PCB design considerations and experience
When it comes to PCB boards, many friends will think that it can be seen everywhere around us, from all household appliances, various accessories in computers, to various digital products, as long as they are electronic products almost all use PCB boards, so what exactly is it? What about the PCB board?
The PCB board is the PrintedCircuitBlock, the printed circuit board, for the placement of electronic components, and the base version of the circuit. By using a printing method, the copper-plated base plate is printed on the anti-corrosion circuit, and the circuit is etched and washed out.
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PCB boards can be divided into single-layer boards, double-layer boards and multi-layer boards. Various electronic components are integrated on the PCB. On the most basic single-layer PCB, the parts are concentrated on one side, and the wires are concentrated on the other side. In this case, 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 parts are soldered to the other side. Because of this, the front and back sides of such a PCB are called the component side (ComponentSide) and the solder side (SolderSide) respectively.
A double-layer board can be regarded as a composition of two single-layer boards that are relatively glued together, and there are electronic components and wiring on both sides of the board. Sometimes it is necessary to connect a single wire on one side to the other side of the board, which requires a via. A via is a small hole filled or coated with metal on the PCB, which can be connected with the wires on both sides. Many computer motherboards now use 4-layer or even 6-layer PCB boards, while graphics cards generally use 6-layer PCB boards. Many high-end graphics cards like the nVIDIAGeForce4Ti series use 8-layer PCB boards. This is the so-called multi-layer PCB board. The problem of connecting the lines between the various layers will also be encountered on a multilayer PCB, which can also be achieved through vias.
Because it is a multi-layer PCB board, sometimes the vias do not need to penetrate the entire PCB board. Such vias are called Buriedvias and Blindvias, because they only penetrate a few layers. Blind holes are to connect several layers of internal PCB to the surface PCB, without having to penetrate the entire board. Buried vias only connect to the internal PCB, so they cannot be seen from the surface. In a multilayer PCB, the entire layer is directly connected to the ground wire and the power supply. So we classify the layers as signal layer, power layer or 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. The more PCB layers used, the higher the cost. Of course, the use of more layers of PCB boards is very helpful to provide signal stability.
The professional PCB board production process is quite complicated, take a 4-layer PCB board as an example. The main board PCB is mostly 4-layer. When manufacturing, the two middle layers are rolled, cut, etched, and oxidized. The four layers are the component surface, power layer, ground layer, and solder pressure layer. Put these 4 layers together and roll them into a motherboard PCB. Then punch and make through holes. After cleaning, print, copper, etch, test, solder mask, silk screen on the outer two layers of circuits. Finally, the entire PCB (including many motherboards) is stamped into a motherboard PCB, and then vacuum packaged after passing the test.
If the copper skin is not well laid during the PCB manufacturing process, there will be loose bonding, which may easily imply short-circuit or capacitive effects (prone to interference). The vias on the PCB must also be paid attention to. If the hole is not in the middle, but to one side, uneven matching will occur, or it will be easy to contact the power layer or ground layer in the middle, which will cause potential short circuits or poor grounding factors.
Copper wiring process
The first step of production is to establish the wiring between the parts. We use negative film transfer method to show the working film on the metal conductor. This technique is to spread a thin layer of copper foil on the entire surface and eliminate the excess. Supplementary transfer is another method that less people use. It is a method of laying copper wires only where needed, but we won't talk about it here. Positive photoresist is made of sensitizer, which will dissolve under lighting. There are many ways to treat the photoresist on the copper surface, but the most common way is to heat it and roll it on the surface containing the photoresist. It can also be sprayed on the head in a liquid way, but the dry film type provides higher resolution and can also produce thinner wires. The hood is just a template for the PCB layer in manufacturing.
Before the photoresist on the PCB board is exposed to UV light, the light shield covering it can prevent the photoresist in some areas from being exposed. These areas covered by 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 the etching solvent or spray the solvent on the board. Generally used as an etching solvent, ferric chloride and the like are used. After the etching, the remaining photoresist is removed.
1. Wiring width and current
Generally the width should not be less than 0.2mm (8mil)
On high-density and high-precision PCBs, the pitch 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 area is generally 80mil, and more attention should be paid to applications with microprocessors.
2. How high is the frequency of a high-speed board?
When the signal's rising/falling edge time
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 ​​Digtal Design>, the time for the signal to rise from 10% to 90% is less than 6 times the wire delay, which is a high-speed signal! ------ That is, even an 8KHz square wave The signal, as long as the edge is steep enough, is also a high-speed signal, and transmission line theory needs to be used when wiring.
3. Stacking and layering of PCB boards
The four-layer board has the following stacking sequences. The advantages and disadvantages of the various stacks are explained below:
The first case
GND
S1+POWER
S2+POWER
GND
Second case
SIG1
GND
POWER
SIG2
The third case
GND
S1
S2
POWER
Note: S1 signal wiring layer one, S2 signal wiring layer two; GND ground layer POWER power layer
The first case should be the best case among the four-layer boards. Because the outer layer is a ground layer, it has a shielding effect on EMI. At the same time, the power supply layer is also very close to the ground layer, which makes the power supply internal resistance smaller and achieves the best results. But the first case cannot be used when the board density is relatively high. Because of this, the integrity of the first layer cannot be guaranteed, and the second layer signal will become worse. In addition, this structure cannot be used in situations where the power consumption of the entire board is relatively large.
The second situation is the most commonly used method we usually use. From the board structure, it is not suitable for high-speed digital circuit design. Because in this structure, it is not easy to maintain a low power source impedance. Take a board of 2 mm as an example: Z0=50ohm is required. The line width is 8mil. The copper foil thickness is 35цm. In this way, the signal layer is 0.14mm between the ground layer. The ground layer and power layer are 1.58mm. This greatly increases the internal resistance of the power supply. In this structure, since the radiation is directed to the space, a shielding plate is needed to reduce EMI.
In the third case, the signal line quality on the S1 layer is the best. S2 comes next. It has a shielding effect on EMI. But the power supply impedance is relatively large. This board can be used when the power consumption of the whole board is large and the board is the source of interference or is close to the source of interference.
4. Impedance matching
The amplitude of the reflected voltage signal is determined by the source reflection coefficient Ïs and the load reflection coefficient ÏL
ÏL = (RL-Z0) / (RL + Z0) and ÏS = (RS-Z0) / (RS + Z0)
In the above formula, if RL=Z0, the load reflection coefficient ÏL=0. If RS=Z0, the source end reflection coefficient ÏS=0.
Since the ordinary transmission line impedance Z0 should usually meet the requirement of 50Ω, the load impedance is usually from several thousand ohms to several tens of thousand ohms. Therefore, it is difficult to achieve impedance matching on the load side. However, since the signal source end (output) impedance is usually relatively small, roughly ten ohms.
Therefore, it is much easier to achieve impedance matching at the source. If a resistor is connected in parallel at the load end, the resistor will absorb part of the signal, which is unfavorable for 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 matching resistance should be added. 13Ω+33Ω=46Ω (approximately 50Ω, weak underdamping helps signal setup time)
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 that you must add source-side matching resistors.
In this way, the signal will be reflected back from the load end, because the source end impedance is matched, the reflected signal will not be reflected back again.
5. Precautions for power cord and ground wire layout
The power cord should be as short as possible and go straight, and it is best to go in a tree shape instead of a loop.
Ground loop problem: For digital circuits, the ground loop caused by ground loops is tens of millivolts, while the anti-interference threshold of TTL is 1.2V, and CMOS circuits can reach 1/2 power supply voltage. , That is to say, the circulation of ground wire will not cause adverse effects on the operation of the circuit at all. On the contrary, if the ground wire is not closed, the problem will be even greater, because the pulse power 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 the ground current of the 74LS161 when the reverse is 1.2A (using Measured by a 2Gsps oscilloscope, the ground current pulse width is 7ns).
Under the impact of a large pulse current, if a branch ground wire (line width 25mil) is used, the potential difference between the ground wires will reach the level of 100 millivolts. After adopting the ground loop, the pulse current will spread to various points of the ground wire, which greatly reduces the possibility of interfering with the circuit. Using a closed ground wire, the measured maximum instantaneous potential difference between the ground wires of each device is one-half to one-fifth of the unclosed ground wire. Of course, the measured data of circuit boards with different densities and speeds are very different. What I said above refers to the level of the Z80 Demo board that comes with Protel 99SE; for low-frequency analog circuits, I think the power frequency after the ground wire is closed. Interference is induced from space, which cannot be simulated and calculated anyway.
If the ground wire is not closed, the ground wire eddy current will not be generated. What is the theoretical basis for Beckhamtao's so-called "but the power frequency induced voltage of the ground wire is open loop will be greater."? To give two examples, 7 years ago, I took over a project of someone else, a precision pressure gauge, which used a 14-bit A/D converter, but the actual measurement only had 11 digits of effective accuracy. After investigation, there was a 15mVp-p on the ground wire. The solution is to divide the analog ground loop of the PCB. The ground wire from the front-end sensor to the A/D is distributed in a branch with flying wires. Later, the mass-produced model PCB is re-produced according to the routing of the flying wires. problem appear.
In the second example, a friend loves to have a fever and DIYs an amplifier by himself, but there is always a humming sound at the output. I suggest that it cut the ground loop to solve the problem. Afterwards, this man consulted dozens of "Hi-Fi famous machines" PCB diagrams and confirmed that none of the machines used ground loops in the analog part.
6. Printed circuit board design principles and anti-interference measures
A printed circuit board (PCB) is a support for circuit components and devices in electronic products. It provides electrical connections between circuit components and devices. With the rapid development of electrical technology, the density of PGB is getting higher and higher. The quality of PCB design has a great influence on the ability to resist interference. Therefore, during PCB design, the general principles of PCB design must be observed, and the requirements of anti-interference design must be met.
General principles of PCB design
To get the best performance of the electronic circuit, the layout of the components and the layout of the wires are very important. In order to design PCBs with good quality and low cost, the following general principles should be followed:
1. Layout
First, consider the PCB size. When the PCB size is too large, the printed lines will be long, the impedance will increase, the anti-noise ability will decrease, and the cost will increase; if the PCB size is too small, the heat dissipation will not be good, and adjacent lines will be easily disturbed. After determining the PCB size, determine the location of the special components. Finally, according to the functional units of the circuit, all the components of the circuit are laid out.
The following principles should be observed when determining the location of special components:
(1) Shorten the wiring between high-frequency components as much as possible, try to reduce their distribution parameters and mutual electromagnetic interference. Components that are susceptible to interference should not be too close to each other, and input and output components should be kept as far away as possible.
(2) There may be a high potential difference between some components or wires, and the distance between them should be increased to avoid accidental short circuits caused by discharge. The components with high voltage should be arranged as far as possible in places that are not easily reachable by hands during debugging.
(3) Components weighing more than 15g should be fixed with brackets and then welded. Those components that are large, heavy, and generate a lot of heat should not be installed on the printed circuit board, but should be installed on the chassis bottom plate of the whole machine, and the heat dissipation problem should be considered. Thermal components should be far away from heating components.
(4) For the layout of adjustable components such as potentiometers, adjustable inductors, variable capacitors, and micro switches, the structural requirements of the whole machine should be considered. If it is adjusted inside the machine, it should be placed on the printed circuit board where it is convenient for adjustment; if it is adjusted outside the machine, its position should match the position of the adjustment knob on the chassis panel.
(5) The position occupied by the positioning hole of the printed board and the fixed bracket should be reserved.
According to the functional unit of the circuit, when laying out all the components of the circuit, the following principles must be met:
(1) Arrange the position of each functional circuit unit according to the circuit flow, so that the layout is convenient for signal circulation, and the signal is kept in the same direction as possible.
(2) Take the core component of each functional circuit as the center and lay out around it. The components should be evenly, neatly and compactly arranged on the PCB. Minimize and shorten the leads and connections between the components.
(3) For circuits operating at high frequencies, the distributed parameters between components must be considered. Generally, the circuit should be arranged in parallel as much as possible. In this way, it is not only beautiful, but also easy to install and weld. It is easy to mass produce.
(4) The components located at the edge of the circuit board are generally not less than 2mm away from the edge of the circuit board. The best shape of the circuit board is rectangular. The aspect ratio is 3:2 to 4:3. When the size of the circuit board is larger than 200x150mm, the mechanical strength of the circuit board should be considered.
2. Wiring
The principle of wiring is as follows:
(1) The wires used for the input and output terminals should try to avoid being adjacent and parallel. It is best to add ground wires between wires to avoid feedback coupling.
(2) The minimum width of the printed wire is mainly determined by the adhesion strength between the wire and the insulating substrate and the current value flowing through them. When the thickness of the copper foil is 0.05mm and the width is 1 ~ 15mm, the temperature will not be higher than 3℃ through 2A current, so the wire width of 1.5mm can meet the requirements. For integrated circuits, especially digital circuits, a wire width of 0.02~0.3mm is usually selected. Of course, as long as possible, use as wide a cable as possible, especially the power cable and the ground cable. The minimum spacing of wires is mainly determined by the worst-case insulation resistance and breakdown voltage between the wires. For integrated circuits, especially digital circuits, as long as the process permits, the pitch can be as small as 5~8mm.
(3) The corners of the printed conductors are generally arc-shaped, and the right angle or the included angle will affect the electrical performance in the high-frequency circuit. In addition, try to avoid using large-area copper foil, otherwise the copper foil will expand and fall off when heated for a long time. When a large-area copper foil is required, it is best to use a grid shape. This will help to eliminate the volatile gas generated by the heating of the adhesive between the copper foil and the substrate.
3. Pad
The center hole of the pad is slightly larger than the diameter of the device lead. If the pad is too large, it is easy to form a false solder. The outer diameter D of the pad is generally not less than (d+1.2) mm, where d is the lead 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-jamming design of the printed circuit board has a close relationship with the specific circuit. Here, only a few common measures of PCB anti-jamming design are explained.
1. Power cord design
According to the size of the printed circuit board current, try to increase the width of the power line to 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 helps to enhance the anti-noise ability.
2. Ground wire design
The principles of ground wire design are:
(1) The digital ground is separated from the analog ground. If there are both logic circuits and linear circuits on the circuit board, they should be separated as much as possible. The ground of the low-frequency circuit should be grounded in parallel at a single point as much as possible. When the actual wiring is difficult, it can be partially connected in series and then connected in parallel. The high-frequency circuit should be grounded at multiple points in series, the ground wire should be short and leased, and the grid-like large-area ground foil should be used around the high-frequency component as much as possible.
(2) The grounding wire should be as thick as possible. If the ground wire uses a very tight line, the ground potential changes with the change of the current, which reduces the anti-noise performance. Therefore, the ground wire should be thickened so that it can pass three times the allowable current on the printed board. If possible, the grounding wire should be 2~3mm or more.
(3) The ground wire forms a closed loop. For printed boards composed only of digital circuits, most of their grounding circuits are arranged in loops to improve noise resistance.
3. Decoupling capacitor configuration
One of the conventional methods of PCB design is to configure appropriate decoupling capacitors on each key part of the printed board.
The general configuration principles of decoupling capacitors are:
(1) Connect a 10 ~ 100uf electrolytic capacitor across the power input. If possible, it is better to connect to 100uF or more.
(2) In principle, each integrated circuit chip should be equipped with a 0.01pF ceramic capacitor. If the gap of the printed board is not enough, a 1-10pF capacitor can be arranged for every 4~8 chips.
(3) For devices with weak anti-noise ability and large power changes when shutting down, such as RAM and ROM storage devices, a decoupling capacitor should be directly connected between the power line and the ground line of the chip.
(4) Capacitor leads should not be too long, especially for high-frequency bypass capacitors.
In addition, you should also pay attention to the following two points:
(1) When there are contactors, relays, buttons and other components in the printed board. Large spark discharges will be generated when operating them, and the RC circuit shown in the figure must be used to absorb the discharge current. Generally, R is 1 ~ 2K, and C is 2.2 ~ 47UF.
(2) The input impedance of CMOS is very high and it is susceptible to induction, so the unused terminal should be grounded or connected to a positive power supply when in use.
7. Design skills and key points for realizing high-efficiency and automatic routing of PCB
Although the current EDA tools are very powerful, as the PCB size requirements are getting smaller and the device density is getting higher and higher, the difficulty of PCB design is not small. How to achieve a high PCB layout rate and shorten the design time? This article introduces the design skills and key points of PCB planning, layout and wiring. Now PCB design time is getting shorter and shorter, smaller and smaller circuit board space, higher and higher device density, extremely demanding layout rules and large-size components make the designer's work more difficult. In order to solve the design difficulties and speed up the launch of products, many manufacturers now tend to use dedicated EDA tools to realize PCB design. However, dedicated EDA tools cannot produce ideal results, nor can they achieve a 100% deployment rate, and are very messy. It usually takes a lot of time to complete the remaining work.
There are many popular EDA tools and software on the market, but they are all the same except for the different terms and the positions of the function keys. How to use these tools to better realize the PCB design? Carry out a careful analysis of the design before starting the wiring and Careful setting of the tool software will make the design more in line with the requirements. The following is the general design process and steps.
1. Determine the number of layers of the PCB
The size of the circuit board and the number of wiring layers need to be determined at the initial stage of the design. If the design requires the use of high-density ball grid array (BGA) components, the minimum number of wiring layers required for wiring these devices must be considered. The number of wiring layers and the stack-up method will directly affect the wiring and impedance of the printed lines. The size of the board helps determine the stacking method and the width of the printed line to achieve the desired design effect.
For many years, people have always thought that the lower the number of layers of the circuit board, the lower the cost, but there are many other factors that affect the manufacturing cost of the circuit board. In recent years, the cost difference between multilayer boards has been greatly reduced. It is best to use more circuit layers and evenly distribute the copper at the beginning of the design, so as to avoid discovering that a small number of signals do not meet the defined rules and space requirements until the end of the design, so that new layers are forced to be added. Careful planning before designing will reduce a lot of troubles in wiring.
2. Design rules and restrictions
The automatic routing tool itself does not know what to do. In order to complete the wiring task, the wiring tool needs to work under the correct rules and restrictions. Different signal lines have different wiring requirements. All signal lines with special requirements must be classified. Different design classifications are different. Each signal class should have a priority, the higher the priority, the stricter the rules. The rules involve the width of the printed lines, the maximum number of vias, the degree of parallelism, the mutual influence between the signal lines, and the limitation of layers. These rules have a great influence on the performance of the wiring tool. Careful consideration of design requirements is an important step for successful wiring.
3. The layout of the components
To optimize the assembly process, design for manufacturability (DFM) rules impose restrictions on component layout. If the assembly department allows the components to move, the circuit can be appropriately optimized, which is more convenient for automatic wiring. The defined rules and constraints will affect the layout design.
The routing channel and via area need to be considered during layout. These paths and areas are obvious to the designer, but the automatic routing tool will only consider one signal at a time. By setting routing constraints and setting the layer of the signal line, the routing tool can be as the designer imagined Complete the wiring like that.
4. Fan-out design
In the fan-out design stage, to enable automatic routing tools to connect component pins, each pin of the surface mount device should have at least one via, so that when more connections are needed, the circuit board can be internally layered Connection, online testing (ICT) and circuit reprocessing.
In order to maximize the efficiency of the automatic routing tool, the largest via size and printed line must be used as much as possible, and the interval is ideally set to 50mil. Use the via type that maximizes the number of routing paths. When carrying out fan-out design, it is necessary to consider the problem of circuit online testing. Test fixtures can be expensive, and they are usually ordered when they are about to go into full production. If only then consider adding nodes to achieve 100% testability, it would be too late.
After careful consideration and prediction, the design of circuit online test can be carried out at the early stage of the design and realized in the later stage of the production process. The type of via fan-out is determined according to the wiring path and circuit online test. The power supply and grounding will also affect the wiring and fan-out design. . In order to reduce the inductive reactance generated by the connection line of the filter capacitor, the vias should be as close as possible to the pins of the surface mount device, and manual wiring can be used if necessary. This may affect the originally envisaged wiring path, and may even cause you to re- Consider which type of via to use, so the relationship between via and pin inductance must be considered and the priority of via specifications must be set.
5. Manual wiring and processing of key signals
Although this article mainly discusses automatic wiring, manual wiring is an important process of printed circuit board design now and in the future. The use of manual wiring helps automatic wiring tools to complete the wiring work. As shown in Figure 2a and Figure 2b, by manually routing the selected nets and fixing them, a path that can be followed during automatic routing can be formed.
Regardless of the number of key signals, these signals should be routed first, either manually or in combination with automatic routing tools. Critical signals usually have to pass careful circuit design to achieve the desired performance. After the wiring is completed, the relevant engineering personnel will check the signal wiring. This process is relatively easy. After passing the inspection, fix these lines, and then start to automatically route the remaining signals.
6. Automatic wiring
The wiring of key signals needs to consider controlling some electrical parameters during wiring, such as reducing distributed inductance and EMC, etc. The wiring of other signals is similar. All EDA vendors will provide a way to control these parameters. After understanding the input parameters of the automatic wiring tool and the influence of the input parameters on the wiring, the quality of the automatic wiring can be guaranteed to a certain extent.
General rules should be used to automatically route signals. By setting restrictions and prohibiting wiring areas to limit the layers used by a given signal and the number of vias used, the wiring tool can automatically route the wires according to the engineer's design ideas. If the number of layers used by the automatic routing tool and the number of vias are not limited, each layer will be used during automatic routing, and many vias will be generated.
After setting the constraints and applying the rules created, the automatic routing will achieve results similar to expectations. Of course, some sorting work may be required, and space for other signal and network wiring needs to be ensured. After a part of the design is completed, fix it to prevent it from being affected by the subsequent wiring process.
Use the same steps to route the remaining signals. The number of wiring depends on the complexity of the circuit and the number of general rules you define. After each type of signal is completed, the constraint conditions of the remaining network wiring will be reduced. But what comes with it is that a lot of signal wiring requires manual intervention. Today's automatic wiring tools are very powerful and can usually complete 100% of the wiring. However, when the automatic wiring tool has not completed all signal wiring, the remaining signals need to be manually wired.
7. The design points of automatic wiring include:
7.1 Slightly change the settings, try a variety of route wiring;
7.2 Keep the basic rules unchanged, try different wiring layers, different printed lines and spacing widths, different line widths, and different types of vias such as blind holes, buried holes, etc., and observe how these factors affect the design results;
7.3 Let the wiring tool process those default networks as needed;
7.4 The less important the signal is, the more freedom the automatic routing tool has to route to it.
8. Arrangement of wiring
If the EDA tool software you are using can list the wiring length of the signal, check these data, you may find that some signal wiring lengths with few constraints are very long. This problem is relatively easy to deal with, and the signal wiring length can be shortened and the number of vias can be reduced by manual editing. In the finishing process, you need to determine which wiring is reasonable and which wiring is unreasonable. Like manual wiring design, automatic wiring design can also be sorted and edited during the inspection process.
9. The appearance of the circuit board
The previous design often paid attention to the visual effect of the circuit board, but now it is different. The automatically designed circuit board is not as beautiful as the manual design, but the electronic characteristics can meet the specified requirements, and the complete performance of the design is guaranteed.
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