"When it comes to the PCB board, many of my friends think it everywhere around us, everything from household appliances, computer accessories inside, to a variety of digital products, electronic products, almost as long as the PCB will be used, then what in the end it is the PCB board? PCB board is PrintedCircuitBlock, i.e. printed circuit boards for placement of electronic components, a base version of the line by using the printing plate on the base plate of copper corrosion lines, is etched and rinsed out of the line.
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PCB board can be divided into single plate, double and multilayer boards. Various electronic components are integrated in the PCB, in a basic single PCB, the parts are concentrated on one side, the wires are concentrated on the other side. This way we need burrows on the board, so pin to get through to the other side of the board, so pin parts are welded to the other side.
Because of this, the front and back so that a PCB are referred to as surface parts (ComponentSide) and the welding plane (SolderSide). The plate can be seen as a double two opposing bonded together to form a single plate, the plate has both sides of the electronic components and traces.
Sometimes you need to face the other side of the line connection to the plate, which would be through the guide hole (via). Guide hole in the PCB, metal filled or coated with holes, which can be connected to both sides of the wire. Are now many computer motherboard with four or even six layer PCB layer, while the graphics are generally used in the 6-layer PCB, many high-end graphics image nVIDIAGeForce4Ti series on the use of 8-layer PCB, which is called a multi-layer PCB board. In the multi-layer PCB board will encounter problems connecting lines between the respective layers, may be realized through the guide hole.
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Since the PCB is a multilayer board, it sometimes does not need a guide hole penetrating the PCB board, such a pilot hole is called buried vias (Buriedvias) and blind holes (Blindvias), wherein several layers because they only penetrate. The blind bore is connected to the surface layers of the internal PCB PCB, through the entire board is not required. Buried vias only internal connection PCB, so that the light from the surface is not apparent.
In the multilayer PCB, the entire layer is directly connected to the ground and the power supply. Therefore, we will classify layers of signal layers (Signal), power level (Power) or ground plane (Ground). If the parts require different power supply PCB, PCB usually have two or more of such power and the wire layer. The more layers PCB board employed, the higher the cost. Of course, the use of more layers of the PCB be helpful to provide a signal stability.
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Professional PCB board manufacturing process rather complex, take Example 4 layer PCB. 4 are large motherboard PCB layers. When manufacturing each of two intermediate rolling is first, after cutting, etching, plating oxide, which are components of the surface layer 4, the power supply layer, formation pressure, and the solder layer. This layer 4 then put together into a motherboard rolling the PCB. Subsequently punch, holes did.
After washing, the outer layers of the printed circuit, copper, etching test, the solder resist layer, screen printing. Finally imposition PCB (including many motherboards) punched into a block motherboard PCB, then after the test by vacuum packaging. If the PCB production process the copper plating is not good, there is the phenomenon of adhesive is not strong, short circuit easily hidden or capacitive effects (causes interference). Vias on the PCB is also important to note. If the hole is not playing in the middle, but to one side, it will have an uneven match, or can be easily in contact with the intermediate ground or power planes, resulting in short circuit or ground potential adverse factors.
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A copper wire cabling procedure
The first step is to establish a production of wiring between parts online. We use transfer negative working manner on the metal conductor film performance. The trick is to the entire surface covered with a thin layer of copper foil, and to eliminate the extra part. Additional type transfer is another way people use less, which is the only method of putting the copper where it is needed, but we are not here to talk about it.
Positive photoresist is made of photosensitizer, it will dissolve under illumination. There are many ways photoresist copper surface treatment, but the most common way, it is heated and rolled on the surface containing photoresist. It can also be used in top spray mode liquid, but provide a relatively high resolution dry film, can produce relatively thin wire. Hood only a template layer PCB manufacturing.
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In the PCB after the photoresist before exposure to UV light, the overlying photoresist hood portion can be prevented from being exposed area. These photoresist covered place, will become wirings. After developing the photoresist, etching to bare copper part of the other. The plates can be immersed in the etching process in an etching solution, or the solvent is sprayed onto the board. Etching using ferric chloride is generally used as a solvent and the like. After completion of the etching of the remaining photoresist is removed.
1. The wiring width and the current
Usually a width of not less than 0.2mm (8mil)
In high-precision high-density PCB, spacing and line width typically 0.3mm (12mil).
When the thickness of the copper foil is about 50um, a width of the wire 1 ~ 1.5mm (60mil) = 2A
The general public 80mil, for the application of microprocessor should pay more attention.
2. How high frequency in the end considered a high-speed board?
When the signal rise / fall time <3-6 times the signal propagation time, i.e., that a high-speed signal.
For digital circuits, the key is to look at the rising edge steepness of the signal, i.e. the signal, fall time,
According to a very classic book "High Speed Digtal Design> theory, signal rises from 10% to 90% less than 6 times the wire delay is the high-speed signal! ------ i.e.! 8KHz the square wave even if signal as long as sufficiently steep edge, as it is the high-speed signal, wiring needs to be used on the transmission line
Stacking plate hierarchical 3.PCB
Four-layer laminated board has the following sequence. The following are the advantages and disadvantages of various laminate stated:
The first case
GND
S1 + POWER
S2 + POWER
GND
The second case
SIG1
GND
POWER
SIG2
The third case
GND
S1
S2
POWER
NOTE: S1 signal wiring layer, S2 signal wiring Layer; the GND power supply layer formation POWER
The first case, the case should be the best four-layer board. Because the outer layer is the formation of EMI shielding effect, while reliably ground the power supply layer with very close, so that power resistance is small, rural best results. But it can not be used when the first case of this board density is relatively large. As a result, can not guarantee the integrity of the first layer, the second layer so that the signal may become worse. Further, this structure can not be used for the entire board is relatively large power consumption.
The second case, is usually the most common way. From the structure of the plate, it does not apply to high-speed digital circuit design. Because in such a configuration, easy to maintain low supply impedance. In an example a 2 mm plate: Z0 = 50ohm line width required as a copper foil thickness of 8mil 35цm... The intermediate layer and the formation of such a signal is 0.14mm. And ground power plane is 1.58mm. This greatly increases the internal resistance of the power supply. In such a structure, because the radiation to the space, the need to add shielding plate, in order to reduce EMI.
The third case, the upper layer signal lines S1 best quality. S2 followed. Shielding effect of EMI. However, the source impedance is large. This board can be used for large power consumption of the entire board is a board where the source of interference or close to the interference source.
4. impedance matching
ΡL is the amplitude of the reflected signal is determined by the voltage source and the reflection coefficient of the load reflection coefficient end ρs
ρL = (RL - Z0) / (RL + Z0) and ρS = (RS - Z0) / (RS + Z0)
In the above formula, when the load RL = Z0 reflection coefficient ρL = 0. If RS = Z0 source reflection coefficient ρS = 0.
Because conventional impedance Z0 of the transmission line should generally meet the requirements of 50 [Omega about 50Ω, and the load impedance is typically several kilohms to several tens kilo-ohms. Therefore, to achieve impedance matching at the load more difficult. However, since the signal source (output) impedance is usually small, approximately ten ohms. Thus to achieve impedance matching at the end of the source is much easier. If the end of the load resistor and the resistance will absorb part of the transmission signal unfavorable (I understand). When selecting TTL / CMOS standard 24mA drive current, its output impedance is approximately 13Ω. If the transmission line impedance Z0 = 50Ω, then the source should be added to a 33Ω matching resistor. 13Ω + 33Ω = 46Ω (approximately 50Ω, the weak setup time helps underdamped signal).
When selecting another transmission standards and the drive current, impedance matching will vary. In the high-speed logic and circuit design, some of the key signals, such as clock, control signals, etc., we recommend that the source must be added matching resistor.
Such signal will then reflected back from the load, because the source impedance match, the reflected signal will not be reflected back.
The layout of power and ground Considerations
Power line as short, straight line, but preferably take the tree, do not go ring.
Ground loop problems: For digital circuits, ground loops caused by ground-level circulation is tens of millivolts, while the TTL immunity threshold is 1.2V, CMOS circuit can also reach 1/2 of the supply voltage , that ground circulation did not adversely affect the circuit will work. Conversely, if the ground is not closed, the problem will be even greater, since the pulse supply current digital circuits at work resulting in uneven ground points, such as I found 74LS161 ground current reversal at 1.2A (with 2Gsps oscilloscope measured, the current pulse width 7ns).
Under the impact of a large pulse current, if the branch ground (line width 25mil) distribution, the potential difference between the ground point of each level will reach one hundred millivolts. After the use of the ground loop, the pulse current will be distributed to each point to the ground, greatly reducing the possibility of interference circuit. In one-half to one-fifth of the ground is closed, found the maximum instantaneous ground potential difference of each device is not closed ground.
Of course, very different speeds of different densities measured data difference circuit boards, I said above, referring to about the same level as Protel 99SE accompanying Z80 Demo plate; for low frequency analog circuits, I believe that the ground work after closing frequency It is sensitive to interference from space, which is in any case not out of calculation and simulation.
If the ground is not closed, no ground vortex, beckhamtao so-called "open-loop ground but the frequency of the induced voltage will be greater and the theoretical basis?
Two examples, a project seven years ago I took someone else's precision pressure gauge, with 14-bit A / D converter, but found only 11 valid precision, and upon inspection, to have 15mVp-p line of work frequency interference, the PCB solution is to cut open loop analog front end to a ground line sensor a / D with fly line as the branch distribution, and later re-production models PCB traces according fly line production, has not problem appear.
The second example, a friend loves fever, their own DIY with a power amplifier, the output always hum, I suggest that it will cut the ground loop, the problem is solved. Later this bit man Now dozens of "Hi-Fi crew" PCB FIG confirmed without using a machine-ground loop in the analog portion.
6. The printed circuit board design principles and measures interference
A printed circuit board (PCB) is an electronic circuit support elements and devices which provide electrical connection between the circuit elements and devices are connected. With the rapid development of technology in electricity, PGB of increasing density. PCB design quality great influence on the anti-jamming capability. Therefore, during the PCB design. PCB design must comply with the general principles of, and shall comply with anti-jamming design.
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Second, the general principles of PCB design
The electronic circuit layout To get the best performance, components and wire cloth and is very important. In order to design good quality, low cost PCB should follow the following general principles:
1. Layout
First, consider the size of the PCB. When the PCB size is too large, the printing line length, the impedance to increase, decrease noise immunity, the cost increases; too small, the heat is not good, and adjacent lines susceptible to interference. Again to determine the position of a particular element in determining the size of the PCB. Finally, the functional units of the circuit, all the components of the circuit layout.
When determining the position of a particular element should comply with the following principles:
(1) as short as possible connections between the high-frequency component, trying to reduce electromagnetic interference between each other and their distribution parameters. Susceptible to interference components is not so close to each other, the input and output elements should be kept away.
(2) may have a higher potential difference between certain components or wires, should increase the distance between them, so as to avoid accidental short discharge lead. Components with a high voltage should be arranged in place of debugging not easily reached.
(3) weighing more than 15g components, should be a fixed support, and then welded. Those big, heavy, heat and more components, should not be mounted on the printed circuit board, but should be mounted on the chassis bottom of the machine, and should consider the heat problem. Thermosensitive element away from the heating element.
(4) For the potentiometer, variable inductor, variable capacitor, micro switch or the like the layout of the adjustable element should be considered the structural requirements of the machine. If adjusting the machine, it should be placed above the printed circuit board to facilitate adjustment; outer if local regulator, whose position is to adjust the position of the knob on the panel of the chassis to adapt.
(5) should leave the printing position fixing bracket and pull the positioning holes occupied.
According to the functional unit when the circuit layout of all circuit components, with the following principle:
(1) according to the flow circuit arrangement positions of the respective functional circuit unit, the signal flow so that the layout convenience, and consistent signal direction as possible.
(2) the core of each functional circuit elements as the center, around which to lay. Components should be uniform, neatly, compactly on the PCB. Minimization and shortening leads and connections between components.
(3) a circuit operating at high frequencies, consideration of the distribution parameter between the components. General circuit components should be arranged in parallel as much as possible. In this way, not only beautiful. And easy assembly and welding. Easy to mass production.
(4) the edge of the circuit board components, from the edge of the board is generally not less than 2mm. The best shape of the circuit board is rectangular. Aspect ratio of 3: 2 to 4: 3. When the circuit board is larger than the size of 200x150mm. Consider the mechanical strength of the circuit board suffered.
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2. wiring
Wiring principles as follows:
(1) with input and output terminals of the adjacent parallel wire should be avoided. Between the ground line is preferably added, in order to avoid feedback coupling.
Minimum width (2) printed conductors of the main subject are determined by the current value of the adhesive strength between the pull wire and the insulating base and flows. When the foil thickness of 0.05mm, a width of 1 ~ 15mm. 2A through current, the temperature is not higher than 3 ℃, therefore. Conductor width of 1.5mm to meet the requirements. forIntegrated circuits, especially digital circuits, typically 0.02 to 0.3mm wire width. Of course, as long as it is allowed, it is as wide as possible as much as possible. Especially the power cord and ground. The minimum spacing of the wire is mainly determined by the line insulation resistance and breakdown voltage of the worst case. For integrated circuits, especially digital circuits, as long as the process is allowed, the spacing is as small as 5 to 8 mm.
(3) Printing wires generally take circular arc, while at right angles or angles can affect electrical properties in high frequency circuits. In addition, try to avoid using large-scale copper foils, otherwise, when heat is hot, it is prone to copper foil expansion and falling. It is necessary to use a large-scale copper foil, it is preferred to use a grid shape. This facilitates the excluding a volatile gas generated by the copper foil and the substrate adhesive.
3. Pad
The pad center hole is slightly larger than the device leads. The pad is too easy to form a duel. The solder outer diameter D is generally not less than (D + 1.2) mm, where D is a lead aperture. For high-density digital circuits, the smallest diameter of the pad can be taken (D + 1.0) mm.
Third, PCB and circuit anti-interference measures
The anti-interference design of the printed circuit board has a close relationship with the specific circuit, which is only some of the common measures for PCB anti-interference design.
Power cord design
Depending on the size of the printed circuit board, the power line width is allowed to reduce the loop resistance. At the same time, the direction of the power cord, the direction of the ground, and the direction of data transfer, which helps to enhance anti-noise ability.
2. Ground design
The principle of ground design is:
(1) Digitally separate from the simulation. If both logic circuits on the board, there are linear circuits, they should make them as separate. The location of the low frequency circuit should be used in a single point and join. When the actual wiring is difficult, it can be partially connected in series and join. High-frequency circuits should be used in series, and the ground should be short and rented, and the high frequency element is surrounded by a grid-like large-area foil.
(2) The grounding should be tried to be bold. If the ground wire is used, the ground potential changes with the current change, and the anti-noise performance is lowered. Therefore, the ground wire should be bold to allow it to pass three times the allowable current on the printed board. If possible, the grounding should be 2 to 3 mm or more.
(3) The ground wire constitutes a closed loop. Printing boards consisting only by digital circuits, which can increase the anti-noise ability.
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3. Rettated capacitance configuration
One of the conventional practices for PCB design is to configure appropriate retaining capacitors in each key part of the printed board.
The general configuration principle of retreat capacitance is:
(1) The power supply input bridges the electrolytic capacitors of 10 to 100 uF. If possible, pick 100UF or more.
(2) In principle, each integrated circuit chip should be disposed of a 0.01 pF of porcelain capacitance. If the printed board is not enough, it can arrange 1 to 10 pF, but capacitances per 4 to 8 chips.
(3) For those of the anti-noise capacity, the power supply changes large when the power is turned off, such as RAM, ROM memory, should be directly accessed between the power cord and the ground line of the chip.
(4) Capacitive leads cannot be too long, especially high frequency bypass capacitors.
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In addition, pay attention to the following two points:
(1) When there is a contactor, relay, buttons such as contactors, relays, buttons. Operate them, larger spark discharges are generated, and the RC circuit shown in the drawings must be used to absorb the discharge current. General R takes 1 to 2 k, C from 2.2 to 47uF.
(2) The input impedance of CMOS is very high, and it is easy to induct, so it is necessary to ground or pick up the power supply without the end of the terminal.
Fourth, implement the design techniques and points of PCB efficient automatic wiring
Although the current EDA tool is very strong, as the PCB size requires less and smaller, the density of the device is getting higher and higher, and the difficulty of PCB design is not small. How to achieve high PCB is high and shortened design time? This article describes the design techniques and points of PCB planning, layout and wiring. Now the PCB design is getting shorter and smaller and smaller, the density of the circuit boards, extremely harsh layout rules and large-sized components make designers' work more difficult.
In order to solve the difficulties in design, accelerate the listing of the product, now many manufacturers tend to adopt a dedicated EDA tool to implement the PCB design. However, dedicated EDA tools cannot produce ideal results, nor can they reach 100% of the openivity rate, and very chaotic, usually take a lot of time to complete the rest of the work.
Nowadays, there are many popular EDA tools software, but in addition to the use of the terms and function keys, the location is different, how to use these tools to better implement PCB design? Before the start of the wiring, the design carefully analyzed and The serious settings for tool software will make the design more compliance. Here is a general design process and steps.
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1. Determine the number of layers of PCB
The number of board sizes and wiring layers need to be determined in the early stage of the design. If the design requires the use of a high-density ball grid (BGA) component, you must consider the minimum number of wiring layers required for these device wirings. The number of wiring layers and the stack-up mode will directly affect the wiring and impedance of the printing line. The size of the board helps to determine the laminated mode and print wire width, and achieve the desired design effect.
Over the years, people always believe that the lower the number of circuit boards, the lower the cost, but there are still many other factors that affect the manufacturing cost of the board. In recent years, the cost difference between multi-layer board has been greatly reduced. It is best to use more circuit layers when designing, making copper-wide distribution to avoid that there is a small number of signals that do not meet the defined rules and space requirements at the end of the design, thereby being forced to add new layers. The serious plan before the design will reduce a lot of trouble in the wiring.
2. Design rules and restrictions
The automatic wiring tool itself does not know what should be done. To complete the wiring task, the wiring tool needs to work under the correct rules and restrictions. Different signal lines have different wiring requirements, and to classify all special requirements, different design classifications are different. Each signal class should have priority, the higher the priority, the more stringent rules. Rules relate to printed wire width, maximum number, parallelism, and signal lines, and limitations of layers, which have great impact on the performance of wiring tools. Seriously considering design requirements is an important step in successful wiring.
3. Component layout
To optimize the assembly process, the manufacturability design (DFM) rules will restrict the component layout. If the assembly department allows the component to move, it can be properly optimized to the circuit, which is more convenient for automatic wiring. The defined rules and constraints will affect the layout design.
The routing channel and via region are considered in the layout. These paths and regions are obvious for designers, but the automatic wiring tool will only consider a signal at a time, by setting the wiring constraint condition and the layers set, the wiring tool can make the wiring tool can be envisioned by the designer. That is to complete the wiring.
4. Fan out design
In the fan-out design phase, to connect the automatic wiring tool to the component pin, each pin of the surface mount device should have a via hole in order to be more connected, so that the circuit board can perform the inner layer when needed Connection, online test (ICT) and circuit processes.
In order to make the automatic wiring tool efficiency, it is necessary to use the maximum via size and print line as much as possible, and the interval is set to 50 mil is ideal. It is to be used to make a via type maximum number of wiring paths. When performing fan-out design, consider the circuit online test problem. Test clamps may be expensive, and usually order when they are about to be put into full production, if you consider adding nodes at this time, it is too late to implement 100% testability.
After careful consideration and prediction, the design of the circuit online test can be carried out in the initial design of the design, and it is achieved in the latter path and circuit online test, and the power supply and ground can also affect the wiring and fan out of the design according to the wiring path and circuit online test. . In order to reduce the induction generated by the filter capacitor cable, the via should be as close as possible to the pin of the surface mounting device, if necessary, can use the manual wiring, which may have an impact on the originally envisaged wiring path, and may even lead you to re- Which via is used to use, so you must consider the relationship between the via and the inductance, the priority of the via specifications must be considered.
5. Manual wiring and the processing of key signals
Although this paper focuses on automatic wiring problems, manual wiring is an important process of designing printed circuit boards in the present and future. The manual wiring will help the automatic wiring tool complete the wiring work. As shown in FIGS. 2A and 2B, by performing a manual wiring and fixing the selected network (NET), it can be based on the path when the automatic wiring can be formed.
Regardless of the number of critical signals, it is first possible to wiring, manually wiring or binding automatic wiring tools. Key signals typically have to achieve desired performance through careful circuit design. After the wiring is completed, these signal wirings are checked by the relevant engineers, which is relatively easy. After checking, secure these lines and then start automatic wiring for the remaining signals.
6. Automatic wiring
The wiring of the key signal needs to be considered to control some of the electrical parameters when the wiring, such as reducing distribution inductance and EMC, etc., the wiring of other signals is similar. All EDA vendors provide a way to control these parameters. After understanding the input parameters of the automatic wiring tool and the input parameters affect the wiring, the quality of the automatic wiring can be guaranteed to a certain extent.
The general rules should be used to automatically wiring the signal. By setting the restriction condition and the prohibition of the wiring zone to define the layer used by the given signal and the number of vias used, the wiring tool can be routed according to the engineer's design idea. If the number of layers and the number of the cloth used for the automatic wiring tool are not limited, each layer will be used when the automatic wiring is used, and there will be many via holes.
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After setting the rules created by the constraints and applications, the automatic wiring will reach the results similar to expected, of course, some sorting work may also be required, and there is also a space for other signals and network wiring. After a portion of the design is completed, it is fixed to prevent the effects of the rear wiring process.
Wiring the remaining signals using the same step. The number of wirings depends on the complexity of the circuit and how much you define. After a class of signals are completed, the constraints of the remaining network wirings are reduced. However, it is that many signal wirings need to be manually intervened. The current automatic wiring tool is very powerful, usually 100% wiring. However, when the automatic wiring tool does not complete all the signal wirings, the remaining signal is required to make a manual wiring.
7. The design points of the automatic wiring include:
1) Slightly change the setting, trial multiple path wirings;
2) Maintaining the basic rules unchanged, trial different wiring layers, different printing lines and spacing widths, and different line width, different types of via holes such as blind holes, buried holes, etc., what is the impact of these factors for design results;
3) Let the wiring tool to process those default networks as needed;
4) The less the signal is, the larger the degree of freedom of the automatic wiring tool.
8. Corruption of wiring
If the EDA tool software you use can list the wiring length of the signal, check the data, you may find that some signal wirings of some constraints have a long length. This problem is relatively easy to handle, shortening the length of the signal wiring and reducing the number of overfores by manually editing. During the finishing process, you need to judge which wiring is reasonable, which wiring is unreasonable. Like the manual wiring design, automatic wiring design can also be organized and edited during the inspection process.
9. Appearance of the board
Previous designs often pay attention to the visual effect of the board, now different. Automatic design circuit boards are not better than manual design, but can meet the requirements of the requirements in electronic properties, and the complete performance of the design is guaranteed. "
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