Speaking of PCB board, many friends will think that it can be seen everywhere around us, from all household appliances, all kinds of accessories in the computer, to all kinds of digital products, almost all electronic products will use PCB board, then what is PCB board
A PCB board is a PrintedCircuitBlock, a printed circuit board for electronic components to sit in, and a circuit board with circuits. The copper-plated substrate is printed with anti-corrosion lines and etched to flush out the lines.
PCB board can be divided into single board, double board and multilayer board. All kinds of electronic components are integrated into the PCB board. In the basic single-layer PCB, the parts are concentrated on one side and the wires are concentrated on the other side. In this way, we need to make holes in the board so that the pins can go through the board to the other side, so the pins of the parts are welded on the other side. Because of this, the PCB’s front and back are called ComponentSide and SolderSide respectively.
A double plate can be thought of as two single plates bonded together, with electronic components and wiring on both sides. Sometimes it is necessary to connect a single wire from one side to the other side of the board through the guide hole (via). A guide hole is a small hole in a PCB, filled with or coated with metal, that can be connected to two wires. Now many computer motherboards are using 4 or even 6 layers of PCB board, and graphics CARDS are generally using 6 layers of PCB board, many high-end graphics CARDS like nVIDIAGeForce4Ti series use 8 layers of PCB board, this is the so-called multi-layer PCB board. In multi-layer PCB, the problem of connecting the lines between the layers can also be realized through the guide hole.
Since the PCB is multi-layer, sometimes the guide hole does not need to go through the entire PCB. This guide hole is called Buriedvias or Blindvias because they only go through a few layers. The blind hole connects several layers of inner PCB with the surface PCB without penetrating the whole board. The buried hole only connects to the PCB inside, so it cannot be seen from the surface. In a multilayer PCB, the entire layer is directly connected to the ground wire and power supply. So we classify each layer as Signal, Power or Ground. If the parts on the PCB need different power supplies, the PCB will usually have more than two power and wire layers. The more layers of PCB used, the higher the cost. Of course, the use of more layers of PCB board will help to provide signal stability.
The production process of professional PCB board is quite complicated, take 4-layer PCB board as an example. The PCB of mainboard is 4 layer mostly. In the manufacturing process, the middle two layers are rolled, cut, etched and oxidized respectively. The four layers are the component surface, power layer, stratum and solder laminate respectively. Then put the four layers together and roll them into a PCB. Then the holes were punched and made. After washing, the outside two layers of the line printed, copper, etching, testing, solder layer, screen printing. Finally, the whole PCB(including many motherboards) is stamped into a PCB of motherboards, which is then vacuum-packed after passing the test.
If the copper skin is not well coated in PCB manufacturing process, there will be the phenomenon of poor adhesion, easy to imply short circuit or capacitor effect (easy to produce interference). The holes in PCB must be paid attention to. If the hole is drilled to one side, rather than right in the middle, it will cause uneven matching or easy contact with the power layer or formation in the middle, resulting in potential short circuit or poor grounding.
Copper wiring process
The first step in manufacturing is to establish the wiring between parts online. We use negative transfer to show the working film on the metal conductor. The trick is to cover the entire surface with a thin layer of copper foil and remove the excess. Additional transfer printing is another less used method, which is to apply copper wire only where it is needed, but we will not talk about it here. Positive photoresists are made of photosensitive agents that dissolve under illumination. There are many ways to treat a photoresist on a copper surface, but the most common way is to heat it up and roll it over a surface containing the photoresist. It can also be sprayed in a liquid form, but the dry film provides higher resolution and can be used to make thin wires. The hood is just a template for the PCB layer in manufacturing.
An overlaid hood prevents some areas from being exposed to UV light before the photoresist on the PCB is exposed. These areas, which are covered by photoresist, will become wiring. Other bare copper parts to be etched after photoresist development. The etching process can dip the plate into the etching solvent or spray the solvent onto the plate. Generally used as etching solvent using ferric chloride and so on. The remaining photoresist is removed after etching.
1. Wiring width and current
General width should not be less than 0.2mm(8mil)
On high-density and high-precision PCB, the spacing and line width are generally 0.3mm(12mil).
When the thickness of copper foil is about 50um, the wire width is 1 ~ 1.5mm (60mil) = 2A
Common ground 80mil, more attention should be paid to applications with microprocessors.
2. How high frequency is high speed board?
When the signal rises/falls along the time
For digital circuits, the key is to see how steep the edge of the signal is, that is, how long it takes for the signal to rise and fall,
According to a very classic book “High Speed Digtal Design> theory, signal from 10% to 90% of the time is less than 6 times of wire delay, is high-speed signal
3. Stack and layer PCB boards
Four layers have the following lamination sequence. The advantages and disadvantages of different layers are described below:
Note: S1 signal wiring layer 1, S2 signal wiring layer 2; GND layer POWER supply layer
The first kind of case, ought to be the best one kind of case in 4 layer board. Because the outer layer is a layer, EMI shielding effect, and the power layer with the layer is also reliable very close, making the power source internal resistance is small, the best fruit. But the first case can not be used when the board density is relatively large. Because there is no way to guarantee the integrity of the first layer, the second layer will get worse. In addition, this structure can not be used in the case of large power consumption.
The second case, is one of the most commonly used way. In terms of the structure of the board, it is not suitable for high-speed digital circuit design. Because in this configuration, it is not easy to maintain low power impedance. In a 2 mm plate, for example: requirements Z0 = 50 ohm. The line width for 8 mil. Copper foil for 35 ц m thick. In this way, the signal layer is 0.14mm in the middle of the formation. The formation and power layer are 1.58mm. This greatly increases the internal resistance of the power supply. In this structure, because the radiation is spatially oriented, shielding plates are needed to reduce EMI.
In the third case, the signal line on layer S1 is of the best quality. S2. EMI has shielding effect. But the power supply impedance is large. This board can be used for full power consumption and the board is the source of interference or close to the source of interference.
4. Impedance matching
The amplitude of the reflected voltage signal is determined by the rho s of the source end and rho p of the load
Rho = (RL – Z0)/L (RL Z0) and rho S = (RS – Z0)/(RS Z0)
In the above equation, RL=Z0 SCP =0 of the load reflection coefficient. If RS=Z0 rho rho S=0.
Due to the common transmission line impedance Z0 usually should meet the requirements of 50 Ω Ω 50 or so, while the load impedance is usually in the thousands of ohm to a few thousand ohms. Therefore, it is difficult to achieve impedance matching at the load end. However, since the impedance at the source end is usually small, it is approximately a few dozen ohms.
It is therefore much easier to achieve impedance matching at the source end. If the load and resistance, resistance can absorb part of the signal against transmission (my understanding). When choosing a TTL/CMOS standard 24 ma drive current, the output impedance is roughly 13 Ω. If the transmission line impedance Z0 = 50 Ω, then you should add a matching 33 Ω source side resistance. 13 Ω 33 Ω = 46 Ω (similar to 50 Ω, weak under-damped contributes to signal the setup time)
When other transmission criteria and drive current are selected, the matching impedance varies. In high speed logic and circuit design, it is recommended to add source matching resistance to some key signals, such as clock and control signal.
In this way, the signal will be reflected back from the load, because the source impedance matches, the reflected signal will not be reflected back.
5. Precautions for the layout of power lines and ground wires
Power line as short as possible, take a straight line, and the best way to go tree, not a circle
Ground loop problem: for digital circuits, the earth circulation caused by the earth loop is tens of millivolts, while the TTL anti-interference threshold is 1.2v, and CMOS circuit can even reach 1/2 of the power supply voltage, which means that the earth circulation will not cause adverse effects on the work of the circuit at all. On the contrary, if the ground wire is not closed, the problem will be greater, because the pulse power current generated by the digital circuit during work will cause the potential imbalance of each point. For example, the ground current of 74LS161 measured by me is 1.2a when reversed (the pulse width of ground current is 7ns measured by 2Gsps oscilloscope).
Under the impact of large pulse current, if the distribution of dendritic ground wire (line width: 25mil) is adopted, the potential difference between each point of ground wire will reach the level of 100 millivolts. After adopting the ground loop, the pulse current will be distributed to each point of the ground, greatly reducing the possibility of interfering with the circuit. Using closed ground, the maximum instantaneous potential difference of ground of each device measured is one half to one fifth of that of closed ground. Of course, the measured data of circuit boards with different densities and speeds vary greatly. By that I mean about the same level as the Z80 Demo board attached to Protel 99SE. For the low-frequency analog circuit, I think the power frequency interference after the earth wire is closed is induced from space, which can not be simulated and calculated anyway.
If the earth wire is not closed, there is no earth eddy current, beckhamtao says, “but the power-frequency induced voltage of the earth wire opening ring is higher.” What is the rationale? Two examples, 7 years ago, I took someone else’s A project, precision pressure gauge, with 14 A/D converter, but only 11 measured precision effectively, as, on the ground with 15 MVP – p power frequency interference, the solution is to put the PCB loop cutting simulation, the front ground of sensor to the A/D dendritic distribution with A jumper wire and then mass production models of PCB again according to the fly line production line, has not yet appeared.
In the second example, a friend loves fever and DIY an amplifier, but the output always has an ac sound. I suggest him to cut open the ground circuit to solve the problem. Later, the man consulted dozens of PCB diagrams of “hi-fi machines” and confirmed that none of the machines used the ground loop in the simulation part.
6. Principles of PCB design and anti-interference measures
Printed circuit board (PCB) is the support of circuit elements and devices in electronic products. It provides the electrical connection between circuit elements and devices. With the rapid development of electrical technology, the density of PGB is getting higher and higher. The ability of PCB design to resist interference is very important. Therefore, in PCB design, the general principles of PCB design must be followed and the requirements of anti-interference design should be met.
General principles of PCB design
In order to achieve the best performance of electronic circuits, it is very important to lay out the components and wires. In order to design PCB with good quality and low cost, the following general principles should be followed:
1. The layout
First, consider PCB size. When PCB size is too large, printed line is longer, impedance increases, noise resistance decreases, and cost increases. Too small, heat dissipation is not good, and adjacent lines easily disturbed. After determining the PCB size, determine the location of the special components. Finally, according to the functional units of the circuit, the layout of all components of the circuit.
The following principles should be followed when determining the location of a particular element:
(1) shorten the connection between high-frequency components as much as possible, and try to reduce their distribution parameters and electromagnetic interference between each other. Easily disturbed components should not be too close to each other, the input and output components should be as far away as possible.
(2) some components or wires may have a higher potential difference, should increase the distance between them, so as not to discharge lead to accidental short circuit. Components with high voltage should be arranged as far as possible in the debugging hand is not easy to reach the place.
(3) components weighing more than 15g shall be fixed with brackets and then welded. Those large and heavy, heat more components, should not be installed in the printed board, and should be installed in the machine chassis bottom plate, and should consider the heat dissipation problem. Heat sensitive elements should be kept away from heating elements.
(4) for the layout of potentiometer, adjustable inductance coil, variable capacitor, micro switch and other adjustable elements, the structural requirements of the whole machine should be considered. If the machine adjustment, should be placed on the PCB upper convenient adjustment place; If the outside adjustment, its position and adjustment knob in the chassis panel position.
(5) the position occupied by the printing spanking hole and the fixing bracket should be set aside.
According to the functional unit of the circuit, the layout of all components of the circuit shall conform to the following principles:
(1) arrange the position of each functional circuit unit according to the circuit flow, so as to facilitate the distribution of signals and keep the signals in the same direction as far as possible.
(2) take the core component of each functional circuit as the center, and arrange the layout around it. Components should be arranged evenly, neatly and compactly on the PCB. Lead and connection between components should be reduced and shortened as much as possible.
(3) for circuits working at high frequency, distribution parameters between components should be considered. General circuits should be as far as possible to make components parallel arrangement. In this way, not only beautiful, but also easy to assemble and weld. Easy to mass production.
(4) the components located at the edge of the circuit board are generally no less than 2mm away from the edge of the circuit board. The best shape of circuit board is rectangle. The ratio of length to width is 3:2 to 4:3. When the circuit board size is greater than 200x150mm, the mechanical strength of the circuit board shall be taken into account.
2. The wiring
Wiring principles are as follows:
(1) the wires used at the input and output terminals should avoid adjacent parallelism as far as possible. It is better to add ground wires between wires to avoid feedback coupling.
(2) the minimum width of printed conductor is mainly determined by the adhesive strength between conductor and insulated substrate and the current value flowing through them. When the thickness of copper foil is 0.05mm and the width is 1 ~ 15mm, the current passing through 2A will not be higher than 3℃, so the wire width is 1.5mm