[Repost] PCB design considerations and experience
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Speaking of PCB boards, many friends will think of it everywhere around us, from all household appliances, various accessories in computers, to various digital products, almost all electronic products will use PCB boards, so what exactly is a PCB board? PCB board is Printed Circuit Block, that is, a printed circuit board with circuits for electronic components to be inserted. The anti-corrosion circuit is printed on the copper-plated base by printing, and then etched to wash out the circuit. PCB boards can be divided into single-layer boards, double-layer boards and multi-layer boards. Various electronic components are integrated on PCB boards. On the most basic single-layer PCB, the components 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 pass through the board to the other side, so the pins of the components are soldered on the other side. Because of this, the front and back sides of such PCBs are called the component side (ComponentSide) and the solder side (SolderSide) respectively. A double-layer board can be regarded as two single-layer boards bonded together, with electronic components and traces on both sides of the board. Sometimes you need to connect a single wire on one side to the other side of the board, and this requires a via. A via is a small hole on the PCB that is filled or coated with metal, and it can connect to the wires on both sides. Now many computer motherboards are using 4-layer or even 6-layer PCBs, and graphics cards generally use 6-layer PCBs. Many high-end graphics cards like the nVIDIAGeForce4Ti series use 8-layer PCBs, which are so-called multi-layer PCBs. On multi-layer PCBs, there will also be problems connecting the lines between the layers, which can also be achieved through vias. Because it is a multi-layer PCB, sometimes the guide hole does not need to penetrate the entire PCB board. Such guide holes are called buried vias and blind vias because they only penetrate a few layers. Blind vias connect several layers of internal PCB to the surface PCB without penetrating the entire board. Buried vias only connect the internal PCB, so they cannot be seen from the surface alone. In a multi-layer PCB, the entire layer is directly connected to the ground and power supply. So we classify each layer as a signal layer, a power layer, or a ground layer. If the parts on the PCB require different power supplies, this type of PCB usually has more than two layers of power and wire layers. The more layers of PCB boards used, the higher the cost. Of course, using more layers of PCB boards is very helpful in providing signal stability. The production process of professional PCB boards is quite complicated. Take a 4-layer PCB board as an example. The PCB of the motherboard is mostly 4-layered. During manufacturing, the two middle layers are first rolled, cut, etched, and oxidized and electroplated. The four layers are the component surface, power layer, ground layer, and solder layer. Then put these four layers together and roll them into a PCB for the motherboard. Then punch and make holes. After cleaning, print the circuits of the two outer layers, apply copper, etch, test, solder mask, and silk screen. Finally, the entire PCB (including many motherboards) is punched into pieces of motherboard PCBs, and then vacuum packed after passing the test. If the copper foil is not well applied during the PCB manufacturing process, it will not stick firmly, which may lead to short circuit or capacitance effect (prone to interference). The vias on the PCB must also be paid attention to. If the hole is not in the middle, but biased to one side, it will produce uneven matching, or it will easily contact the power layer or ground layer in the middle, resulting in potential short circuit or poor grounding. Copper wire routing process The first step in fabrication is to create the wiring for the connections between the parts. We use negative transfer to transfer the work to the metal conductor. This technique involves laying a thin layer of copper foil over the entire surface and removing the excess. Additive transfer is another lesser-used method that only applies copper where it is needed, but we won't discuss it here. Positive photoresist is made of a photosensitive agent that dissolves when illuminated. There are many ways to treat the photoresist on the copper surface, but the most common method is to heat it and roll it over the surface containing the photoresist. It can also be sprayed on as a liquid, but dry film methods provide higher resolution and can also produce thinner wires. The light mask is just a template for the PCB layer being manufactured. Before the photoresist on the PCB is exposed to UV light, the light shield covering it can prevent some areas of the photoresist from being exposed. These areas covered by the photoresist will become wiring. After the photoresist is developed, the other bare copper parts to be etched. The etching process can immerse the board in an etching solvent or spray the solvent on the board. Generally, ferric chloride is used as an etching solvent. After the etching is completed, the remaining photoresist is removed. 1. Wiring width and current The general width should not be less than 0.2mm(8mil) On high-density and high-precision PCBs, the spacing and line width are generally 0.3mm(12mil). When the thickness of the copper foil is about 50um, the wire width is 1~1.5mm (60mil) = 2A The common ground is generally 80mil, and it is more important for applications with microprocessors. 2. How high a frequency is considered a high-speed board? When the rising/falling edge time of the signal is < 3~6 times the signal transmission time, it is considered a high-speed signal. [color=rgb(51, 51, For digital circuits, the key is to look at the steepness of the signal edge, that is, the rise and fall time of the signal. According to the theory of a very classic book "High Speed Digital Design", if the time for the signal to rise from 10% to 90% is less than 6 times the wire delay, it is a high-speed signal! That is, even an 8KHz square wave signal, as long as the edge is steep enough, it is also a high-speed signal, and the transmission line theory needs to be used when wiring. "]3. Stacking and layering of PCB boards There are several stacking sequences for four-layer boards. The advantages and disadvantages of various stacking sequences are explained below: The first case GND S1+POWER S2+POWER GND The second case SIG1 GND POWER SIG2 The third situation GND S1 S2 POWER Note: S1 signal wiring layer, S2 signal wiring layer 2; GND Ground layer POWER Power layer The first case should be the best case among the four-layer boards. Because the outer layer is the ground layer, it has a shielding effect on EMI, and the power layer and the ground layer are also very close, so that the internal resistance of the power supply is small and the best effect is achieved. However, the first case cannot be used when the density of the board is relatively large. Because in this way, the integrity of the first layer of ground cannot be guaranteed, so the signal of the second layer will become worse. In addition, this structure cannot be used when the power consumption of the whole board is relatively large. The second case is the most common method we usually use. From the perspective of board structure, it is not suitable for high-speed digital circuit design. Because in this structure, it is not easy to maintain low power supply impedance. Take a 2mm board as an example: Z0=50ohm is required. The line width is 8mil. The copper foil thickness is 35цm. In this way, the distance between the signal layer and the ground layer is 0.14mm. The distance between the ground layer and the power layer is 1.58mm. This greatly increases the internal resistance of the power supply. In this structure, since the radiation is toward space, a shielding plate is required to reduce EMI. In the third case, the signal line quality on the S1 layer is the best. S2 is second. It has a shielding effect on EMI. But the power supply impedance is large. This board can be used when the power consumption of the entire board is high and the board is an interference source or is close to an interference source. 4. Impedance Matching The amplitude of the reflected voltage signal is determined by the source reflection coefficient ρs and the load reflection coefficient ρL. ρL = (RL - Z0) / (RL + Z0) and ρS = (RS - Z0) / (RS + Z0) 51)]In the above formula, if RL=Z0, the load reflection coefficient ρL=0. If RS=Z0, the source reflection coefficient ρS=0. Since the ordinary transmission line impedance Z0 should usually meet the requirement of 50Ω, and the load impedance is usually in the range of several thousand ohms to tens of thousands of ohms. Therefore, it is difficult to achieve impedance matching at the load end. However, since the signal source end (output) impedance is usually relatively small, roughly a dozen ohms. Therefore, it is much easier to achieve impedance matching at the source end. If a resistor is connected in parallel at the load end, the resistor will absorb part of the signal and be detrimental to transmission (my understanding). When the TTL/CMOS standard 24mA drive current is selected, the output impedance is approximately 13Ω. If the transmission line impedance Z0=50Ω, then a 33Ω source-end matching resistor should be added. 13Ω+33Ω=46Ω (approximately 50Ω, weak underdamping helps the setup time of the signal) When other transmission standards and drive currents are selected, the matching impedance will be different. In high-speed logic and circuit design, for some key signals such as clocks and control signals, we recommend adding source matching resistors. In this way, the signal will be reflected back from the load end, because the source impedance is matched, and the reflected signal will not be reflected back again. 5. Notes on the layout of power and ground wires The power wire should be as short as possible and run in a straight line. It is best to run in a tree shape rather than a loop. "]Ground loop problem: For digital circuits, the ground loop current caused by the ground loop is only at the level of tens of millivolts, while the anti-interference threshold of TTL is 1.2V, and the CMOS circuit can reach 1/2 of the power supply voltage, which means that the ground loop current will not have any adverse effect on the operation of the circuit. On the contrary, if the ground wire is not closed, the problem will be even greater, because the pulse power supply current generated by the digital circuit when it is working will cause the ground potential of each point to be unbalanced. For example, I have measured that the ground current of 74LS161 is 1.2A when it is reversed (measured with a 2Gsps oscilloscope, the ground current pulse width is 7ns). Under the impact of large pulse current, if a branch-like ground wire (line width 25mil) is used for distribution, the potential difference between each point between the ground wires will reach the level of hundreds of millivolts. After adopting the ground wire loop, the pulse current will be spread to various points of the ground wire, greatly reducing the possibility of interfering with the circuit. With the closed ground wire, the maximum instantaneous potential difference of the ground wire of each device is measured to be one-half to one-fifth of that of the open ground wire. Of course, the measured data of circuit boards with different densities and speeds vary greatly. What I said above refers to the level of the Z80 Demo board attached to Protel 99SE. For low-frequency analog circuits, I think the power frequency interference after the ground wire is closed is induced from space, which cannot be simulated or calculated in any way. If the ground wire is not closed, no ground wire eddy current will be generated. Beckhamtao said, "But the power frequency induced voltage will be greater when the ground wire is open. What is the theoretical basis of "? Let me give you two examples. Seven years ago, I took over a project from someone else, a precision pressure gauge, which used a 14-bit A/D converter, but the actual measurement showed only 11-bit effective accuracy. After investigation, it was found that there was 15mVp-p power frequency interference on the ground line. The solution was to split the analog ground loop of the PCB and use flying wires to make branch-like distribution of the ground line from the front-end sensor to the A/D. Later, the mass-produced model PCB was re-produced according to the routing of the flying wire, and no problems have occurred so far. For the second example, a friend of mine loves audiophiles and built a DIY amplifier. However, there is always hum in the output. I suggested that he cut the ground loop and the problem was solved. Afterwards, this guy checked dozens of PCB diagrams of "famous Hi-Fi machines" and confirmed that none of them used ground loops in the analog part. 6. PCB design principles and anti-interference measures Printed circuit board (PCB) is the support for circuit components and devices in electronic products. It provides electrical connections between circuit components and devices. With the rapid development of electronic technology, the density of PCB is getting higher and higher. The quality of PCB design has a great impact on the anti-interference ability. Therefore, when designing PCB, the general principles of PCB design must be followed and the requirements of anti-interference design must be met. General principles of PCB design To achieve the best performance of electronic circuits, the layout of components and wires is very important. In order to design a high-quality, low-cost PCB, the following general principles should be followed: 1. Layout First, consider the size of the PCB. When the PCB size is too large, the printed lines are long, the impedance increases, the anti-noise ability decreases, and the cost also increases; if it is too small, the heat dissipation is not good, and the adjacent lines are susceptible to interference. After determining the PCB size, determine the location of special components. Finally, layout all the components of the circuit according to the functional units of the circuit. The following principles should be followed when determining the location of special components: (1) Keep the connections between high-frequency components as short as possible, and try to reduce their distributed parameters and mutual electromagnetic interference. Components susceptible to interference should not be placed too close to each other, and input and output components should be kept as far apart as possible. (2) There may be a high potential difference between some components or wires. The distance between them should be increased to avoid discharge and cause accidental short circuit. Components with high voltage should be placed in places that are not easily touched by hands during debugging. (3) Components weighing more than 15g should be fixed with a bracket and then soldered. Large, heavy and heat-generating components should not be installed on printed circuit boards, but should be installed on the chassis bottom plate of the whole machine, and heat dissipation should be considered. Thermistors should be kept away from heat-generating components. (4) The layout of adjustable components such as potentiometers, adjustable inductors, variable capacitors, and micro switches should take into account the structural requirements of the entire machine. If they are adjusted inside the machine, they should be placed in a convenient place on the printed circuit board for adjustment; if they are adjusted outside the machine, their position should be consistent with the position of the adjustment knob on the chassis panel. (5) Space should be reserved for the positioning holes of the printed circuit board and the space occupied by the fixing bracket. When laying out all the components of a circuit according to its functional units, the following principles must be followed: (1) Arrange the positions of each functional circuit unit according to the circuit flow, so that the layout facilitates signal flow and keeps the signal in the same direction as much as possible. (2) Take the core component of each functional circuit as the center and arrange the layout around it. The components should be arranged evenly, neatly and compactly on the PCB. Try to reduce and shorten the leads and connections between the components. (3) For circuits working at high frequencies, the distribution parameters between components should be considered. In general, the components of the circuit should be arranged in parallel as much as possible. This will not only be beautiful, but also easy to assemble and solder, and easy to mass produce. (4) Components located at the edge of the circuit board should generally be no less than 2mm away from the edge of the circuit board. The best shape for a circuit board is a rectangle. The aspect ratio is 3:2 to 4:3. When the circuit board surface size is larger than 200x150mm, the mechanical strength of the circuit board should be considered. 2. Wiring The wiring principles are as follows: (1) The wires used for input and output should be kept away from being parallel to each other. It is best to add a ground wire between the wires to avoid feedback coupling. (2) The minimum width of the printed conductor is mainly determined by the adhesion strength between the conductor and the insulating substrate and the current flowing through them. When the copper foil thickness is 0.05mm and the width is 1 to 15mm, the temperature will not be higher than 3℃ when a current of 2A passes through it. Therefore, a conductor width of 1.5mm can meet the requirements. For integrated circuits, especially digital circuits, a conductor width of 0.02 to 0.3mm is usually selected. Of course, as long as it is allowed, wide wires should be used as much as possible, especially power lines and ground lines. The minimum spacing of the conductors is mainly determined by the insulation resistance and breakdown voltage between the wires in the worst case. For integrated circuits, especially digital circuits, the spacing can be as small as 5 to 8mm as long as the process allows. "](3) The bends of printed conductors are generally arc-shaped, while right angles or included angles will affect the electrical performance in high-frequency circuits. In addition, try to avoid using large areas of copper foil, otherwise, when heated for a long time, the copper foil is prone to expansion and falling off. When large areas of copper foil must be used, it is best to use a grid shape. This is conducive to removing the volatile gases generated by the heat of the adhesive between the copper foil and the substrate. 3. Soldering pads The center hole of the soldering pad should be slightly larger than the diameter of the device lead. A soldering pad that is too large is prone to cold soldering. The outer diameter D of the pad is generally not less than (d+1.2)mm, where d is the lead hole diameter. For high-density digital circuits, the minimum diameter of the pad can be (d+1.0)mm. PCB and circuit anti-interference measures The anti-interference design of printed circuit boards is closely related to the specific circuits. Here we will only explain some common measures for PCB anti-interference design. 1. Power line design According to the current of the printed circuit board, try to increase the width of the power line and reduce the loop resistance. At the same time, make the direction of the power line and ground line consistent with the direction of data transmission, which will help enhance the anti-noise ability. 2. Ground wire design The principles of ground wire design are: (1)Digital ground and analog ground are separated. If there are both logic circuits and linear circuits on the circuit board, they should be separated as much as possible. Low-frequency circuit ground should be connected in parallel at a single point as much as possible. If the actual wiring is difficult, it can be connected in series and then in parallel. High-frequency circuits should be connected in series at multiple points. The ground wire should be short and loose. A large area grid-shaped ground foil should be used around high-frequency components. (2)The ground wire should be as thick as possible. If the ground wire is made of very thin wire, the ground potential will change with the change of current, which will reduce the noise resistance performance. Therefore, the ground wire should be thickened so that it can pass three times the allowable current on the printed circuit board. If possible, the ground wire should be more than 2~3mm. (3) The grounding wire forms a closed loop. For printed circuit boards consisting only of digital circuits, the grounding circuits are usually arranged in a cluster loop to improve noise resistance. 3. Decoupling capacitor configuration One of the common practices in PCB design is to configure appropriate decoupling capacitors at various key locations on the printed circuit board. The general configuration principle of decoupling capacitors is: (1) Connect a 10~100uf electrolytic capacitor across the power input. If possible, it is better to connect a 100uF or higher capacitor. (2) In principle, each integrated circuit chip should be equipped with a 0.01pF ceramic capacitor. If there is not enough space on the printed circuit board, a 1~10pF capacitor can be arranged for every 4~8 chips. (3)For devices with weak noise immunity and large power supply changes when turned off, such as RAM and ROM storage devices, a decoupling capacitor should be directly connected between the power line and ground line of the chip. (4)The capacitor leads should not be too long, especially high-frequency bypass capacitors should not have leads. In addition, the following two points should be noted: (1) When there are contactors, relays, buttons and other components on the printed circuit board, large spark discharges will be generated when they are operated. The RC circuit shown in the attached figure must be used to absorb the discharge current. Generally, R is 1 ~ 2K and C is 2.2 ~ 47UF. (2)CMOS input impedance is very high and susceptible to induction, so when in use, the unused ends should be grounded or connected to the positive power supply. 7. Design skills and key points for efficient automatic PCB routing Although today's EDA tools are very powerful, with the requirements for PCB size becoming smaller and smaller and the device density becoming higher and higher, PCB design is not easy. How to achieve a high PCB routing rate and shorten the design time? This article introduces the design skills and key points of PCB planning, layout and routing. Nowadays, the time for PCB design is getting shorter and shorter, the board space is getting smaller and smaller, the device density is getting higher and higher, the extremely demanding layout rules and large-sized components make the designer's work more difficult. In order to solve the design difficulties and speed up the product launch, many manufacturers now tend to use dedicated EDA tools to implement PCB design. However, dedicated EDA tools cannot produce ideal results, nor can they achieve 100% routing pass rate, and they are very messy, and usually take a lot of time to complete the remaining work.
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