regular. The component foot is separated from the solder joint and we call it the pad. Why are other copper wire patterns not tinned? Because the surface of the pad except soldering is required, the surface of the remaining portion has a solder mask that is resistant to wave soldering. The surface solder mask is mostly green, and a few are yellow, black, blue, etc., so the solder resist oil is often called green oil in the PCB industry. Its function is to prevent bridging during wave soldering, improve soldering quality and save solder. It is also a permanent protective layer for printed boards that protects against moisture, corrosion, mildew and mechanical abrasion. Viewed from the outside, the surface is smooth and bright green solder mask, which is a photo-curing green oil for the film. Not only does the appearance look better, but it is important that the pad has a higher accuracy, which improves the reliability of the solder joint.
We can see from the computer board that there are three ways to install components. A plug-in mounting process for a drive that inserts electronic components into the vias of a printed circuit board. Thus, it is easy to see that the through holes of the double-sided printed circuit board are as follows: one is a simple component insertion hole; the other is a component insertion and a double-sided interconnection via; and the third is a simple double-sided guide. The through hole; the fourth is the substrate mounting and positioning hole. The other two installation methods are surface mounting and direct chip mounting. In fact, the direct mounting technology of the chip can be considered as a branch of the surface mounting technology. It is to directly bond the chip to the printed board, and then interconnected by wire bonding or carrier tape method, flip chip method, beam lead method and other packaging technologies. On the board. Its welding surface is on the component surface.
Surface mount technology has the following advantages:
1) Since the printed board largely eliminates the large via or buried via interconnection technology, the wiring density on the printed board is increased, and the printed board area is reduced (generally one of the three steps of the plug-in installation), It also reduces the number of layers and cost of the printed board.
2) Reduced weight and improved seismic performance, using gelatinous solder and new welding technology to improve product quality and reliability.
3) Due to the increased wiring density and shortened lead length, 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 automate than plug-in installation, increasing installation speed and labor productivity, and correspondingly reducing assembly costs.
It can be seen from the above surface security technology that the improvement of the circuit board technology is improved with the improvement of the package technology and surface mounting technology of the chip. Nowadays, the surface sticking rate of the computer boards we are watching is constantly rising. In fact, such a circuit board reuse transmission screen printing circuit graphics can not meet the technical requirements. Therefore, the ordinary high-precision circuit board, the line pattern and the solder resist pattern basically adopt the photosensitive line and the photosensitive green oil manufacturing process.
With the development trend of high density of 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 the like. The above is just a superficial introduction to some surfaces. There are still many things in the production of circuit boards that are not specified due to space limitations, such as blind buried holes, wound plates, Teflon plates, lithography and so on. If you want to conduct in-depth research, you need to work hard.