PCB Blog - PCB printed circuit board knowledge

PCB board is the abbreviation of English (Printed Circuit Board) printed circuit board. Usually, a conductive pattern made of printed circuits, printed components or a combination of the two on the insulating material according to a predetermined design is called a printed circuit. The conductive pattern that provides electrical connection between components on an insulating substrate is called a printed circuit. In this way, the printed circuit or the finished board of the printed circuit is called a printed circuit board, also known as a printed circuit board or a printed circuit board.

The PCB board is inseparable from almost all electronic equipment we can see, ranging from electronic watches, calculators, general-purpose computers, to computers, communication electronic equipment, and military weapon systems. As long as there are electronic devices such as integrated circuits, they The electrical interconnection between them all use the PCB board. It provides mechanical support for the 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, etc. At the same time, it provides solder mask graphics for automatic soldering; provides identification characters and graphics for component insertion, inspection, and maintenance.

How are PCB boards made? When we open the health disk of the general-purpose computer, we can see a soft film (flexible insulating substrate), printed with silver-white (silver paste) conductive graphics and positioning graphics. Because this kind of pattern is obtained by the general screen printing method, we call this printed circuit board flexible silver paste printed circuit board. The printed circuit boards on various computer motherboards, graphics cards, network cards, modems, sound cards and household appliances that we see in the computer city are different. The substrate it uses is made of paper base (usually used for single-sided) or glass cloth (usually used for double-sided and multi-layered), pre-impregnated phenolic or epoxy resin, and the surface layer is pasted with copper cladding on one or both sides and then laminated and cured. made. This kind of circuit board copper-clad sheet, we call it a rigid board. After making a printed circuit board, we call it a rigid printed circuit board. A printed circuit board with a printed circuit pattern on one side is called a single-sided printed circuit board, a printed circuit board with a printed circuit pattern on both sides, and a printed circuit board formed by double-sided interconnection through the metallization of the holes, we call it a double-sided board. If a printed circuit board with double-sided inner layer, two single-sided outer layer, or two double-sided inner layer and two single-sided outer layer is used, the positioning system and insulating bonding material are alternated together and The printed circuit board with the conductive pattern interconnected according to the design requirements becomes a four-layer and six-layer printed circuit board, also known as a multi-layer printed circuit board. There are now more than 100 layers of practical printed circuit boards.

The production process of PCB boards is relatively complex, and it involves a wide range of processes, from simple machining to complex machining, ordinary chemical reactions, photochemical electrochemical thermochemical processes, computer-aided design CAM and many other processes. Knowledge. Moreover, there are many process problems in the production process, and new problems will be encountered from time to time, and some problems will disappear without finding out the cause. Because the production process is a discontinuous assembly line form, any problem in any link will cause the entire line to stop production. Or the consequences of a large number of scrapped, if the printed circuit board is scrapped, it cannot be recycled and reused, and the work pressure of process engineers is high, so many engineers leave this industry and turn to printed circuit board equipment or material suppliers to do sales and technical services. . In order to further understand the PCB board, it is necessary to understand the production process of the usual single-sided, double-sided printed circuit boards and ordinary multi-layer boards, so as to deepen our understanding of it.

Single-sided rigid printed board: → single-sided copper clad laminate → blanking → (brushing, drying) → drilling or punching → screen printed circuit anti-etching pattern or using dry film → curing inspection repair plate → etching copper → anti-corrosion removal Printing material, drying → brushing, drying → screen printing solder mask pattern (commonly used green oil), UV curing → screen printing character mark graphics, UV curing → preheating, punching and shape → electrical open, short circuit test → brushing, drying → Pre-coating with anti-oxidants for soldering (drying) or spraying with tin and hot air for leveling → inspection and packaging → finished product delivery.

Double-sided rigid printed board: → double-sided copper clad laminate → blanking → stacking → CNC drilling through holes → inspection, deburring and brushing → chemical plating (through hole metallization) → (full board electroplating of thin copper) → inspection Brushing→screen printing negative circuit pattern, curing (dry or wet film, exposure, development)→inspection, repair plate→circuit pattern plating→electroplating tin (resist nickel/gold)→printing material (photosensitive film)→etching Copper → (tin stripping) → cleaning and brushing → screen printing solder mask pattern commonly used heat-curing green oil (photosensitive dry film or wet film, exposure, development, heat curing, commonly used photosensitive heat-curing green oil) → cleaning, drying → screen printing Marking character graphics, curing → (spray tin or organic solder protection film) → shape processing → cleaning, drying → electrical continuity testing → inspection and packaging → finished product delivery.

Process flow of through-hole metallization to manufacture multilayer board → double-sided cutting of inner copper clad laminate → brushing → drilling positioning hole → sticking photoresist dry film or coating photoresist → exposure → development → etching and removal Film→Inner layer roughening, deoxidation→Inner layer inspection→(Outer layer single-sided copper clad laminate circuit production, B-stage bonding sheet, board bonding sheet inspection, drilling positioning holes)→Lamination→Numerical control drilling→ Hole inspection → hole pretreatment and electroless copper plating → thin copper plating on the whole board → coating inspection → paste photoresist electroplating dry film or coating photoresist electroplating agent → surface layer base plate exposure → development, repair plate → circuit pattern plating → Electroplating tin-lead alloy or nickel/gold plating → film removal and etching → inspection → screen printing solder mask pattern or photoresist pattern → printing character pattern → (hot air leveling or organic solder protection film) → CNC shape washing → cleaning , drying → electrical on-off detection → finished product inspection → packaging and delivery.

It can be seen from the process flow chart that the multi-layer board process is developed on the basis of the double-face metallization process. In addition to the double-sided process, it has several unique contents: metallized hole inner layer interconnection, drilling and de-epoxy drilling, positioning system, lamination, special materials. Our common computer boards are basically epoxy resin glass cloth-based double-sided printed circuit boards, one of which is the plug-in component and the other side is the component foot welding surface. It can be seen that the solder joints are very regular. We call it the pad for the discrete soldering surface of the component feet. Why are other copper wire patterns not tinned? Because in addition to the pads that need to be soldered, the surface of the rest has a solder mask that is resistant to wave soldering. Most of the surface solder masks are green, and a few are yellow, black, blue, etc., so the solder mask oil is often called green oil in the PCB board industry. Its function is to prevent bridging during wave soldering, improve soldering quality and save solder. It is also the protective layer of the printed board, which can prevent moisture, corrosion, mildew and mechanical scratches. From the outside, the green solder mask with smooth and bright surface is a green oil for film-to-board photosensitive heat curing. Not only does the appearance look good, but it is also important that the pads have a higher degree, thereby improving the reliability of the solder joints.

We can see from the computer board that there are three ways to install components. A plug-in installation process for transmission, inserting electronic components into the through holes of the printed circuit board. In this way, it is easy to see that the via 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 double-sided interconnection via hole; The fourth is the substrate mounting and positioning holes. The other two installation methods are surface mounting and direct chip mounting. In fact, the direct chip mounting technology can be considered as a branch of surface mounting technology. It is to directly stick the chip on the printed board, and then use the wire bonding method or the tape carrier method, the flip chip method, the beam lead method and other packaging technologies to interconnect to the printed circuit board. board. The welding surface is on the component surface. Surface mount technology has the following advantages:
1) Due to the elimination of large through-hole or buried-hole interconnection technology on the printed board, the wiring density on the printed board is improved, and the printed board area is reduced (usually one of the third steps of plug-in installation), and at the same time It can also reduce the design layers and cost of the printed board.
2) The weight is reduced, the shock resistance is improved, and the gel solder and new welding technology are adopted to improve the product quality and reliability on PCB board.