Pcb wiring skills, easy to get the wiring, layout, including: First, the basic rules of component layout; Second, the component wiring rules; to increase the system's anti-electromagnetic interference capacity; 3, reduce some noise and electromagnetic interference experience.
First, the basic rules of component layout1. According to the circuit module layout, the related circuit that realizes the same function is called a module, and the components in the circuit module should adopt the nearest set*, and the digital circuit and the analog circuit are separated;
2. Positioning holes, standard holes, etc. shall not be placed within 1.27mm around the mounting holes. Components such as components and screws shall not be placed around 3.5mm (for M2.5) and 4mm (for M3) mounting holes.
3. Avoid the over-holes under the components such as the horizontal resistors, inductors (inserts), and electrolytic capacitors to avoid short-circuiting the vias and component housings after wave soldering;
4. The distance between the outer side of the component and the edge of the board is 5mm;
5. The outer side of the mounting component pad and the outer side of the adjacent interposing component are greater than 2 mm;
6. Metal shell components and metal parts (shield boxes, etc.) cannot be in contact with other components, and should not be in close contact with the printed wires or pads. The spacing should be greater than 2mm. The positioning hole, the fastener mounting hole, the elliptical hole and the outer side of the other hole in the plate are larger than 3 mm;
7. The heating element cannot be in close proximity to the wire and the heat-sensitive element; the high-heat device should be evenly distributed;
8. The power socket should be placed around the printed circuit board as much as possible. The power socket and the bus bar terminal connected to it should be placed on the same side. Special care should be taken not to place power outlets and other solder connectors between the connectors to facilitate soldering and power cable design and cable ties for these sockets, connectors. The spacing between the power socket and the soldering connector should be considered to facilitate the insertion and removal of the power plug;
9. Arrangement of other components:
All IC components are unilaterally aligned, and the polarity of the polar components is clearly marked. The polarity on the same printed board shall not be more than two directions. When two directions are present, the two directions are perpendicular to each other;
10, the board surface wiring should be properly dense, when the difference between the density is too large, it should be filled with mesh copper foil, the grid is larger than 8mil (or 0.2mm);
11. There should be no through holes on the chip pads to avoid solder joint loss and solder joints. Important signal lines are not allowed to pass between the socket feet;
12, the patch is unilaterally aligned, the characters are in the same direction, and the package direction is consistent;
13. Polarized devices should be as consistent as possible in the direction indicated by the polarity on the same board.
Second, the component wiring rules1. Draw wiring in the area where the wiring area is ≤1mm from the edge of the PCB board and within 1mm around the mounting hole.
2, the power cord should be as wide as possible, should not be lower than 18mil; signal line width should not be lower than 12mil; cpu input and output should not be lower than 10mil (or 8mil); line spacing is not less than 10mil;
3, the normal through hole is not less than 30mil;
4, double in-line: pad 60mil, aperture 40mil;
1/4W resistor: 51*55mil (0805 surface mount); 62 mil pad when in-line, 42 mil aperture;
Infinite capacitance: 51*55mil (0805 surface mount); pad 50mil when in-line, aperture 28mil;
5. Note that the power and ground wires should be as radial as possible, and the signal wires should not have loopback traces.
Third, the PCB design processThe general PCB basic design flow is as follows: pre-preparation - PCB structure design - PCB layout - wiring - wiring optimization and silk screen - network and DRC inspection and structural inspection - plate making.
First: preliminary preparation. This includes preparing the component library and schematics. "If you want to do something good, you must first sharpen your tools." To make a good board, in addition to designing the principles, you must draw well. Before proceeding with PCB design, first prepare the component library of the schematic SCH and the component library of the PCB. The component library can use the library that comes with peotel, but it is generally difficult to find a suitable one. It is better to do the component library yourself according to the standard size data of the selected device. In principle, first do the component library of the PCB, and then do the component library of the SCH. PCB component library requirements are higher, it directly affects the board installation; SCH component library requirements are relatively loose, as long as you pay attention to define the pin properties and the corresponding relationship with the PCB components. PS: Pay attention to the hidden pins in the standard library. Then there is the design of the schematic, and when it is ready, it is ready to start PCB design.
Second: PCB structure design. This step draws the PCB surface in the PCB design environment according to the determined board size and various mechanical positioning, and places the required connectors, buttons/switches, screw holes, assembly holes, etc. according to the positioning requirements. And fully consider and determine the wiring area and non-wiring area (such as the extent of the screw hole is a non-wiring area).
Third: PCB layout. The layout is white is to put the device on the board. At this time, if the preparations mentioned above are all done, you can generate a network table (Design-"Create Netlist) on the schematic, and then import the network table (Design-"Load Nets) on the PCB. I saw the full stack of devices, and there are flying lines between the pins. Then you can lay out the device.
The general layout is based on the following principles:
1. According to the reasonable electrical division, it is generally divided into: digital circuit area (that is, interference and interference), analog circuit area (fear of interference), power drive area (interference source);
2. The circuit that completes the same function should be placed as close as possible, and the components should be adjusted to ensure the most compact connection. At the same time, adjust the relative position between the function blocks to make the connection between the function blocks the most concise;
3. For high quality components, the installation position and installation strength should be considered; the heating elements should be placed separately from the temperature sensitive components, and thermal convection measures should also be considered when necessary;
4. The I/O driver device is as close as possible to the edge of the printed board, close to the lead-out connector;
5. The clock generator (such as crystal or clock) should be as close as possible to the device that uses the clock;
6. A decoupling capacitor (usually a monolithic capacitor with high frequency performance) is required between the power input pin of each integrated circuit and ground. When the board space is dense, one can be added around several integrated circuits. Tantalum capacitors.
7. A discharge diode should be added to the relay coil (1N4148); The layout requirements should be balanced, orderly and orderly, not top-heavy or sinking
——Special attention should be paid to the actual size (area and height) of the components and the relative position between the components when placing the components to ensure the electrical performance of the circuit board and the feasibility of production and installation. At the same time of convenience, under the premise of ensuring that the above principles can be embodied, the placement of the device should be properly modified to make it neat and tidy. For example, the same device should be placed neatly and in the same direction, and it should not be placed in a "staggered manner".
This step is related to the overall image of the board and the difficulty of wiring in the next step, so it takes a lot of effort to consider. When laying out the layout, you can make preliminary wiring for the place that is not sure.
Fourth: wiring. Wiring is the most important process in the overall PCB design. This will directly affect the performance of the PCB. In the design process of the PCB, the wiring generally has three kinds of boundaries: the first is the connection, the most basic requirements of the PCB design. If the line is not laid, and the line is flying, it will be an unqualified board. It can be said that it has not yet started. Second is the satisfaction of electrical performance. This is a measure of the eligibility of a printed circuit board. This is after the connection, carefully adjust the wiring to achieve the best electrical performance. Then it is beautiful. If your wiring is connected, there is no place that affects the performance of the electrical appliances, but at a glance, in the past, with a lot of colorful, colorful, then how good your electrical performance is, in the eyes of others is still a piece of garbage. This brings great inconvenience to testing and maintenance. The wiring should be neat and uniform, and there should be no rules. These must be achieved while ensuring the electrical performance and meeting other individual requirements, otherwise it will be the end.
The wiring is mainly carried out according to the following principles:
1. In general, the power and ground wires should be routed first to ensure the electrical performance of the board. Within the scope of the conditions, try to widen the power supply and ground line width. It is better to ground the ground line than the power line. Their relationship is: ground line > power line > signal line. Usually the signal line width is 0.2~0.3mm. The finest width can reach 0.05~0.07mm, and the power cord is generally 1.2~2.5mm. For the PCB of the digital circuit, a wide ground wire can be used to form a loop, that is, a ground net is used (the ground of the analog circuit cannot be used in this way)
2. Wires that require more stringent requirements (such as high-frequency lines) are pre-wired, and the edges of the input and output ends should be avoided to avoid adjacent reflections to avoid reflection interference. If necessary, ground wire should be isolated. The wiring of two adjacent layers should be perpendicular to each other, and parasitic coupling is easy to occur in parallel.
3. The oscillator case is grounded and the clock line is as short as possible and cannot be pinned everywhere. Below the clock oscillating circuit, the special high-speed logic circuit part should increase the area of ​​the ground, and should not take other signal lines, so that the surrounding electric field approaches zero;
4. Use 45o of polyline wiring as much as possible, and 90o fold lines should not be used to reduce the radiation of high frequency signals; (higher lines require double arcs)
5. Do not form a loop on any signal line. If it is unavoidable, the loop should be as small as possible; the number of vias of the signal line should be as small as possible;
6. The key lines are as short and thick as possible, with protective ground on both sides.
7. When transmitting a sensitive signal and a noise field band signal through a flat cable, it is extracted by means of "ground-signal-ground".
8. Key signals should be reserved for test points to facilitate production and maintenance testing.
9. After the schematic wiring is completed, the wiring should be optimized. At the same time, after the initial network inspection and the DRC inspection are correct, the grounding of the unwired area is performed, and the large-area copper layer is used as the grounding line, and the printed board is not used. The places used are connected to the ground as ground. Or make a multi-layer board, power supply, ground line each occupy a layer.
——PCB wiring process requirements 1. line
In general, the signal line width is 0.3mm (12mil), the power line width is 0.77mm (30mil) or 1.27mm (50mil); the distance between the line and the line and between the line and the pad is greater than or equal to 0.33mm (13mil) ), in practical applications, the conditions should be considered to increase the distance;
When the wiring density is high, it is considered (but not recommended) to use two wires between the IC pins. The width of the wire is 0.254 mm (10 mil) and the line spacing is not less than 0.254 mm (10 mil). In special cases, when the device pins are dense and the width is narrow, the line width and line spacing can be appropriately reduced.
2. Pad (PAD)
The basic requirements for pads (PAD) and transition holes (VIA) are: the diameter of the disk is larger than the diameter of the hole by more than 0.6 mm; for example, general-purpose pin type resistors, capacitors, and integrated circuits, with a disk/hole size of 1.6 mm / 0.8 Mm (63 mil / 32 mil), socket, pin and diode 1N4007, etc., using 1.8mm / 1.0mm (71mil / 39mil). In practical applications, it should be determined according to the size of the actual components. When conditions are met, the pad size can be appropriately increased.
The component mounting aperture on the PCB should be approximately 0.2 to 0.4 mm larger than the actual size of the component pins. 3. Via (VIA)
Generally 1.27mm / 0.7mm (50mil / 28mil);
When the wiring density is high, the via size can be appropriately reduced, but it should not be too small, and 1.0 mm/0.6 mm (40 mil/24 mil) can be considered.
4. Pad, line, via spacing requirements PAD and VIA: ≥ 0.3mm (12mil) PAD and PAD : ≥ 0.3mm (12mil) PAD and TRACK : ≥ 0.3mm (12mil) TRACK and TRACK : ≥ 0.3mm (12mil) When the density is high:
PAD and VIA : ≥ 0.254mm (10mil) PAD and PAD : ≥ 0.254mm (10mil) PAD and TRACK : ≥ 0.254mm (10mil) TRACK and TRACK : ≥ 0.254mm (10mil)
Fifth: wiring optimization and silk screen printing. "There is no best, only better!" No matter how you dig into the design, after you finish painting, if you look at it again, you will still feel that many places can be modified. The general design experience is that the time to optimize the wiring is twice the time of the initial wiring. Feel that there is no place to modify, you can lay the copper (Place-"polygon Plane). Copper is generally laid on the ground (note the separation of the analog ground and the digital ground), and the power supply may be required when the multi-layer board is used. For silk screen, be careful not to be blocked by the device or removed by vias and pads. At the same time, the design faces the component surface, and the underlying words should be mirrored to avoid confusion.
Sixth: Network and DRC inspection and structural inspection. Firstly, under the premise that the circuit schematic design is correct, the network check (NETCHECK) of the physical connection relationship between the generated PCB network file and the schematic network file is performed, and the design is corrected according to the output file result to ensure the wiring. The correctness of the connection relationship;
After the network check is passed correctly, the PCB design is DRC checked, and the design is corrected according to the output file results to ensure the electrical performance of the PCB layout. Finally, the mechanical installation structure of the PCB needs to be further checked and confirmed.
Seventh: plate making. Before that, it is best to have a review process.
PCB design is a work of thought, who has a good mind and high experience, and the board is designed. Therefore, the design should be extremely careful, and fully consider the factors of various aspects (for example, many people will not consider it for easy maintenance and inspection). If you are improving, you will be able to design a good board.
Fourth, pcb wiring experience electromagnetic compatibilityHow to improve anti-interference ability and electromagnetic compatibility when developing electronic products with processors?
1, the following systems should pay special attention to anti-electromagnetic interference:
(1) A system with a particularly high clock frequency and a particularly fast bus cycle.
(2) The system contains high-power, high-current drive circuits, such as spark-generating relays, high-current switches, and so on.
(3) A system containing a weak analog signal circuit and a high-precision A/D conversion circuit.
2. To increase the system's anti-electromagnetic interference capability, take the following measures:
(1) Select a low frequency microcontroller:
The use of a microcontroller with a low external clock frequency can effectively reduce noise and improve the system's anti-interference ability. For square waves and sine waves of the same frequency, the high frequency components in the square wave are much more than the sine waves. Although the amplitude of the wave of the high-frequency component of the square wave is smaller than the fundamental wave, the higher the frequency, the easier it is to emit the noise source. The most influential high-frequency noise generated by the microcontroller is about three times the clock frequency.
(2) Reduce distortion in signal transmission
Microcontrollers are primarily manufactured using high speed CMOS technology. The static input current of the signal input terminal is about 1mA, the input capacitance is about 10PF, and the input impedance is quite high. The output of the high-speed CMOS circuit has considerable load capacity, that is, a considerable output value, and the output of one gate passes a very long period. The long line leads to the input with a relatively high input impedance, and the reflection problem is very serious, which causes signal distortion and increases system noise. When Tpd>Tr, it becomes a transmission line problem, and signal reflection, impedance matching, etc. must be considered.
The delay time of the signal on the printed board is related to the characteristic impedance of the lead, which is related to the dielectric constant of the printed wiring board material. It can be roughly assumed that the transmission speed of the signal on the printed circuit board leads is between about 1/3 and 1/2 of the speed of light. The Tr (standard delay time) of a logical telephone component commonly used in a system composed of a microcontroller is between 3 and 18 ns.
On the printed circuit board, the signal passes through a 7W resistor and a 25cm lead, and the line delay time is approximately 4~20ns. That is to say, the shorter the lead of the signal on the printed circuit, the better, the longest should not exceed 25cm. Also, the number of vias should be as small as possible, preferably no more than two.
When the rise time of the signal is faster than the signal delay time, it must be processed according to fast electronics. In this case, the impedance matching of the transmission line should be considered. For the signal transmission between the integrated blocks on a printed circuit board, the situation of Td>Trd should be avoided. The larger the printed circuit board, the less the speed of the system should be too fast.
Use the following conclusions to summarize a rule for printed circuit board design:
The signal is transmitted on the printed board and the delay time should not be greater than the nominal delay time of the device used.
(3) Reduce the interference between signal lines:
At step A, a step signal with a rise time of Tr is transmitted through the lead AB to the B terminal. The delay time of the signal on the AB line is Td. At point D, due to the forward transmission of the A-point signal, the signal reflection after reaching point B and the delay of the AB line, a page pulse signal of width Tr is induced after Td time. At point C, due to the transmission and reflection of the signal on the AB, a width of twice the delay time of the signal on the AB line, that is, a positive pulse signal of 2Td, is induced. This is the interference between the signals. The strength of the interfering signal is related to the di/at of the C-point signal and to the distance between the lines. When the two signal lines are not very long, what is seen on AB is actually the superposition of two pulses.
The micro-control made by CMOS technology has high input impedance, high noise and high noise tolerance. The digital circuit is superimposed with 100~200mv noise and does not affect its operation. If the AB line in the figure is an analog signal, this interference becomes intolerable. For example, if the printed circuit board is a four-layer board, one of which is a large-area ground or a double-panel, and when the reverse side of the signal line is a large-area ground, the interference between the signals becomes small. The reason is that the large-area ground reduces the characteristic impedance of the signal line, and the reflection of the signal at the D-end is greatly reduced. The characteristic impedance is inversely proportional to the square of the dielectric constant of the medium between the signal line and the ground, and is proportional to the natural logarithm of the thickness of the medium. If the AB line is an analog signal, to avoid the interference of the digital circuit signal line CD to AB, there should be a large area below the AB line, and the distance from the AB line to the CD line is greater than 2 to 3 times the distance between the AB line and the ground. A local shield can be used to ground the left and right sides of the leads on the lead side.
(4) Reduce noise from the power supply
While the power supply supplies power to the system, it also adds its noise to the power supply that is being supplied. The reset lines, interrupt lines, and other control lines of the microcontroller in the circuit are most susceptible to external noise. Strong interference on the electrical network enters the circuit through the power supply. Even in battery-powered systems, the battery itself has high frequency noise. Analog signals in analog circuits are more resistant to interference from the power supply.
(5) Pay attention to the high frequency characteristics of printed wiring boards and components
In the case of high frequency, the leads, vias, resistors, capacitors, and the distributed inductance and capacitance of the printed circuit board cannot be ignored. The distributed inductance of the capacitor is not negligible, and the distributed capacitance of the inductor cannot be ignored. The resistor produces a reflection of the high frequency signal, and the distributed capacitance of the lead acts. When the length is greater than 1/20 of the corresponding wavelength of the noise frequency, an antenna effect is generated and the noise is emitted outward through the lead.
The via of the printed wiring board causes a capacitance of approximately 0.6 pf.
The packaging material of an integrated circuit itself introduces a 2~6pf capacitor.
A connector on a board with a distributed inductance of 520nH. A two-column, 24-pin integrated circuit pedestal that introduces a distributed inductor of 4 to 18 nH.
These small distribution parameters are negligible for this line of microcontroller systems at lower frequencies; special attention must be paid to high speed systems.
(6) Component layout should be properly partitioned
The position where the components are arranged on the printed circuit board should fully consider the problem of electromagnetic interference resistance. One of the principles is that the leads between the components should be as short as possible. In the layout, the analog signal part, the high-speed digital circuit part, the noise source part (such as relay, high-current switch, etc.) are reasonably separated, so that the signal coupling between them is minimized.
Handle the ground wire
Power lines and ground lines are the most important on printed circuit boards. The main means of overcoming electromagnetic interference is grounding.
For the double-panel, the grounding wire arrangement is particularly stressful. By adopting the single-point grounding method, the power source and the ground are connected to the printed circuit board from both ends of the power supply, and the power supply has one contact and one ground. On the printed circuit board, there must be multiple return ground lines, which will be gathered back to the contact point of the power supply, which is called single point grounding. The so-called analog ground, digital ground, high power device opening points, the wiring is separated, and finally gathered to this grounding point. Shielded cables are commonly used when connected to signals other than printed circuit boards. For high frequency and digital signals, both ends of the shielded cable are grounded. A shielded cable for low-frequency analog signals is grounded at one end.
Circuits that are very sensitive to noise and interference or circuits with particularly high frequency noise should be shielded with a metal cover.
(7) Use decoupling capacitors.
Good high frequency decoupling capacitors remove high frequency components up to 1 GHz. Ceramic chip capacitors or multilayer ceramic capacitors have good high frequency characteristics. When designing a printed circuit board, a decoupling capacitor is added between the power supply of each integrated circuit and the ground. The decoupling capacitor has two functions: one is the storage capacitor of the integrated circuit, which provides and absorbs the charge and discharge energy of the integrated circuit when the door is opened and closed; on the other hand, the high frequency noise of the device is bypassed. The decoupling capacitor with a typical decoupling capacitor of 0.1uf in a digital circuit has a 5nH distributed inductor. Its parallel resonant frequency is about 7MHz, which means better decoupling for noise below 10MHz, and above 40MHz. The noise hardly works.
1uf, 10uf capacitor, parallel resonance frequency above 20MHz, the effect of removing high frequency noise is better. It is often advantageous to have a 1uf or 10uf de-HF capacitor where the power supply enters the printed board, even for battery-powered systems.
For every 10 or so ICs, add a charge and discharge capacitor, or a storage capacitor. The capacitor size is 10uf. It is best not to use an electrolytic capacitor. The electrolytic capacitor is rolled up by two layers of enamel film. The rolled structure is an inductor at high frequencies, and it is preferable to use a bile capacitor or a polycarbonate capacitor.
The selection of the decoupling capacitor value is not strict, and can be calculated according to C=1/f; that is, 10 Hz is taken as 0.1 uf, and the system composed of the microcontroller can be between 0.1 and 0.01 uf.
3. Some experience in reducing noise and electromagnetic interference.
(1), high-speed chips can be used in low-speed chips, and high-speed chips are used in key places.
(2), a string of resistors can be used to reduce the jump rate of the upper and lower edges of the control circuit.
(3) Try to provide some form of damping for relays and the like.
(4) Use the lowest frequency clock that meets the system requirements.
(5) The clock generator should be as close as possible to the device using the clock. The quartz crystal oscillator housing should be grounded.
(6) Circle the clock area with the ground wire and keep the clock line as short as possible.
(7), I / O drive circuit as close as possible to the side of the printed board, let it leave the printed board as soon as possible. The signal entering the printed board should be filtered, and the signal from the high-noise area should be filtered. At the same time, the signal of the string termination is used to reduce the signal reflection.
(8), MCD useless terminal to be connected, or grounded, or defined as the output terminal, the end of the integrated circuit should be connected to the power supply ground, do not hang.
(9) The input terminal of the unused circuit should not be left floating. The unused input terminal of the op amp is grounded, and the negative input terminal is connected to the output end. (10) The printed board should use 45-fold lines instead of 90-fold lines to reduce the external transmission and coupling of high-frequency signals.
(11) The printed board is divided according to the frequency and current switching characteristics, and the noise component is farther away from the non-noise component.
(12) Single-panel power supply and single-point grounding, single-point grounding, power supply line and grounding wire for single-panel and double-panel are as thick as possible. If the economy can withstand, use multi-layer board to reduce the power supply and grounding inductance.
(13) The clock, bus, and chip select signals should be kept away from the I/O lines and connectors.
(14), analog voltage input line, reference voltage end should be as far as possible from the digital circuit signal line, especially the clock.
(15) For A/D type devices, the digital part and the analog part are more uniform and do not pay *.
(16) The clock line is perpendicular to the I/O line and has less interference than the parallel I/O line. The clock component pins are far from the I/O cable.
(17) The component pins are as short as possible, and the decoupling capacitor pins are as short as possible.
(18) The key lines should be as thick as possible and the protected areas should be added on both sides. The high speed line should be short and straight.
(19) The noise sensitive line should not be parallel to the high current, high speed switch line.
(20) Do not route underneath the quartz crystal and below the noise sensitive device.
(21), weak signal circuit, do not form a current loop around the low frequency circuit.
(22), do not form a loop for any signal, if it is unavoidable, make the loop area as small as possible.
(23) One decoupling capacitor for each integrated circuit. A small high frequency bypass capacitor is added to each electrolytic capacitor.
(24) Use a large-capacity tantalum capacitor or a condenser capacitor instead of an electrolytic capacitor for the circuit to charge and discharge the storage capacitor. When using a tubular capacitor, the housing should be grounded.
Lithium Battery Pack For Hybrid Electric Locomotive
Lithium Battery For Electric Car,Cylindrical Battery,2000-2500Kw Lifepo4 Battery System,Ce Certificated Lithium Battery
Henan Xintaihang Power Source Co.,Ltd , https://www.taihangbattery.com