1、 Introduction
PCB (printed circuit board), also known as printed circuit board, printed circuit board, or printed circuit board for short, or PCB (printed circuit board) or PWB (printed wiring board) for short, is cut to a certain size with insulating board as the base material, on which at least one conductive figure is attached, and holes (such as component holes, fastening holes, metallized holes, etc.) are arranged to replace the chassis of electronic components of previous devices, And realize the interconnection between electronic components. Because this kind of board is made by electronic printing, it is called 'printed circuit board'. It is inaccurate to call 'printed circuit board' as 'printed circuit', because there is no 'printed component' on the printed circuit board and only wiring.
2、 Basic composition
The current circuit board mainly consists of the following:
1. Line and pattern: the line is a tool for conducting between the elements. In design, a large copper surface will be designed additionally as the grounding and power layer. Lines and drawings are made at the same time.
2. Dielectric layer: used to maintain the insulation between the lines and layers, also known as substrate.
3. Hole (Through hole/via): The through hole can make more than two layers of lines connected to each other. The larger through hole is used as part plug-in. In addition, the non-pTH hole (nPTH) is usually used for surface mounting and positioning, and is used for fixing screws during assembly.
4. Solder resistant/holder mask: not all copper surfaces need to eat tinned parts, so a layer of material (usually epoxy resin) will be printed in the area that does not eat tin to prevent short circuit between lines that do not eat tin. According to different processes, it can be divided into green oil, red oil and blue oil.
5. Legend/Marking/Silk screen: This is an unnecessary component. Its main function is to mark the name and position box of each part on the circuit board for easy maintenance and identification after assembly.
6. Surface finish: Because the copper surface is easy to oxidize in the general environment, it cannot be tinned (poor solderability), so it will be protected on the copper surface to be tinned. The protection methods include HASL, ENIG, Silver, Tin and OSP. Each method has its advantages and disadvantages, which are collectively referred to as surface treatment.
3、 Brief History of Development
Before the emergence of printed circuit boards, the interconnection between electronic components was based on the direct connection of wires to form a complete circuit. At present, the circuit breadboard exists only as an effective experimental tool, while the printed circuit board has become the absolute dominant position in the electronic industry.
At the beginning of the 20th century, in order to simplify the production of electronic machines, reduce the wiring between electronic parts, and reduce production costs, people began to study the method of replacing wiring with printing. Over the past three decades, engineers have proposed to use metal conductors on insulated substrates as wiring.
The most successful is that in 1925, Charles Ducas of the United States printed the circuit pattern on the insulating substrate, and then successfully established the conductor for wiring by electroplating.
Until 1936, Austrian Paul Eisler published the foil film technology in Britain, and he used printed circuit boards in a radio device; In Japan, Miyamoto Hiroshi successfully applied for a patent by spraying the wiring method 'メ tata リ コ ン blowing the wiring method (concession No. 119384)'. Paul Eisler's method is the most similar to that of today's printed circuit board. This kind of method is called subtraction method, which is to remove unnecessary metal; Charles Ducas and Miyamoto's approach is to add only the required wiring, which is called additive method. However, because of the high heat generated by electronic parts at that time, it was difficult to use the two substrates together, so there was no formal use, but it also made the printed circuit technology further.
In 1941, the United States painted copper paste on the talcum for wiring to make proximity fuse.
In 1943, Americans used this technology extensively in military radios.
In 1947, epoxy resin began to be used as a manufacturing substrate. At the same time, NBS began to study the manufacturing technology of forming coils, capacitors, resistors, etc. with printed circuit technology.
In 1948, the United States officially recognized the invention for commercial use.
Since the 1950s, vacuum tubes have been largely replaced by transistors with low heat output, and printed circuit board technology has been widely used. At that time, etching foil film technology was the mainstream.
In 1950, Japan used silver paint on glass substrate as wiring; And copper foil on paper phenolic substrate (CCL) made of phenolic resin.
In 1951, with the emergence of polyimide, the heat resistance of the resin was further improved, and the polyimide substrate was also manufactured.
In 1953, Motorola developed a double-sided plate with electroplated through-hole method. This method is also applied to the later multilayer circuit board.
In the 1960s, after 10 years of widespread use of printed circuit boards, their technology became increasingly mature. Since the advent of Motorola's double-sided board, multi-layer printed circuit boards have appeared, making the ratio of wiring to substrate area even higher.
1960, V Dahlgreen made a flexible printed circuit board by sticking the metal foil film printed with the circuit in the thermoplastic plastic. In 1961, Hazeltine Corporation of the United States referred to the electroplated through-hole method to produce multilayer plates.
In 1967, 'Plated-up technology', one of the layer-adding methods, was published.
In 1969, FD-R manufactured flexible printed circuit boards with polyimide.
In 1979, Pactel published the 'Pactel method', one of the layer-increasing methods.
In 1984, NTT developed the 'Copper Polymide method' for thin film circuits.
In 1988, Siemens developed Microwiring Substrate's multiplayer printed circuit board.
In 1990, IBM developed the 'Surface Laminar Circuit' (SLC) multiplayer printed circuit board.
In 1995, Panasonic Electric developed ALIVH's multiplayer printed circuit board.
In 1996, Toshiba developed Bit's multiplayer printed circuit board.
At the end of the 1990s, when many schemes for adding layers of printed circuit boards were put forward, the added layers of printed circuit boards have also been formally applied in a large number until now.
4、 Importance It is an important electronic component
It is the support of electronic components. PCB is not a general terminal product, and the definition of its name is slightly confused.
For example, the motherboard used for personal computers is called the motherboard, not the circuit board directly. Although there are circuit boards in the motherboard, they are not the same. Therefore, when evaluating the industry, they cannot be said to be the same.
Another example: because there are integrated circuit parts loaded on the circuit board, the news media called it IC board, but it is not equivalent to printed circuit board in essence. We usually refer to the printed circuit board as the bare board - that is, the circuit board without the upper component.
5、 Classification
According to the number of circuit layers of PCB printed circuit board, PCB printed circuit board is divided into single-panel, double-sided board and multi-layer board. The common multilayer board is generally 4 or 6 layers, and the complex multilayer board can reach dozens of layers.
There are three main types of PCB:
1. Single panel
Single-Sided Boards are on the most basic PCB, with parts concentrated on one side and wires concentrated on the other side. Because wires only appear on one side, this kind of PCB is called single-sided. Because the single panel has many strict restrictions on the design of the circuit (because there is only one side, the wiring cannot cross and must be around a separate path), only the early circuits used this type of board.
2. Double-sided board
Double-sided boards have wiring on both sides of the circuit board, but to use wires on both sides, it is necessary to have proper circuit connections between the two sides. The 'bridge' between such circuits is called a guide hole (via). The pilot hole is a small hole filled or coated with metal on the PCB, which can be connected with the wires on both sides. Because the area of the double-sided panel is twice as large as that of the single panel, the double-sided panel solves the difficulty of staggered wiring in the single panel (it can be connected to the other side through vias), and it is more suitable for more complex circuits than the single panel.
3. Multi-layer board
In order to increase the area that can be wired, multilayer boards use more single-sided or double-sided wiring boards. With one double-sided printed circuit board as the inner layer, two single-sided printed circuit boards as the outer layer, or two double-sided printed circuit boards as the inner layer, two single-sided printed circuit boards as the outer layer, the printed circuit boards that are alternately connected by the positioning system and the insulating bonding materials and the conductive patterns are interconnected according to the design requirements become four-layer and six-layer printed circuit boards, also known as multilayer printed circuit boards.
The number of layers of the board does not mean that there are several independent wiring layers. In special cases, empty layers will be added to control the thickness of the board. Usually, the number of layers is even and includes the two outermost layers. Most mainboards are 4-8 layer structures, but in theory, nearly 100 layers of PCB can be achieved. Large-scale supercomputers mostly use multi-tier mainboards, but because such computers can be replaced by clusters of many ordinary computers, ultra-multi-tier boards are gradually not used. Because the layers in the PCB are closely combined, it is not easy to see the actual number, but if you carefully observe the motherboard, you can still see it.
6、 Panel specification
1. Width of circuit board splicing ≤ 260mm (SIEMENS line) or ≤ 300mm (FUJI line); If automatic dispensing is required, PCB panel width × Length ≤ 125mm × 180mm。
2. The shape of the panel should be as close to the square as possible, and 2 is recommended × 2、3 × 3. ... panel; But don't make up yin and yang plates.
3. The outer frame (clamping edge) of the circuit board panel shall be designed with closed loop to ensure that the PCB panel will not deform after being fixed on the fixture.
4. The center distance between small plates shall be controlled between 75mm and 145mm.
5. There should be no large components or protruding components near the connection point between the outer frame of the panel and the inner small plate, and between the small plate and the small plate. The edge of the components and the PCB should have a space of more than 0.5mm to ensure the normal operation of the cutting tool.
6. Open four locating holes at the four corners of the outer frame of the panel, with an aperture of 4mm ± 0.01mm; The strength of the hole should be moderate to ensure that it will not break during the process of loading and unloading; The hole diameter and position accuracy shall be high, and the hole wall shall be smooth without burr.
7. Each small board in the circuit board assembly shall have at least three positioning holes, 3mm ≤ aperture ≤ 6mm, and no wiring or patch is allowed within 1mm of the edge positioning hole.
8. In principle, the QFP with spacing less than 0.65mm shall be set at its diagonal position for the positioning of the whole board of the circuit board and for the positioning of fine-spaced devices; The positioning reference symbols used for assembling circuit boards shall be used in pairs and arranged at the opposite corners of the positioning elements.
9. When setting the datum locating point, generally leave a weld free zone 1.5 mm larger than the locating point around
7、 Appearance
Bare board (without parts on the board) is also often called 'Printed Wiring Board (PWB)'. The base plate of the board itself is made of insulating and non-flexible materials. The fine circuit material visible on the surface is copper foil, which was originally covered on the whole board, but some of it was etched during the manufacturing process, and the remaining part became a mesh of fine circuit. These lines are called conductor patterns or wiring and are used to provide circuit connections for parts on PCBs.
Generally, the color of PCB is green or brown, which is the color of solder mask. It is an insulating protective layer, which can protect the copper wire, prevent short circuit caused by wave soldering, and save the amount of soldering tin. A layer of silk screen will also be printed on the solder mask. Generally, words and symbols (mostly white) are printed on it to mark the position of each part on the board. The screen printing surface is also called the legend.
When the final product is made, integrated circuits, transistors, diodes, passive components (such as resistors, capacitors, connectors, etc.) and other various electronic parts will be installed on it. Through wire connection, electronic signal connection and proper function can be formed.
8、 Key Benefits
The main advantages of using printed boards are:
1. Due to the repeatability (reproducibility) and consistency of graphics, errors in wiring and assembly are reduced, and time for equipment maintenance, commissioning and inspection is saved;
2. The design can be standardized to facilitate interchangeability;
3. High wiring density, small size and light weight are conducive to miniaturization of electronic equipment;
4. It is conducive to mechanized and automated production, improves labor productivity and reduces the cost of electronic equipment.
5. The bending resistance and precision of FPC flexible board should be better on high-precision instruments (such as cameras, mobile phones, video cameras, etc.).
9、 Market status
In the past ten years, China's printed circuit board manufacturing industry has developed rapidly, with its total output value and total output ranking first in the world. As electronic products are changing with each passing day, the price war has changed the structure of the supply chain. China has become the most important production base of printed circuit boards in the world with both industrial distribution, cost and market advantages.
The printed circuit board has developed from single layer to double-sided board, multilayer board and flexible board, and is continuously developing towards high precision, high density and high reliability. The continuous reduction of volume, cost reduction and performance improvement make the printed circuit board still maintain strong vitality in the future development of electronic products.
The development trend of production and manufacturing technology of printed circuit board in the future is to develop in the direction of high density, high precision, fine aperture, fine wire, small spacing, high reliability, multi-layer, high-speed transmission, light weight and thin shape.
According to the survey data of the Market Outlook and Investment Strategic Planning Analysis Report of China's PCB Manufacturing Industry, there were 908 PCB manufacturing enterprises above designated size in China in 2010, with total assets of 216.176 billion yuan; The sales revenue was 225.796 billion yuan, up 29.16% year on year; The total profit was 9.403 billion yuan, up 50.08% year on year.
10、 Design
The design of the printed circuit board is based on the circuit schematic diagram to realize the functions required by the circuit designer. The design of printed circuit board mainly refers to layout design, which needs to consider various factors such as the layout of external connections, the optimal layout of internal electronic components, the optimal layout of metal connections and through holes, electromagnetic protection, heat dissipation, etc. Excellent layout design can save production costs and achieve good circuit performance and heat dissipation performance. Simple layout design can be realized by hand, while complex layout design needs to be realized by computer aided design (CAD).
1. Ground wire design
Grounding is an important method to control interference on circuit boards, circuit boards and PCB boards in electronic equipment. If the grounding and shielding can be properly combined, most of the interference problems can be solved. The ground wire structure in electronic equipment generally includes system ground, enclosure ground (shielding ground), digital ground (logic ground) and simulation ground.
The following points should be noted in the design of ground wire:
(1) Correct selection of single-point grounding and multi-point grounding
In the low-frequency circuit, the working frequency of the signal is less than 1MHz, its wiring and the inductance between the components have a small impact, while the circulating current formed by the grounding circuit has a large impact on the interference, so the one-point grounding should be adopted. When the signal working frequency is greater than 10MHz, the ground wire impedance becomes large. At this time, the ground wire impedance should be reduced as much as possible, and the nearest multipoint grounding should be adopted. When the working frequency is between 1 and 10 MHz, if one point grounding is adopted, the length of the ground wire shall not exceed 1/20 of the wavelength, otherwise the multi-point grounding method shall be adopted.
(2) Separate digital circuit from analog circuit
There are both high-speed logic circuits and linear circuits on the circuit board. They should be separated as far as possible, and the ground wires of the two should not be mixed, and they should be connected with the ground wires of the power supply terminal. The grounding area of linear circuit shall be increased as much as possible.
(3) Try to thicken the grounding wire
If the ground wire is very thin, the ground potential will change with the change of current, resulting in unstable timing signal level of electronic equipment and poor anti-noise performance. Therefore, the grounding wire should be thickened as much as possible so that it can pass three times the allowable current of the printed circuit board. If possible, the width of the grounding wire should be greater than 3mm.
(4) Make the ground wire into a dead cycle loop
When designing the ground wire system of printed circuit board composed only of digital circuits, making the ground wire into a dead loop can significantly improve the anti-noise ability. The reason is that there are many integrated circuit components on the printed circuit board, especially when there are components with high power consumption, due to the limitation of the thickness of the ground wire, a large potential difference will be generated on the ground junction, resulting in a decline in anti-noise ability. If the ground structure is looped, the potential difference will be reduced and the anti-noise ability of electronic equipment will be improved.
2. High-speed multi-layer
Under the premise that electronic products tend to be multi-functional and complex, the contact distance of integrated circuit components will be reduced, and the speed of signal transmission will be relatively improved. The following will be the increase in the number of wiring, and the local reduction in the length of inter-point wiring, which requires the application of high-density wiring configuration and micropore technology to achieve the purpose. Wiring and bridging are basically difficult to achieve for single-sided and double-sided boards, so the circuit board will tend to be multi-layer, and due to the continuous increase of signal lines, more power layers and ground layers are necessary means of design, all of which make multi-layer printed circuit board more common.
For the electrical requirements of high-speed signals, the circuit board must provide impedance control with AC characteristics, high-frequency transmission capability, and reduce unnecessary radiation (EMI). With the structure of Stripline and Microstrip, multi-layer design becomes necessary. In order to reduce the quality problem of signal transmission, insulating materials with low dielectric coefficient and low attenuation rate will be used. In order to cooperate with the miniaturization and array of electronic components, the density of circuit boards will also be continuously increased to meet the demand. The emergence of BGA (BallGrid Array), CSP (Chip Scale Package), DCA (Direct ChipAttachment) and other group parts assembly methods has pushed the printed circuit board to an unprecedented high-density level.
The holes with diameter less than 150um are called microvias in the industry. The circuit made by using the geometric structure technology of this microhole can improve the efficiency of assembly, space utilization, and so on. At the same time, it is also necessary for the miniaturization of electronic products.
For circuit board products with this structure, the industry has used many different names to refer to such circuit boards. For example, European and American companies used to call this kind of product SBU (Sequence Build UpProcess), which is generally translated as 'Sequence Build UpProcess', because the process they produced is in a sequential way. As for Japanese manufacturers, because the pore structure produced by such products is much smaller than the previous ones, the manufacturing technology of such products is called MVP (Micro Via Process), which is generally translated as 'Micro Via Process'. Some people also call this kind of circuit board BUM (Build Up Multilayer Board) because the traditional multilayer board is called MLB (Multilayer Board), which is generally translated as 'multiplayer board'.
11、 Manufacturing
1. Assembly
PCB design is completed because the PCB shape is too small to meet the production process requirements, or a product is composed of several PCBs, so it is necessary to assemble several small boards into a large board with an area meeting the production requirements, or to assemble multiple PCBs used in a product to facilitate production and installation. The former is similar to the stamp board, which can not only meet the PCB production process conditions, but also facilitate the installation of components. It is very convenient to separate them when using; The latter is to assemble several sets of PCB boards of a product together, which is convenient for production, and also convenient for a complete set of products.
2. Data generation
The basis of PCB production is film version. When making a film version in the early stage, it is necessary to make a film base map first, and then use the base map for photography or reproduction. The accuracy of the base drawing must be consistent with that required by the printed board, and compensation for the deviation caused by the production process should be considered. The base drawing can be provided by the customer or produced by the manufacturer, but both parties should cooperate and negotiate closely to make it not only meet the user's requirements, but also adapt to the production conditions. If the user provides the base drawing, the manufacturer shall inspect and approve the base drawing, and the user can evaluate and approve the original version or the first printed board product. The production methods of base map include manual drawing, mapping and CAD drawing. With the development of computer technology, printed board CAD technology has made great progress, and the level of printed board production technology has also been rapidly improved in the direction of multi-layer, thin wire, small aperture, and high density. The original film plate-making process can no longer meet the design needs of printed boards, so the photo-drawing technology has emerged. The photo plotter can directly send the PCB graphics data file designed by CAD into the computer system of the photo plotter, and control the photo plotter to draw graphics directly on the negative film using light. Then the film version is obtained by developing and fixing. The printed board film base plate made by photo-drawing technology has high speed, high accuracy and good quality, and avoids possible human errors in manual mapping or drawing the base map, greatly improves the work efficiency and shortens the production cycle of the printed board. The laser photo-drawing machine can complete the work that many people have been able to complete for a long time in a very short time, and its thin wire and high-density base plate are also incomparable to manual operation. According to the different structures of laser photoplotters, they can be divided into flat plate type, internal drum type and external drum type. The standard data format used by photo plotter is Gerber-RS274 format, which is also the standard data format in the PCB design and production industry. The name of Gerber format is quoted from Gerber Company of the United States, a pioneer in the design and production of photo-drawing machines. The generation of optical drawing data is to convert the design data generated by CAD software into optical drawing data (mostly Gerber data), which is modified and edited by the CAM system, and complete the optical drawing pretreatment (mosaic, image, etc.), so as to meet the requirements of the production process of printed boards. Then, the processed data is sent to the photo plotter, which is converted into raster data by the Raster image data processor of the photo plotter. This raster data is sent to the laser photo plotter through a high power fast compression and reduction algorithm to complete photo painting.
3. Photo data format
The photo-drawing data format is developed based on the Gerber data format of the vector photo-drawing machine, and has expanded the data format of the vector photo-drawing machine, and is compatible with the HPGL HP plotter format, Autocad DXF, TIFF and other special and general graphics data formats. Some CAD and CAM developers have also extended Gerber data.
Here is a brief introduction to Gerber data. The official name of Gerber data is Gerber RS-274 format. Each symbol on the code disk of the vector photo plotter has a corresponding D code (D-CODE) in the Gerber data. In this way, the photo plotter can control and select the code disk through the D code to draw the corresponding graphics. List the shape, size and size of the corresponding symbols of D code and D code to get a D code table. This D-code table will become a bridge from CAD design to photo-drawing machine using this data for photo-drawing. When providing Gerber photo data, users must provide corresponding D code table. In this way, the photoplotter can determine which symbol disk should be selected for exposure according to the D code table, so as to draw the correct figure. In a D code table, it should generally include D code, the shape and size of the code disk corresponding to each D code, and the exposure mode of the code disk.
12、 Function test
More intensive PCBs, higher bus speeds and analog RF circuits all pose unprecedented challenges to testing. Functional testing in this environment requires careful design, thoughtful test methods and appropriate tools to provide reliable test results.
When dealing with fixture suppliers, we should keep these problems in mind and also think about where the products will be manufactured. This is a place that many test engineers will ignore. For example, we assume that the test engineer is located in California, while the product is manufactured in Thailand. The test engineer will think that the product needs expensive automated fixtures, because the price of the plant in California is high, and the tester is required to be as few as possible, and the automated fixture is also required to reduce the employment of high-tech and high-wage operators. But in Thailand, neither of these two problems exists. It is cheaper to solve these problems manually, because the labor cost here is very low, the land price is also very cheap, and large factories are not a problem. Therefore, sometimes first-class equipment may not be popular in some countries.
1. Technical level
In high-density UUT, if calibration or diagnosis is required, it is likely to need to be detected manually. This is because the contact of the needle bed is limited and the test is faster (using the probe to test the UUT can quickly collect data rather than feedback the information to the edge connector), so the operator is required to detect the test point on the UUT. Wherever it is, make sure that the test points are clearly marked.
Probe types and common operators should also pay attention to the following issues:
1. Is the probe larger than the test point? Is the probe in danger of short-circuiting several test points and damaging UUT? Is there electric shock hazard to operators?
2. Can each operator quickly find out the test points and check them? Is the test point large and easy to identify?
3. How long does it take for the operator to press the probe on the test point to get an accurate reading? If the time is too long, there will be some problems in the small test area, such as the operator's hand will slide due to the test time is too long, so it is recommended to expand the test area to avoid this problem.
After considering the above problems, the test engineer should re-evaluate the type of test probe, modify the test file to better identify the location of test points, or even change the requirements for operators.
2. Automatic detection
In some cases, automatic detection will be required. For example, when it is difficult to detect the PCB manually, or the test speed is greatly reduced due to the limitation of the operator's technical level, the automatic method should be considered.
Automatic detection can eliminate human error, reduce the possibility of short circuit at several test points, and speed up the test operation. However, we should know that there may be some limitations in automatic detection, which vary according to the design of the supplier,
include:
(1) , size of UUT
(2) , number of synchronous probes
(3) How close are the two test points?
(4) , test the positioning accuracy of the probe
(5) Can the system detect both sides of UUT?
(6) , how fast does the probe move to the next test point?
(7) What is the actual interval required by the probe system? (Generally speaking, it is larger than offline function test system)
Automatic detection usually does not contact other test points with the needle bed clamp, and generally it is slower than the production line, so two steps may need to be taken: if the detector is only used for diagnosis, the traditional functional test system can be considered on the production line, and the detector can be placed on the production line as a diagnostic system; If the purpose of the detector is UUT calibration, the only real solution is to use multiple systems, which is much faster than manual operation.
How to integrate into the production line is also a key issue that must be studied. Is there room for the production line? Can the system be connected to the conveyor belt? Fortunately, many new detection systems are compatible with the SMEMA standard, so they can work in an online environment.
3. Boundary scan
This technology should be discussed as early as the product design stage, because it requires special components to perform this task. In UUT, which is mainly based on digital circuits, devices with IEEE 1194 (boundary scan) support can be purchased, so that most diagnostic problems can be solved with little or no detection. Boundary scan will reduce the overall functionality of the UUT, because it will increase the area of each compatible device (each chip adds 4 to 5 pins and some lines), so the principle of selecting this technology is that the cost should be able to improve the diagnosis results. It should be remembered that boundary scan can be used to program flash memory and PLD devices on UUT, which further increases the reason for selecting this test method.
How to deal with a limited design?
If the UUT design has been completed and determined, the options are limited at this time. Of course, you can also ask for modification in the next revision or new product, but process improvement always takes some time, and you still need to deal with the current situation.